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RESEARCHNOTE

AN INDEX TO ASSESS THE REPRODUCTIVE TICKS

FITNESS OF FEMALE

NEIL B. CHILTON* School of Biological Sciences, Flinders University of South Australia, GPO Box 2100, Adelaide, South Australia 5001, Australia (Received 9 August 199 1; accepted 22 November 1991)

AbSb~Ct?CHILTON N. B. 1992. An index to assess the reproductive fitness of female ticks. Internarianaf Journalfir Parasitology 22: 109-I 1 I. This paper describes an index that assesses the reproductive fitness of

female ticks. The Reproductive Fitness Index (RFI) is calculated by the number of eggs that hatch into larvae as a function of the body weight of the engorged female at the time of detachment from the host. The RFI has an advantage over existing reproductive indices because it takes into account the viability of eggs laid by females. This is of ecological significance because it provides a more accurate and biologically relevant measurement in intraspecific and/or interspecific comparisons of the reproductive fitness of females. INDEX KEY WORDS: Reproductive Fitness Index; ixodid ticks; Amblyomma limbatum; oviposition; egg viability.

THE fitness

of animals must be measured in terms of their ability to survive and to produce viable offspring

(Andrewartha & Birch, 1954). Parasites produce large numbers of progeny, compensating for low survival rates before finding a host (Price, 1977). It is known that females of some tick species lay up to 20,000 eggs in a single egg clutch (Diehl, Aeschlimann & Obenchain, 1982). In ixodid ticks, females die after opposition and therefore have only one opportunity to produce viable offspring after mating (i.e. an ‘all or none’ event). For many species of ixodid tick, the number of eggs produced is related to the weight of the fully engorged female, with heavier females producing more eggs (Diehl et af., 1982). Such a relationship occurs for Amblyomma limbatum and Aponomma hydrosuuri, two Australian species of reptile tick (Chilton & Bull, I99 I). Drummond & Whetstone (1970) constructed the Reproductive Efficiency Index (REI = number of eggs/weight of engorged female at the time of host detachment) to compare the reproductive fitness of different females. This index, which calculates the conversion of female body weight into egg production, has been applied in intraspecific comparisons of the reproductive fitness of females left undistur-

*Present address: Evolutionary Biology Unit, South Australian Museum, North Tee, Adelaide, South Australia 5000, Australia.

bed or disturbed during oviposition (D~mmond & Whetstone, 1970; Drummond, Whetstone, Ernst & Gladney, 1971; Koch, 1982), between females that had fed on different host species (Koch, 1982) or for females on different days of the ovipositional cycle (Koch & Dunn, 1980). This index ignores the viability of eggs, that is the proportion of eggs that successfully hatch into larvae under optimal conditions. This could be biologically misleading in comparisons of intraspecific and/or interspecific female reproductive fitness. For example, if two females of the same engorged weight and under the same environmental conditions produce the same number of eggs, but 90% of the eggs laid by the first female hatch, while none laid by the second female do, then the two females with the same REI value clearly have unequal fitness. A modified version of Drummond and Whetstone’s REI is proposed. The Reproductive Fitness Index (RFI) is defined as the number of eggs that hatch into larvae/weight of the engorged female at the time of host detachment. This provides a more biologically meaningful determination of the reproductive success of females. Based on this index, females that fail to lay eggs or females that lay non-viable eggs have a reproductive fitness of zero. This is illustrated by data in Table 1 showing the reproductive output of 26 engorged Amblyomma limbatum females and comparing the fitness values obtained using the REI and RFI. Recordings were made of the number of eggs laid 109

N. B. CHILTON

110 TABLE

I-THE

REPRODUCTIVE OUTPUT OF USING THE

Female No.

26

ENGORGED

Amblyomma limbatum

(NO. OF EGGS PER G FEMALE WEIGHT) AND

REI

No. of

Engorged weight

No. of

eggs laid

(mg) Group 1 2 3

I: No eggs

Group 4 5 6

II: Few eggs and low viability 352.7 689.2 803.9

Group 7 8 9

III: Few eggs and high viability 422.4 479.4 511.0

389.6 484.6 485.5

eggs hatched

RF1

FEMALES AND A COMPARISON

OFTHE

VALUES OBTAINED

(NO. OF EGGS HATCHED PER G FEMALE WEIGHT)

% egg hatch

REI (x 10’)

RF1 (x 10’)

0.00 0.00 0.00

0 0 0

0 0 0

_

0.00 0.00 0.00

35 157 387

0 0 0

0.0 0.0 0.0

0.10 0.23 0.48

0.00 0.00 0.00

932 226 1935

796 150 1704

85.4 66.4 88.2

2.21 0.47 3.79

1.88 0.31 3.33

Group 10 11 12 13

IV: Many eggs and low viability 435.3 2403 462.8 2670 496.4 2403 679.2 3513

90 964 148 304

3.7 36.1 6.2 8.7

5.52 5.77 4.84 5.17

0.21 2.08 0.30 0.45

Group 14 15 16 17 18 19 20 21 22 23 24 25 26

V: Many eggs and high viability 194.3 899 204.1 1187 347.8 1935 383.6 2211 394.2 2733 437.4 2877 437.8 2745 441.2 2589 444.5 2314 518.6 3057 545.4 3380 691.9 3878 733.9 5167

861 1047 1826 2112 2572 2716 2560 2359 2295 2923 3307 3809 4111

95.8 88.2 94.4 95.5 94.1 96.0 93.3 91.1 99.2 95.6 97.8 98.2 79.6

4.63 5.82 5.56 5.76 6.93 6.58 6.27 5.87 5.21 5.89 6.20 5.61 7.04

4.43 5.13 5.25 5.51 6.52 6.21 5.85 5.35 5.16 5.64 6.06 5.51 5.60

by each successfully

female

and

hatched.

the The

number methods

of used

eggs

which

to raise

and

breed ticks in the laboratory have been described previously (Chilton & Andrews, 1988; Chilton & Bull, 1991). These females were subdivided into five groups according to their reproductive output; females that failed to lay eggs (group I), females that laid significantly fewer eggs than females of equivalent weight and where eggs had low or high levels of viability (groups II and III, respectively), females that laid many eggs but with very few eggs subsequently hatching (group IV), and females that laid many eggs and where egg viability was high (group V). Regardless of which index (RF1 or REI) is used,

Amb. Iimbatum females that failed to oviposit had a fitness value of zero (Table 1). Females which produce a small number of eggs relative to their body weight but which fail to hatch (group II) have a fitness value according to the REI. Despite this, these females do not contribute any progeny to the tick population, and thus have a reproductive fitness of zero, the value obtained using the RFI. Similar fitness values were obtained for group III females when comparing between indices, because of the relatively high proportion of eggs that hatched. The most obvious difference between the two indices is reflected in the fitness values obtained for groups IV and V. Females of both groups laid the expected number of eggs

111

Research Note relative to their body weight (see relationship in Chilton & Bull, 1991), however, the viability of eggs laid by group IV is sig~~can~y lower. Based on the REI, the mean fitness value of group IV females (5.33 i 0.20 x 10’) is similar to that of group V females (5.95 rt 0.18 x 103), yet fewer than 40% of the eggs laid by group IV females subsequently hatched. For three females (nos. 10, 12 and 13), they make only a minor contribution in terms of the progeny produced towards the tick population. In contrast, when the RF1 is used, the mean fitness value of group IV (0.76 f 0.44 x 10’) is considerably lower than the mean value of group V females (5.56 f 0.15 x 103). Failure to consider the viability of eggs produced by females has irn~~nt ecological r~ifications, for example, in models that calculate the reproductive success of colonizing females at the edge of the species distributional range. For instance, the time when Am&. limbatum females are mated by conspecific males has no effect on the number of eggs produced, however, females delayed in mating, a potential problem for colonizing females, produce significantly fewer viable progeny (Chilton, Andrews & Bull, unpublished data). Clearly, it is important to consider the viability of eggs laid by female ticks (and indeed other parasite taxa) in assessing their contribution to the population. The Reproductive Fitness Index provides a more biologically relevant assessment of the reproductive fitness of females.

REFERENCES AND~WARTHAH. G. & BIRCHL. C. 1954. The distribution and Abundance of Animals. University of Chicago Press, Chicago. CHILT~N N. B. & ANDREWS R. H. 1988. Mating behaviour and parapatry in two species of Australian reptile ticks. Oecologia (Berlin) 75: 146-152. CHILTONN. B. & BULLC. M. 1991. A comparison of the reproductive parameters of females of two reptile tick species. International Journalfor Parasitology 21: 907911.

DIEHLP. A., AESCHLIMANN A. St OBENCHAIN F. D. 1982. Tick reproduction: oogenesis and oviposition. In: Physiology of Ticks (Edited by OBENCHAIN F. D. & GALUHR.), pp. 277350. Pergamon Press, New York. DRUMMOND R. 0.62 WHETSTONE T. M. 1970. Ovi~sition of the Gulf Coast tick. Annals of the ~ntumolog~c~ Society of America 63: 1547-I 55 I. DRUMMOND R. O., WHETSTONE T. M., ERNSTS. E. & GLADNEY W. J. 1971. Oviposition of the American dog tick (Acarina: Ixodidae). Annals of the Entomological Society of America 64: 1305-1309.

DRUMMOND R. O., WHETSTONE T. M. & GLADNEY W. J. 1971. Oviposition of the lone star tick. Annals of the Entomological Society of America 64: 191-194.

KOCHH. G. & DUNNJ. C. 1980. Egg production efficiency of female lone star ticks ofdifferent engorgement weights. The Southwestern

Entomologist

J: 179-182.

KOCHH. G. 1982. Oviposition of the brown dog tick (Acariz Ixodidae) in the laboratory. Annals of the entomological Sociefy of America 75: 583-586.

research was funded by a grant from the Flinders University Research Budget. Dr Ross Andrews and Dr Michael Bull provided useful comments on the manuscript. Acknowledgements-This

PRICEP. W. 1977. General concepts on the evolutionary biology of parasites. Evolution 31: 405-420.

An index to assess the reproductive fitness of female ticks.

This paper describes an index that assesses the reproductive fitness of female ticks. The Reproductive Fitness Index (RFI) is calculated by the number...
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