Copyright 0 1992 by the Genetics Society of America

Evolution of Multilocus Genetic Structurein an Experimental Barley Population R. W. Allard,* Qifa Zhang? M. A. Saghai Maroof* and0. M. Muonas *Department of Genetics, University of Calfornia, Davis, Calqornia 95616, +BiotechnologyCenter, Huazhong Agricultural University, Wuhan 430070, People’s Republic of China, $Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, and #Department of Genetics, University of Oulu, SF-90570, Finland Manuscript received September16, 1991 Accepted for publication May 2, 1992

ABSTRACT Data from 31 1 selfed families isolated from four generations (Fa, Fls, Fz5, F45) of an experimental barley population were analyzed to determine patterns of change in character expression for seven quantitative traits, and in single-locus allelic frequencies, and multilocus genetic structure, for 16 Mendelian loci that code for discretely recognizable variants. The analyses showed that large changes in single-locusallelic frequencies and major reorganizations in multilocus geneticstructure occurred in eachof the generation-to-generation transitions examined. Although associations among a few traits persisted over generations, dynamic disassociations and reassociations occurred among several traits in each generation-transition period. Overall, the restructuring that occurred was characterized by gradual decreases in the number of clusters of associated traits and increases in the number of traits within each cluster. T h e observed changesin single-locus frequencies andin multilocus genetic structure were attributed to interplayamongvariousevolutionaryfactorsamong which natural selection acting in a temporally heterogeneous environmentwas the guiding force.

G

ENETIC changes associated with the evolution of adaptedness have been studied in detail in experimental populations of several species of cultivated plants and also in natural populations of the wild ancestors of these domesticated species (reviews in ALLARD 1965, 1975, 1977, 1988, 1990; ALLARD, JAIN andWORKMAN1968;and references cited in these reviews). Composite cross I1 (CCII), an experimental population of cultivated barley (Hordeum uulgure ssp. uulgure), has been studied most intensively. CCII was synthesized in 1928 by pooling equal numbers of F2 seeds obtained by selfing F1 hybrid plants derived from the 378 possible pairwise intercrosses among 28 barley cultivars chosen to represent all of the major barley growing areasof the world (HARLAN and MARTINI1929). This population has subsequently been grown annuallyat Davis, California, under standard agricultural conditions. Each generation was allowed toreproduce by its naturalmating system (-99% of selfing andonepercent of outcrossing, WAGNERand ALLARD1991),harvested in bulk at maturitywithout conscious selection, and the next generation sown from a randomsample of seeds from the previous harvest. Two otherbroadly based experimentalpopulations of barley, CCV andCCXXI, which differ from each other and from CCII in parentage and in method of synthesis, have also been studied in detail with respect to allelic frequency changes which occurred over generationsat a number of Mendelian loci that code for discretely recognizable Genetics 131: 957-969 (August, 1992)

variants, as well as for numerous continuously distributed quantitative traits often supposed to be components of adaptedness. Observed genetic changes in each trait followed closely similar patterns in all three populations. Population size exceeded 15,000 reproducing adults in each generation; consequently statistically significant genetic changes that occurredin the populations are attributable to natural selection and not to genetic drift. Considering their broadly based hybrid origin it is notsurprising thatCCII, CCV and CCXXIwere conspicuously more variable genetically, especially in early generations, than landraces or modern cultivars of barley. Genetic variability was especially obvious when families derived by selfing numerous randomly chosen single plants from early, intermediateand late generations of theexperimentalpopulations were grown side-by-side in family rows. Under such conditions it could readily be seen that different families varied in continuous series for such traits as flowering time, height, spike size and dimensions, and leaf size and shape. Measurements revealed that steady unidirectional changes occurred for only two of the quantitative traits studied,number of seeds/spike and spike weight, both of which are direct components of fecundity. These two quantitative characters each increased by more than 20 percent from the earliest generationstogeneration F ~ o ;increases continued steadily thereafter to the latest generations, but at a slower rate. Increases in these two traits over genera-

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R. W. Allard et al.

tions were parallel to increases in grain yield, determined by measuringgrain yieldsin replicated plot trials, repeated several years, and comparing the grain yields of early, intermediate and late generations of the experimental populations with the grain yields of several widely grown commercial cultivars entered as standards in the same nurseries. Measurements made on other quantitative traits indicated that changes in the direction of shorter and morecompact spikes were also steadily unidirectionalbutthatchanges inall other quantitative traits, e.g., height, flowering time, awn length and culm thickness, were erratic in direction and not significantly correlated with fecundity as measured by number of seeds/spike and spike weight. CCII, CCV and CCXXI have also beenstudied intensively with respect to single-locus allelic and genotypic frequency changes at more than 20 loci that code for discretely recognizable variants,including morphological variants, resistance us. susceptibility to specific pathotypes of diseases, allozyme variants and DNA restriction fragment variants (e.g.,JAIN and ALL A R D 1960;ALLARDand KAHLER 1971,1972; ALLARD, KAHLERand WEIR 1972; KAHLER1973; SAGHA1 MAROOFet al. 1984; WEBSTER,SAGHAI MAROOF andALLARD1986;ALLARD1988).These studies showed that alleles present in very high frequencies in the parents of these experimental populations consistently remained in high frequency into advanced generations of the populations;this suggests that these prominent alleles are “generalist” alleles favored in virtually all environments worldwide. Patterns of population behavior were different, however, forloci that were moderately to highly polymorphic in the parents of the experimental populations: frequency changes of the more frequentalleles of such loci exceeded two standard error units in approximately one-half of single-generation transitions and very large single-generation frequency changes(>5 SE units) were common for the more frequentalleles of such loci in the Davis environment. Also single-generation allelic frequency changes foralleles ofthe highly polymorphic loci were often in opposite directions and correlated with annual environmental fluctuations, especially with shifts from conditions of drought to conditions of ample rainfall, and vice versa. Reversals persisting for two o r sometimes several generations were also observed; such patterns were also often associated with periods in which two or more years of drought alternated with several years of more orless ample rainfall. These results suggested that specific alleles of the polymorphic loci have pleiotropic effects ononeormore physiological processes that affect survivability and/ o r reproductive success under specific environmental circumstances and that alternative alleles have favorable pleiotropic effects under differing environmental circumstances. This hypothesis was tested by extensive

progeny testing of selfed families, and also of iosgenic lines, eachdescendedfrom single plants extracted from early, intermediate or late generations of CCII or CCV. Comparisons between alternative homozygous variants of single Mendelian loci revealed that specific alleles of morphological, allozyme and disease resistance ‘us. susceptibility loci had statistically significant effects on the expression of several quantitative traits, including ability to survive to reproductive maturity and the number of seeds produced/spike by the surviving plants. Thus each Mendelian locus tested was, in addition to the descriptive effects for which it was named, also a locus for several quantitative traits. These pleiotropic effects often varied significantly in magnitude, and sometimes in direction in different years, so that testing of alternative alleles in more than one year was required to obtain reliable estimates of the pleiotropic effects of a given locus. Such testing revealed that superior ability to survive to reproductive maturity, that larger numbersof seeds/spike, and that greater weight of spike were nearly always positively associated with alleles that increased in frequency under population conditions in the same year. No other quantitative traits were consistently associated with increases in allelic frequencies. It is also well documented that barleys from different ecogeographical regions nearly always differ significantly fromoneanother in multilocus genetic structure (e.g., WARD 1962; BROWN,FELDMAN and NEVO1980; KAHLERand ALLARD 198 1; ZHANG, SAGHAI MAROOFand ALLARD 1990; ALLARD et al. 1990, SAGHAIMAROOF,ALLARD and ZHANG 1990). Studies of the population dynamics of experimental populations of cultivated barleys have broughtto light a number of additional features of multilocus genetic structure, among which the following are particularly germane to the present study: (1) statistically significant nonrandom associations among alleles of different loci developed within a few generations in each of the populations studied (e.g., WEIR, ALLARD KAHand LER; 1972, 1974, CLEGG, ALLARD and KAHLER1972; ALLARD 1975, 1977, 1988); (2) the internal structure of two-locus, three-locus, four-locus and five-locus associations frequentlychangedovergenerations, often in concert with shifts in environmental factors and/or shifts in the multilocus background genotype of the population (e.g., CLEGG, ALLARD and KAHLER 1972,1978; SAGHAIMAROOFet al. 1984;ALLARD 1988, ALLARDet al. 1990); (3) different populations grown in the same environment soon developed similar multilocus structures (e.g., ALLARD andKAHLER 1972; ALLARD1975, 1977, 1988); (4)subpopulations of the same experimental population soon developed different multilocus structures when transferredto different environments. Thus, subpopulations transferred to the North Plains region of the United States

Barley

in

Evolution Multilocus

and Canada, or tolocations in Californiathat are morexeric or more mesic than Davis, developed multilocusstructuresdifferentfromthat of Davis within a few generations, e.g., the four-locus gametic type for esterase loci 1 , 2, 3 a n d 4 shiftedinthe direction of the gametic type thatis most common of the North Plains region, whereas the four-locus gametic types that are favored in years of drought vs. ample rainfall in Davis increased in frequency in subpopulations transferred to xeric and mesic sites, respectively, in California (ALLARD1988; JANA,ZHANG and SAGHAIMAROOF 1989). Similarlyquantitative traits shifted in degree of character expression, e.g., toward earlier maturity, shorter stature and smaller kernel sizein xericCaliforniahabitatsandtoward later maturity, taller stature and larger kernel size in mesic Californian habitats. O u r goal in the present experiment was to obtain a comprehensive profile of the evolution of multilocus structure in CCII through simultaneous analyses of genetic change for numerous diverse traits over 45 generations. We selected 22 traits, each of which had beeninvestigated in detail in previousstudies,for inclusion in the present study. Our results show: (1) character expression for most of the seven quantitative traits studied changed significantly over generationsindicatingthatthesetraitswereaffected by natural selection; (2) that at least 14 of 15 discretely inherited traits were under strong directional selection; (3) that nonrandom associations amongtraits had developed by generation Fs; (4) that major reorganizations of associations among traits occurred in generation transitions Fs t o Fl3, Fl3 t o F23 a n d F23 t o Fd5. Overall these reorganizations led to increases in the numberof traits in each cluster of associated traits accompanied by decreases in the number of clusters. We concluded that the changes in quantitative trait expressions, in allelic frequencies of Mendelian loci governing discretely recognizable variants, as well as changes in multilocus population structure, resulted from complex interplay among several evolutionary factors, among which natural selection acting in the temporally heterogeneous environment of Davis was the guiding force in organizing the allelic resources o f CCII into increasingly larger multilocus complexes that enhance overall population fitness. MATERIALS AND METHODS The genetic materials of this study were 31 1 families derived from random samples of stored kernels of generations Fg, F13, F23 and Fq5of CCII. Kernels of these generations were planted at wide spacing (30 X 30 cm) in a field nursery to minimize opportunity for intercrossing between plants and to optimize the number of kernels produced by each plant. Each plant was allowed toreproduce by its natural mating system of self pollination and, at maturity, all kernels were harvested from individual plants and kept separate to establish 80 self-fertilized families derived from

959 each generation Fs, Fls, FZ3and F45. Each of these families was subsequently progeny tested to characterize it for the 22 characters described in the following sections. Ultimately the number of families with complete records for all 22 characters were 80, 77, 78 and 74 for generations F8, Fls, Fzs, and F45, respectively. Quantitativecharacter: Quantitative character expression was calculated from measurements made on single plants grown in replicated field plantings (2 blocks/family), repeated intwosuccessiveyears.Eachblock (replicate) contained 17 plants of each family, spacedat intervals of 30 cm in rows 30 cm apart in the first year, and 10 plants of each family planted at the same spacing in the second year. The quantitative characters measured were: ( 1 ) heading date (date of emergence of the first spike of each plant, measured as number of days after March 23); (2) the number of reproductive tillers produced by each plant; (3) plant height (height in cm of the tallest tiller); (4) spike length (cm); (5) number of spikelets/row; (6) grain yield (weightof kernels/plant, g); and (7) kernel size index (g/100 kernels). The data were subjected to a standard analysis of variance in which the effects of replications, test years, generations and families, and interactions among these main effects on the expression of each of the seven quantitative traits were computed and tested for statistical significance.Correlation analyses were based on family means obtained by summing the values of the 54 plants within each family and dividing by the number of plants. Measurements made under experimentally convenient wide spacings, such as those of the present experiment, often differ from measurements made on plants grown in dense stands inwhich plant-to-plant competition is intense. Thus the changes in quantitative character expression observed in the present experiment, as well as the correlations between pairs of quantitative characters with each other and with discretely inherited traits, may not reflect accurately patterns of change as expressed in the dense plantings in which CCII evolved. Morphologicalvariants: All adult plantsin our field plantings were also scored for phenotype for the following morphological variants (the first named variant is dominant and the second recessive ineach case): blue ( B l )vs. non-blue (61) aleurone color (located on chromosome 4 ) ; rough ( R ) us. smooth ( r )awn (chromosome 7); long ( S ) vs. short-haired (s) rachilla (chromosome 7); two-row ( V ) vs. six-row ( v ) spike (chromosome 2 ) . As expected, nearly all families were uniform (homozygous) for one of the alternative variants for each locus but about 2% of families segregated in ratios of approximately 3: 1 fordominant us. recessive variants. These three types of family were classified, respectively, as homozygous dominant, homozygous recessive, or heterozygous for each of the four loci. Reactiontoscalddisease: Two-week-oldseedlingsof each family were inoculated in a mist chamber with one or another of four pathotypes (races 40, 61, 72, 74) of Rhynchosporium secalis (the fungus that causesscalddisease), following the methods of JACKSON and WEBSTER(1976). Individual seedlings were scored to determine the family reaction (homozygous resistant, homozygous susceptible or segregating) to each of the four races. Resistancevs. susceptibility to races 40, 61 and 74 is governed by a single Mendelian locus (resistance dominant to susceptibility for races 40 and 74 but recessive for race 6 1 ) whereas resistance us. susceptibility to race 72 is governed by a pair of loci: individuals with at least one dominant allele at each locus (RI-R2-) are resistant, whereas individuals with genotypes R I - ~ z ~rIrIR2z, and rIrIrzrzare susceptible (GURUSINGHE 1984). Adequate numbers of plants were scored to allow each familyto be classified unambiguously(probability 0.01)

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R. W. Allard et al.

as homozygous resistant, homozygous susceptible or segregating in ratios of approximately 3R: 1 s for races 40 and 74 race 6 1. Individuals resistant to race 72 were and 3s:1 R for infrequent in all four generations of CCII studied and all families within which resistant plants were observed segregated in a ratioof approximately 9R:7S, indicating that each of these families was heterozygous for both of the two resistance alleles. The loci governing resistance vs. susceptibility to races 40, 61 and 74 are loosely linked on chromosome 3. The twoloci governing resistance to race 72 segregate independently of each other and the three other resistance loci; the chromosome locationsof this pair of complementary loci are not known. Allozyme variants: Seedlings of each family were assayed electrophoretically for four esterase loci ( E s t l , EstZ, Est3, Est4) (KAHLERand ALLARD 1970) and foracid phosphatase HEATH-PAGLUISO and ALLARD1981). locus Acpl (KAHLER, Sample sizes permitted families to be classified unambiguously (probability 0.01) as homozygous or heterozygous for specific variants of these codominant loci. E s t l , Est2 and Est3 are very tightly linked on chromosome 3 (Est24-0,003+ Est1+0~004X-+Est3), whereas Est4 is on chromosome 1 and Acpl is on chromosome 6 . RibosomalDNA (rDNA) variants: Seedlingsof each family were assayed for phenotype for two Mendelian loci, R r n l and R m 2 which code for 12 and 8alleles, respectively MAROOFet al. 1984; ALLARD et al. 1990). Sample (SAGHAI sizeswere adequate to allow each family to beclassified unambiguously (probability 0.01) as homozygous or heterozygous for specific codominant alleles of these two loci. R r n l and R m 2 are located on chromosomes 6 and 7, respectively. In summary the 22 traits studied were: (trait 1) heading date; (2) tillers/plant; (3) plant height; (4) spike length; ( 5 ) spikelets/row; (6) yield; (7) kernel size; (8) Estl; (9) EstP; ( 10) Est3; (1 1) Est4; (12) Acp 1 ; (13) reaction to race 74; (14) reaction to race 72; (15) reaction to race 61; (16) reaction to race 40; (17) aleurone color (Bl bl); 18) awn texture ( R r ) ; (1 9)rachilla hairiness (Ss);(20) two-row vs. six-row (Vv);(2 1) R r n l ; (22) R m 2 . The 16 Mendelian loci that code for traits 8-22 are distributed as follows on the seven chromosomes of barley: one locus is on each of chromosomes 1, 2 and 4, two loci are on chromosome 6 , three loci are on chromosome 7, six loci are on chromosome 3, the location of two loci is unknown and no loci are on chromosome 5. Each of these 16 Mendelian loci is known from previous studies to have statistically significant effects on at least one andusually on several among quantitative traits 1-7; presumably many other loci also affect each of these seven quantitative traits but neither the numbers nor the chromosome locations of these presumed additional loci are known. We note that linkage among loci, and also the mating system of predominant self fertilization, both have major implications concerning the coalescenceofloci into associated adaptive complexes as well as the disassociation of complexes of loci and (WRIGHT1933; JAIN and ALLARD 1966;COCKERHAM WEIR 1973; WEIRand COCKERHAM 1973; ALLARD1975). Linkage restricts recombination between loci that are physically close on the same chromosome but it does not affect recombination between loci on different chromosomes. Inbreeding, in contrast, affects recombination among all loci whether they are on the same or different chromosomes. In theory the -99% of selfing that occurs in barley causes all loci, whether situated far apart on the same chromosomes, o r located on different chromosomes, to behave as if they are linked with crossover values of c < 0.01. Thus, theory indicates that inbreeding in barley is a highly effective mechanism for organizing theentire gene pool into an

integrated system and also for reducing recombination to the point where little selection is required to protect favorable combinations of alleles from disassociation once they have formed (ALLARD 1975). Product-moment correlations and cluster analysis: Correlation studies with quantitative characters 1-7 were based on family means obtained by summing the measurement values of all plants in each family from the field plantings described above and dividing by the number (54) of plants scored. Correlation studies with the four sets of discretely recognizable characters were based on thefollowing scoring system: familiesthat were homozygous for the most frequent variant (allele) of each Mendelian locus were assigned the value 1.O, families that were homozygous for an alternative allele were assigned the value 0.0, and families that were segregating were assigned value0.5. When there were more than two alleles at any locus the most frequent allele was designated allele 1 and theremaining alleles werecombined into a "synthetic" allele designated allele 2; this procedure can result in some loss of information (WEIR and COCKERHAM 1978). Product-moment correlations were computed for all character pairs of generations F8, Fls, FZ5 and F45. The number of character pairs is large in each generation; consequently cluster analyses of correlations between character pairs were performed within each generation, using the correlation matrix as input,to serve as a guide for identifying groups of associated characters. Clusters were formed following an average linkage procedure (JOHNSON and WICHERN1982) inwhich the two clusters with the largest average similarity, as measured by the average correlation between all members of the clusters, were merged to form a new cluster. The critical values of the correlation coefficients, at probability level 0.05, varied from 0.21 to 0.22 from generation to generation, dueto unequal sample sizes in different generations. For simplicity we adopted r = 0.21 as the criterion for termination of these agglomerative processes withineach of the four generations.

RESULTS

Quantitative characters:Only the main conclusions reached from analyses of data for the seven quantitative traits will be given due to the large number of data points and the complexitiesof interrelationships of variance. The main among various components conclusions a r e t h efollowing. (1) Differences in character expression within the 2 test years were either nonsignificant or marginally significant; this indicates that the environment was essentially the same in the two replications grown in the first test year and in the second test year. (2) Character expression in test year 1 differed significantly from character expression in test year 2 for nearly all characters; this indicates that the environment was generally not the same for most characters in the two test years. (3) Character expression changed significantly from generation to generof the sevencharacters;changesin ation for most characterexpressionwere usuallysmall a n d sometimes in opposite directions in different generationto-generationintervals.Theseresultsindicatethat significant genetic changes occurred for most traits in the intervals from Fs to F13,F13 t o F23, and from F23 t o F45,and that the changes often tended to cancel

96 1

Multilocus Evolution in Barley

TABLE 1 Frequencies of the ultimately most frequent variant(allele) in the parents andin four generations of CCII Generation Trait

8 Est I 9 Est2 10 Est3 1 1 Est4 12 A c p l 13 Race 74 14 Race 72 15 Race 6 1 16 Race 40 17 blue 18 rough 19 short 20 six-row 21 Rrnl 22 R m 2

Parental’

Fe

FI,

Fzs

0.29 0.89 0.46 0.14 0.39 0.11 0.04 0.18 0.21 0.40 0.93 0.61 0.75 0.86 0.26

0.50 0.88 0.55 0.24 0.60 0.05 0.02 0.04 0.07 0.51 0.88 0.69 0.96 0.96 0.51

0.59 0.90 0.50 0.19 0.70 0.05 0.02 0.06 0.10 0.33 0.89 0.76 0.98 0.99 0.52

0.45 0.93 0.42 0.16 0.68 0.14 0.03 0.16 0.16 0.71 0.93 0.85 1.00 0.98 0.53

F~s

0.92 0.98 0.91 0.73 0.69 0.68 0.03 0.44 0.74 0.76 1.00 0.91 1.00 1.00 0.69

Parental allelic frequencies given in this table are from earlier studies of the parents of CCII [traits 8, 9, 10, 11) (R. W. ALLARD and A . L. KAHLER,unpublished); traits 13, 14, 15, 16 (WEBSTERet al. 1986); traits 17, 18, 19, 20 (R. W. ALLARDand A. L. KAHLER unpublished); traits 21, 22 (SAGHAI MAROOFet al. 1984)].

earlier changes. (4) Character expression often differed significantly among families within each generation within the 2 test years; this indicates that some families within each generation differed from each other genetically and hence that CCII remained genetically variable for metric characters into the late of generations.(5) Family X testyearcomponents variance were often statistically significant in generations Fe, Fls, FZ3 and F45; this indicates that higherorder genotype X environmentinteractions play a role in the evolution of population structure in CCII in most generations and years. Changes in single-locus allelic frequencies: Allelic frequencies for the 15 discretely recognizable Mendelian traits in the 28 parents, andin four generations of CCII, aregiven in Table 1. The frequencies in this table arethose of the alleles that were ultimately most frequent in the later generations of CCII. In a population with known parents and known mating system expected allelic and genotypic frequencies can becalculated for any selectively neutral locus in any generation, n, from WRIGHT’S(1921) well known equilibrium equation,

( p 2 + F’”)pq)[RR]+ 2 pq

(1

- F’”))

[Rr]

+ (q2 + F(”))[rr]= I ,

as illustrated by the following computations for resistance vs. susceptibility torace 40 (trait 16). Six (2 1.43%)of the 28 parents of CCII are homozygous for resistance (genotype R R ) and 22 are homozygous for susceptibility (genotype rr) to race 40. Thus frequencies, p and q, of alleles for resistance us. suscep-

tibility in the parental generation were p = 0.2143 and q = 0.7857 respectively. It can be deduced that 15 of the 378 pairwise F1 hybrids from which CCII was synthesized resultedfrom crosses between RR parents, 132 from RR X rr crosses and 231 from rr X rr crosses, thatthefrequencies of RR, Rr and rr individuals in the F1 generation of CCII were 0.04, 0.35and0.61, respectively, and that the expected frequencies of resistant and susceptible plants were 0.39 and 0.61, respectively. Changes in F over generations in CCII, in which the amount of selfing, s = 0.994, and the amount of outcrossing t = 1 - s = 0.006 (KAHLER,CLEGGand ALLARD1975), can be calculated from F’”)

= [(l

- t)/(l

+ t ) ] [1 - (s/2)‘”’] + (s/2)‘”)F’0’,

in which F’”) is the theoretical inbreeding coefficient in any generation n. By generation F7 the theoretical inbreeding coefficient is expected to approach its equilibrium value F, = (1 - t)/(1 + t ) 1.OO. Expected frequencies of RR, Rr and rr genotypes can be calculated insuccessive generations by substituting p , q, and I””) into WRIGHT’Sequilibriumequation. The frequency v) of plants resistant to race 40 is expected to behighest cf= 0.39) in the F1 generation when the frequency of plants with Rr genotypes is maximal; thereafter the frequencyof resistant plants is expected to decrease as selfing reduces the number of heterozygotes and it is expected to stabilize at f = 0.2 1 (E the parental value) as F approaches its equilibrium value. The above theory thus provides anull hypothesis for comparisons of expected with observed allelic and genotypic frequencies and for the development of estimators of selective values, and their standard errors, based on genotypic recurrence formulae (e.g., ALLARD and WORKMAN 1962; ALLARD, HARDING and KAHLER WEHRHAHN 1966; WEIR, ALLARD and 1972, 1974). The theory has subsequently been expended to include parameters for describing two-locus behavior (COCKERHAM and WEIR 1973, WEIR and COCKERHAM 1973) and for developing two-locus estimators of selective values (WEIR, ALLARD and KAHLER1972, 1974). CLEGG,ALLARDand KAHLER(1972,1978) have shown that interactions affectingselective values occur not only at the two-locus but also at the threeand four-locus levels. SAGHAIMAROOF et al. (1992) have shown that interactions also occur between the nuclear and cytoplasmic genomes of barley. We now focus on two main patterns of allelic frequency change that have been observed in previous studies of experimentalplantpopulations,patterns that are also readily apparent in the present study (Table 1). Four of the lociof Table 1 (Est2, awn texture, two-row vs. six-row spike, R m l ) follow the first pattern. These four loci were weakly polymorphic in all generations of the present study, as has been the case in all previous studies: a single allele of each locus

-

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R. W. AIlard et al.

was present in very high frequency cf> 0.75) in the parents of CCII and thesepredominant alleles increased in frequency over generations. T h e high frequencies of these alleles in the parents of CCII, which represent all of the major barley growing areasof the world, suggest that these predominant alleles have a selective advantage over all other alleles of these four loci in a great majority of the environments throughout the world in which cultivated barleys are grown. T h e consistent increases in frequency of these alleles that have also been observedovergenerations in previous studies of CCII, CCV and CCXXI indicate that these alleles have been favored by selection over all other alleles in all or nearly all of the many years that these populations have beengrown inDavis. Evidently the cumulative effect of such directional selection was to reduce genetic variability and ultimately cause three of the four loci to go to fixation for the favoredalleles (six-row, rough awn, allele 112 of R m l ) and to cause the favored allele of Est2 to closely approach fixation (Table 1). The second main pattern of population behavior is brought out by allelic frequency changes (Table 1) at 10 Mendelian loci that were moderately to highly polymorphic in the parents of CCII (traits 8, 10, 1 1 , 12,13,15,16,17,19,22). Thispattern of allelic frequency change differs from the first pattern in four ways: ( 1 ) one or more reversals in direction of allelic frequency change occurred for most of the loci; (2) despite these reversals cumulative changes were large for all 10 loci; (3) in 9 of 10 cases (all except trait 19) the ultimately most frequent allele was not originally the most frequent; (4) all 10 loci remained moderately to highly polymorphic (usually for only two alleles, including the originally most frequent or the originally second most frequent allele) into generation F45. T h e most facile explanation for the above patterns of allelic frequency change is that various features of the Davis environment are temporally heterogeneous with the result that selective advantage shifts from one allele to a different allele with shifts in the environment. Evidence favoring this explanation is most clear-cut for traits 13, 15, and 16 (resistance us. susceptibility to races 74, 61 and 40 ofscald disease). Scald disease causes little damage to barley in dry years but often reduces grain yields dramatically in wet years (SCHALLER 195 1). Among the 28 parents of CCII six (21%), five (18%), three (11 %), and one (4%) are resistant to races 40, 61, 74 and 72, respectively. Expected frequencies of resistant plants in the F1 and later generations of CCIJ are given by the sum of the frequencies of RR and Rr genotypes for races 40 and 74 and by the frequency of the rr genotype for race 61. Thus expectations under neutral theory are that the frequencies of plants resistant to races 40 and 74 will

gradually decrease from F1 levels of 0.39 and 0.21 to equilibrium levels of 0.2 1 and 0.1 1, respectively, during generations F1 to F7 as inbreeding reduces the proportion of heterozygotes in the population to their equilibrium frequencies. In contrast, the frequencyof plants resistant to race 6 1 is expected to increase from the FI level (f= 0.03) to the equilibrium level (f= 0.18) as inbreeding gradually reduces the frequency of susceptible Rr heterozygotes during generations F2 to F7. It can be seen from Table 1 that the changes expectedunderneutral theorydidnotoccur; observed frequencies of plants resistant to races 40, 61 and 74 (0.07, 0.04 and 0.05) were much lower in generation Fa than the frequencies expected under neutral theorycf= 0.2 1 , O . 18, and 0.1 l),respectively. However, the frequencies of resistant plants then began to increase and by generation F45frequencies had reached levels (0.74,0.44,0.68)far higher thanthose predicted by neutral theory (0.39, 0.18, 0.11 , respectively); these results are closely similar to those of earlier studies based on larger samples than those of the present study (JACKSONet al. 1978; MUONA, ALLARD and WEBSTER 1982; SACHAI MAROOF,WEBSTER and ALLARD1983; WEBSTER, SACHAIMAROOF and ALLARD1986). The years in which the first 10 generations of CCII were grown were all years of moderate to severe drought in which little scald was observed. In contrast, rainfall was generally ample and scald disease was prevalent in most of the years when generations Fa to F 1 3 , F13 to F23 and F23 to F45 were grown. WEBSTER, SACHAI MAROOF and ALLARD (1986) estimated selective values by comparing observed allelic frequencies with frequencies expected assuming selective neutrality. The observed decreases in the frequencies of alleles for resistance that had occurred by generation Fs indicated that the average selective disadvantage of resistant compared to susceptible plants was approximately 10% per generation in that period, whereas observed increases in the frequency of resistant plants that occurred in later generations indicated that resistant plants had average selective advantages oversusceptible plants of approximately 10% during those generations. Thus, the direction of change in the frequency of resistance us. susceptibility alleles was correlated with environmental conditions: the frequency of resistance alleles decreased in dry years when resistance to the disease was apparently irrelevantto survival and/or fecundity, whereas the frequency of resistance alleles increased in wet years favorable for scald when resistant plants exceed susceptible plants in survival ability and in fecundity. Detailed studies of the effects of resistance us. susceptibility on pairs of isogenic lines have provided additional insights concerning the nature of the correlations between environmentand allelic frequency changes(review in ALLARD 1990). These stud-

Multilocus Evolution in Barley

ies show that: (1) under conditions favorable for scald (wet years) infection by races 40 or 61 leads to increases in juvenile mortality and reduces seed yields of susceptible materials, including susceptible members of resistant-susceptible pairs of isogenic lines, relative to resistant materials; (2) under dry conditions unfavorable for scald, susceptible members produce more seeds/plant than resistant members of isogenic pairs. This latter result suggests that host alleles for resistance have innate properties that reduce the reproductive capacity of the host. T h e relationship betweenenvironmentand selective advantage isless clear-cut for race 72. It can be seen from Table 1 that plants susceptible to race72 were frequent in the parents and in all generations of CCII. Race 72 is known to differ from races 40 and 61 in two ways that may have a bearing on its differing population behavior. First, race 72 is highly erratic in its ability to infect susceptible materials (including susceptible members of isogenic pairs) under conditions that are normally highly favorable for infection. Second, this race often causes very little damage even when it succeeds in infecting and producing typical lesions. These two attributes suggest that race 72 causes so little damage to susceptible plants that resistance may be of little benefit to the host. Studies of isogenic lines have shown that the member of isogenic pairs resistant to race 72is usually inferior to thesusceptible member in juvenile survival and further that resistant plants that survive to reproductive maturityare also inferior to the susceptible member in seed producingcapacity. These attributes of host alleles for resistance are expected, under population conditions, to reduce the frequency of resistant plants below the already low equilibrium frequency = 0.0013) predictedby neutral theory. Yet resistant plants appeared in all generations(Table1) at frequencies 15to30 times greater than expected under the assumption that all resistance alleles came into CCII from the single resistant parent. In this connection we note that some of the 27 susceptible parents of CCII may have been genotypically RlRlrlr, or r2r2RzR2 and hence that the true equilibrium value of susceptible plants in CCII may have been much higher than 0.0013. We also note that substantially higher frequencies of plants resistant to race 72 have been observed in occasional generations in earlier studies of CCII, e.g., WEBSTER, SACHAI MAROOF and ALLARD(1986) reported frequencies of resistant plants of 0.09 and 0.14 (69 and 108 times larger than the equilibrium value) in generations F17 and FIB. Itis possible that such sporadic increases in frequency result from environmental circumstances under which race 72 is very damaging and that this race occasionally exerts exceptionally strong selective pressuresfavoring resistant plants (McDERMOTT et al. 1989).

ue

963

We now turn to the seven remaining loci of Table 1 (Estl, Est3, Est#, Acpl, aleurone color,rachilla hairiness, Rrn2). Allof these loci were moderately to highly polymorphic in the parents and in all generations of CCII. Yet the ultimately most frequent allele of each of these loci followed patterns of allelic frequency change which differ from those of the four nearlymonomorphic loci (Est2, awn texture, row number, R r n l ) , as well as from the patterns for the alleles for resistance to races 40,61, 74 and72 (Table l), as follows: (1) In the interval from parents to F8 (drought years) the ultimately predominant allele of each of these seven loci increased sharply in frequency; (2) In the interval F8-F13 the ultimately predominant alleles of loci Estl, Acpl and rachilla hairiness increased further in frequency whereas the ultimately most frequent alleles of Est3, Est#, aleurone color,and R r n l decreased or remained essentially unchanged in frequency; (3) In the interval Fls to Fp3 the ultimately most frequent allele of two loci (aleurone color and rachilla hairiness) increased in frequency whereas the ultimately most frequent allele of five loci remainedunchanged or decreased in frequency;(4)In the interval F23 to F45 increases occurred in the frequency of the predominant allele of all loci except A c p l . The above results suggest that several environmental factors in addition to total rainfall affect selective values of the different loci. Attempts to identify these environmental factors have been successful only to the extent of suggesting: (1) that the distribution of rainfall during different parts of the life cycle, (2) that the occurrence of high or low temperatures (especially during flowering and/or maturation),(3)thatcompetition associated with high stand densities, and possibly still other stresses occasionally exert selective pressures favoringalleles other than the predominant allele. Generation FB correlation analysis: Cluster analysis identified four clusters defined by the 0.05 probability criterion. T h e largest cluster, which formed at an average linkage of 0.24, included three quantitative characters(headingdate,plantheight,kernel weight), two morphological loci (rachilla hairiness, awn texture),andone allozyme locus ( A c p l ) . The arithmetic mean of the absolute value of the correlation coefficients, M, for all character pairs in the cluster was calculated to measure the averagestrength of associations between membersof the cluster; the value ofM for thesix members of the cluster was 0.32 (Table 2). The correlationmatrix(Table 2) suggests that interrelationships among the six characters of this cluster were such that plants with short-haired rachillas, smooth awns and allele 2 of Acpl had heavier kernels, were later heading andtaller than plants with long-haired rachillas, rough awns and allele 1 of A c p l . T h e second largest cluster of generation F8, which

R. W. Allard et al.

m 3

0

I

formed at anaverage similarity of 0.29, was made up of thefour esterase loci. The correlationmatrix (Table 2) suggests that associations of Estl, Est2 and Est3 with each other were stronger than the associations of these three loci with Est4 and that allele 1 of each Estl, Est2 and Est3 was associated with allele 2 of Est4. KAHLER (1973)reportedthat r values (WRIGHT1933) were statistically significant in generation F7 (the earliest generation for which seeds of CCII are available) for allsix pairwise comparisons among the fourloci; he also reported that values of r were nonsignificant in generation F2 (values of r for this generation were inferred from the properties of the28 parents). Similar rapidbuildup ofpairwise associations have been reported in other experimental populations of barley (CLEGG,ALLARD andKAHLER 1972,1978;WEIR,ALLARDand KAHLER1972; 1974). Theseearlierstudies, which were based on much larger samples than the present study, differ primarily in indicating that associations between Est2 and Est3 are weaker thanthoseamong theother loci, e.g., KAHLER(1973) reported thatr was 0.07 for theEst2Est3 association in generation F7 whereas values of r for the five other pairwise associations varied from 0.27 to 0.49. The third largest cluster of generation F g , which formed at average similarity 0.25, included numbers of tillers/plant, yield, and reaction to race 40 of R. secalis. The correlation matrix indicates that families resistant to race 40 had fewer tillers and were lower yielding than families that were susceptible to race 40. T h e fourthcluster, which formed with average similarity 0.63, also included three characters, resistance toraces 6 1 and 74 andtwo-row vs. six-row spike. The correlation matrix indicates that two-rowed families were more resistant to these two races than sixrowed families. Generation Fls correlation analysis:Cluster analysis identified four clusters with five, five, four and three characters in this generation.These clusters formed with average similarities of 0.21, 0.27, 0.50 and 0.33. The first clusterincludedheadingdate, plantheight, rachilla hairiness, kernel weight, and A c p l . This cluster differs from the largest cluster of generation Fs in that awn texture is no longer significantly associated with any other character. The correlation matrix (Table 3) indicates that families with short-haired rachillas that also carry allele 2 of Acpl were more likely to be later in heading, taller and to have heavier seeds than families with long-haired rachillas and allele 1 of A c p l . The second cluster included number of tillers/ plant, yield, spike length, number of spikelets/row, and R r n 2 . Among the 10 pairwise associations between these five characters only one, that between yield and number of tillers/plant, traced back to gen-

EvolutionMultilocus

I

in Barley

965

eration Fa. In cluster 2 of Fa, yield and number of tillers/plant were associated with each other and with resistance to race 40. Thus thebuildup of this second cluster of generation F13 was brought about mainly by the joining together of characters that had not been significantly associated with any other character in Fa. T h e correlation matrix indicates that the associations which developed in the five generation interval from Fa to F13featured the developmentof higher yielding families that had shorter spikes with fewer spikelets/row, more tillers and carriedallele 2 of Rm2. The third largest cluster of generation F13 was comprised of the four esterase loci associated with each other in essentially the same way as in generation Fa (Tables 2 and 3). Thus, the second largest cluster of generation Fa was transmitted over the five generation interval from Fs to F13in basically intact form. The smallest cluster of generation Fl3 included resistance to race 72, two-row us. six-row spike and R m 1. Comparisons of the correlation matricesof generations Fa and F13 show: (1) that two-row vs. six-row spike was significantly associated with resistance to races 61 and 74 in generation Fa but that resistance to race 72 and toRrnl was not significantly correlated with other characters in that generation; (2) that the signs of the correlations changed, indicating that the internal organization of the cluster changed over generations. T h e formation of this cluster thus involved both the break down of an old relationship and the buildup of new relationships. The correlation matrix (Table 3) shows that families with allele 1 of Rrnl were likely to be six-rowed and that families with sixrowed spikes were likely to be susceptible to race 72. Generation F p g correlation analysis: The largest of the three clusters of this generation, containing eightcharacters, was formedataverage similarity 0.23; both of the smaller clusters included three characters, one formed at average similarity 0.34 and the other at 0.27. The morphological character two-row us. six-row spike had become fixed for six-row by this generation and hence did not enter any of the clusters. The characters of the largest cluster were heading date, plant height, A c p l , Est3, Est4 and resistance to races 40, 61 and 74. These eightcharacterscame from different clusters of generation F13 (Table 3): heading date, plant height, and Acpl came from cluster 1, Est3 and Est4 from cluster 3, whereas resistance to races 40,61 and 74 were not significantly associated with each other or with any other characters in generation F13. Thus the formation of the largest cluster of F23 resulted from disassociations and build up of relationships among characters that were in different clusters, or were unassociated, in generation Fl3. It can be seen from Table 4 that families carrying allele 1 of Est3 were likely to be later in heading, taller, and more resistant to races 40, 61 and 74 than families

R. W. Allard et al.

966

TABLE 4 Correlations between characters within clustersof Fos (see footnotes Table 2) 1

Clxll.;tcters

1 ?J

12 10 11 13

1.5 16 8

9 14

17 0.30

18 19

Heading date Plant height Acpl Est3 Est4

74 Race Race 61 Race 40 Est1

3

12 16

10

0.62 -0.29 -0.40 0.27 0.27 -0.21 -0.05 -0.23 0.04 -0.44 0.33 0.27 0.01 0.29 0.49 0.38 -0.11 0.33 0.28 0.35 -0.14 0.31

141 1

1i.l

14

17

18

1i.l

19

= 0.16

= 0.33

-0.50 -0.37 -0.29

Est2

Race Aleurone color Awn texture Rach. hair.

9

8

13

-0.44

carrying allele 2, whereas Est4 was less closely associated with resistances to these three races; also families with allele 1 of Acpl were likely to beearlier in heading and shorter than families with allele 2. The second cluster of generation F23 was comprised of Estl, Est2 and resistance to race 72. Comparisons of Tables 3 and 4 show that formation of this cluster also resulted from a break down and buildup of relationships amongcharactersthatwere in different clusters in generation F13. Also Estl and Est2 were no longer associated with Est3 and Est4 in generation F23; this is consistent with the results of KAHLER(1973) who found that the correlations between these two pairs of loci were either nonsignificant or marginally significant in generations F17-F23. The third cluster of generation F23 included aleurone color, awn texture andrachilla hairiness. Rachilla hairiness was associated with heading date, plant height and awn texture in Fs and with heading date and plant height in generation F13, whereas aluerone color and awn texture were not significantly correlated with each other orwith other charactersin either generation Fs or F13. Thus, break downs of earlier associations and the joining of breakdown products with previously unassociated characters were involved in the assemblyof this thirdcluster of associated characters of generation 23. The associations were such that families with blue aleurone were likely to have rough awns and short-haired rachillas. Generation F45correlation analysis: Cluster analysis identified two large clusters, both at average similarity 0.21, in this generation. Two additional loci, awn textureand R r n l , weremonomorphic(rough awn and allele 1) in F45; thus alongwith six-rowspikes, these loci were not recovered in either of the two clusters. The larger cluster of F45 included the four esterase loci and reaction to races 40, 61 and 74. Five of these

0.78 0.60

0.70 0.39 -0.28

1i.l

= 0.37

1i.l -0.23

= 0.28 -0.31

seven characters (Estl, Est3,resistance to races 40, 61 and 74) were present in the major clusterof F23. Thus the changes that occurred in the 22 generation interval from F23 to F45 featured the disassociation of two quantitativecharacters, plant height and heading date, from theeight component cluster of FZ3and the reassociation of Estl and Est2 with the five holdover members of the F23 cluster. The data of Table 5 indicate that the four esterase loci were closely associated in generation F45;however, the results of KAHLER (1973) indicatethat Est2 and Est3 were only weakly associated in generations Fs9 to F42 of CCII. The generation matrix (Table 5 ) indicates that families carrying allele 1 of Estl, Est2, Est3and allele 2 of Est4 were more resistant to races 40, 61 and 74 than families carrying alternativealleles of the esterase loci. The second cluster of generation F45 included six characters, numberof tillers/plant, yield, plant height, A c p l , aleuronecolorand R m 2 . Comparisons of Tables 2-5 show that theevolution of this cluster over generations featured a series of breakdowns of associations and assembly of new associations from breakdown products with previously unassociated characters. In generation F45the interrelationships of these six characters were such that families with allele 2 of A c p l , allele 1 of R r n 2 and white aleurone were higher yielding than families carrying alternative alleles of these loci. DISCUSSION

Twenty-two traits were monitored over 45 generations to determine patterns of genetic change in single-locus allelic frequencies, in metrical character expression for quantitatively inherited traits, and in the multilocus genetic structure of CCII, a broadly based population of barley. Two main patternsof single-locus allelic frequency changes were observed. Four loci followed the first pattern: the predominant

Multilocus Evolution in Barley

967

TABLE 5 Correlations between characters within clusters of F ~ (see S footnotes Table2) 8

Characters

8 9 10 11 13 15 16

2

Aleurone

6 3 12 17 22

9

10

11

13

15

16

2

6

17 3

12

22

Estl

Est2 0.49 Est3 0.56 Est4 -0.39 -0.37 Race 74 0.29 0.28 0.22 0.08 Race 0.13 0.0161 Race 40 0.04 Tillers/plant Yield Plant height Acpl color Rrn2

( i l = 0.12

lil = 0.33

0.57 -0.69 -0.56 -0.23 0.14 -0.36 0.25

0.41 0.35

0.22

-0.22

allele of each of these loci was nearly monomorphic worldwide and this predominant allele remained in highfrequencyinto the latest generation of CCII. These fouralleles appear to be “generalist” alleles that are highly favored overall other alleles in all or nearly all environments worldwide in which cultivated barleys are grown, including the environments encountered by CCII in each of the 45 years covered by the present experiment. Ten loci, all of which are moderately to highly polymorphic on a worldwide basis, followed the second main pattern of allelic frequency changes. This second pattern differsfrom the first pattern in several ways, e.g., one or more reversals in direction ofallelic frequencychangeoccurredfor most of the loci, cumulative frequency changes were large over the 45 generation period monitored, all of the loci remained moderately to highly polymorphic intothe latest generation,andthe originally most frequent allele was often replaced by a different allele as most frequent allele in latergenerations. These patterns suggest thatthe Davis environment varies over time inways that cause selective advantage to shift back and forth, usually between only two different alleles of a locus. Associations between environmental conditions favoring development of epiphytotics of scald disease, and increase in the frequency of alleles for resistance to races 40,61 and 74, were high. Attempts to associate allelic frequencychanges at other loci with natural environmental perturbations were successful only in the sense of suggesting that two or more environmental factors often appeared to interact complexly in exerting selective pressures on different alleles of various loci. Analyses of quantitative trait data showed that degree of character expression changed significantly for all of the seven traits monitored in at least one and usuallyin several of the generation intervals monitored. Degree of character expression also differed significantly among families within each generation

0.52 0.74 0.33 -0.28 -0.38 -0.14 -0.09 -0.29 -0.02 -0.34 0.15 0.23 -0.28

lil = 0.27

0.35

within the two test years, indicating that CCIIremained genetically variable for quantitative characters into the latest generation. Family X test year components of variance were often statistically significant; this indicates that metric character expression differs from test year to test year and hence that testing is required in more than one test-year environment to characterize each genotype.Reproductive success, measured as numbers of seeds/plant, increased in each of the three generation intervals monitored (Fs to Fls, Fls to F23, F 2 3 to F45). However, changes in character expression wereerraticfor each of the six other quantitative traits monitored (heading date, numbers of reproductive tillers/plant, plant height, spike length, number of spikelets/row, kernel weight) and character expression of none of these traits was associated with numbers of seeds/plant. This suggests that any effects these six characters have on reproductive capacity may be indirect rather than direct. Analysesof product-momentcorrelationsamong of traits were the 22 traits showed thatanumber associated in generation Fs butthat few of these associations persisted into later generations. Instead extensive restructuring occurred featuring partialdisassociations of clusters of associated traits and the assembly of newclusters from thedisassociation products, and from previously unassociated traits, to form progressively larger clusters. Our results can be summarized as follows. Large changes in single-locus allelic frequencies, and in frequencies of multilocus complexes of alleles, occurred over generations in CCII. Parallel changes have also occurred in other large experimental populations of barley grown at Davis. Observed frequency changes, which were often much too large to be attributable to genetic drift were, for many loci, closely associated with natural environmental perturbations, especially with periods of drought interspersed among longer periods of ample rainfall. Variations in survivorship

968

R. W. Allard et al.

and/or reproductive success associated with environmental fluctuations often appeared to affect change through first-order effects on individual traits; however, survivorship and reproductivesuccess sometimes appearedtoactthrough pairwise or higher-order interactions among traits in ways that led to complex and shifting patterns of associations among traits. Each year selection appeared to move the population in the direction of a multilocus genetic structure appropriate to theset of environmental factors actingin that year. However, no environmentalregime and hence no single pattern of selection continued for long and the direction of selection constantly changed, moving the population genotype first toward one genetic structure and then toward other genetic structures. CCII remained highly variable genetically and its multilocus organizationappeared to fluctuate about a rarely if ever realized “adaptive peak” corresponding more or less to thegenetic structure appropriate to a weighted mean of the environmental conditions that prevailed during preceding generations. Thus, thegenetic structure of CCIIcontinuedto undergo dramatic reorganizations throughout the45 generation period monitored, sometimes shifting in directions that improved its ability to cope with the syndrome of challenges imposed by moisture stress butmoreoften shifting inways that allowed the population to takeadvantage of ample supplies of moisture. It is implicit in the above model of evolutionary dynamics that, had the amplitude of environmental fluctuations been sufficiently small at Davis, there would have been only one leptokurtic “adaptive peak” and CCII would have moved toward the summit of such an “adaptive peak.” In this connection we note that, in an earlier study of a subpopulation of CCII which had been transferredtoa site in California where rainfall is rarely limiting, alleles and multilocus associations of alleles that are favored in wet years in Davis increased steadily in frequency. Conversely, in subpopulations of CCII which had been transferred to sites in California that are characterized by moderateto severemoisture stress nearly every year, alleles and multilocus associations of alleles that are favored in years of severe drought in Davis increased steadily in frequency. These results suggest that environmental perturbationsplayed a smaller role in the more consistently mesic or more consistently arid sites to which thesubpopulationshadbeentransferred than in the Davis environment. Nevertheless all of the transportedsubpopulationsremained substantially polymorphic during 10 or more generations in their new and presumably environmentally more stable sites. The above results lead us to two final conclusions, first, that natural environmental perturbations play a central role in the evolutionary dynamics of populations of annual plants, and second, that studies

based on only one or a few generations, traits or test environments are unlikely to provide adequate characterizations of the nature and consequences of genetic change in such populations. LITERATURE CITED ALLARD, R. W., 1965 Genetic systems associated with colonizing ability in predominantly self-pollinated species, pp 49-76 in The Genetics of Colonizing Species, edited by H. G. BAKERand G. L. STEBBINS, Academic press, New York. ALLARD,R. W., 1975 The mating system and microevolution. Genetics 79: 115-126. ALLARD, R. W., 1977 Coadaptation in plant populations, pp. 223231 in Genetic Diversity in Plants, edited by A. MUHAMMAD,R. AKSELand R. C. VON BORSTEL. Plenum, New York. ALLARD, R. W., 1988 Genetic changes associated with the evolution of adaptedness in cultivated plants and their wild progenitors. J. Hered. 79: 225-238. ALLARD, R. W., 1990 The genetics of host-pathogen coevolution: Implications in genetic resource conservation. J. Hered. 81: 16. ALLARD, R. W., J. HARDINC andC. WEHRHAHN, 1966 The estimation and useof selective valuesin predicting population change. Heredity 21: 547-563. ALLARD,R. W., S. K. JAIN and P. L. WORKMAN, 1968 The genetics of inbreeding populations. Adv. Genet. 14: 55-131. ALLARD,R. W., and A. L. KAHLER,1971 Allozyme polymorphisms in plant populations. Stadler Symp. 3: 9-24. ALLARD, R. W., and A. L. KAHLER,1972 Patterns of molecular variation in plant populations. Proc. Sixth BerkeleySymp. Math. Stat. Probab. 5: 237-254. ALLARD, R. W., A. L. KAHLERand B. S. WEIR,1972 The effect of selection on esterase allozymesin barley. Genetics 72: 489503. ALLARD, R. W., and P. L. WORKMAN, 1962 Population studies in predominantly self-pollinated plants. IV. Seasonal fluctuations in estimated values of genetic parameters in lima bean populations. Evolution 17: 470-480. ALLARD,R. W., M. A. SACHAIMAROOF, Q. ZHANC and R. A. JORCENSEN, 1990 Genetic and molecular organization of ribosomal DNA (rDNA) variants in wild and cultivated barley. Genetics 126: 743-751. BROWN, A. H. D., M. FELDMAN and E. NEVO,1980 Multilocus structure of natural populations of Hordeum spontuneum. Genetics 96: 523-536. CLECC,M. T., R. W. ALLARDand A. L. KAHLER,1972 Is the gene the unit of selection? Evidence from two experimental populations of barley. Proc. Natl. Acad. Sci. USA 69: 24742478. CLECC,M. T., R. W.ALLARDandA. L. KAHLER, 1978 Estimation of life-cycle components of selection in an experimental barley population. Genetics 89: 765-792. COCKERHAM, C. C., and B. S. WEIR, 1973 Descent measures for two loci with some applications. Theor. Popul. Biol. 4: 300330. GURUSINCHE, P., 1984 The inheritance of scald (Rhynchosporium seculis) resistance in experimental populations of barley (Hordeum uulgare L.). Ph.D. dissertation, University of California, Davis. HARLAN, H. V., and M. L. MARTINI,1929 A composite hybrid mixture. J. Am. SOC.Agron. 21: 407-409. JACKSON, L. F., and R. K. WEFJSTER, 1976 Race differentiation, distribution, and frequency of Rhynchosporium secalis in California. Phytopathology 66: 726-728. JACKSON, L. F., A. L. KAHLER, R. K. WEFJSTER and R. W. ALLARD, 1978 Conservation ofscald resistance in barley composite

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1983 Evolution of resistance to scald, powdery mildew, and net blotch in barley composite cross 11. Theor. Appl. Genet. 66: 279-283. SAGHAI MAROOF, M. A., K. M. SOLIMAN, R. A. JORGENSEN and R. W. ALLARD, 1984 RibosomalDNA spacer-length polymorphismsin barley: Mendelian inheritance, chromosomal location, and population dynamics. Proc. Natl. Acad. Sci. USA 81: 8014-8018. SAGHAI MAROOF, M. A,, Q. ZHANG,D. B. NEALEand R. W. ALLARD, 1992 Associations between nuclear loci and chloroplast DNA genotypes in wild barley. Genetics 131: 225-231. SCHALLER, C. W., 1951 The effect of mildew and scald infection on yield and quality of barley. Agron. J. 43: 183-188. WAGNER, D. G., and R. W. ALLARD,1991Pollen migration in predominantly self-pollinated plants: barley. J. Hered. 82: 302304. WARD,D. J., 1962 Some evolutionary aspects of certain morphological characters in a world collection of barley. U. S. Dept. Agric. Tech. Bull. 1276: 1-1 12. WEBSTER,R. K., M. A. SAGHAIMAROOF and R. W. ALLARD, 1986 Evolutionary response of barley composite cross I1 to Rhynchospoirum secalis analyzed by pathogenic complexity and gene-by-race relationships. Phytopathology 76: 661-668. WEIR,B. S., R. W. ALLARDand A. L. KAHLER,1972 Analysis of complex allozyme polymorphisms in a barley population. Genetics 72: 505-523. WEIR, B. S., R. W. ALLARDand A. L. KAHLER,1974 Further analysis of complex allozyme polymorphisms in a barley population. Genetics 7 8 91 1-919. WEIR, B. S., and C. C. COCKERHAM, 1973 Mixed selfing and random mating at two loci. Genet. Res. 21: 247-262. WEIR, B. S., and C. C. COCKERHAM, 1978 Testing hypotheses about linkage disequilibrium with multiple alleles. Genetics 8 8 633-641. WRIGHT,S., 1921 Systems of mating. Genetics 6 1 1 1-1 75. WRIGHT,S., 1933 Inbreeding and recombination. Proc. Natl. Acad. Sci. USA 19: 420-433. ZHANG, Q., M. A. SAGHAIMAROOF and R. W. ALLARD, 1990 Worldwide pattern of multilocus structure in barley determined by discrete log-linear multivariate analyses. Theor. Appl. Genet. 8 0 121-128. Communicating editor: B. S. WEIR