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Biol. Rev. (2017), pp. 000–000. doi: 10.1111/brv.12367

New-age ideas about age-old sex: separating meiosis from mating could solve a century-old conundrum Michael Brandeis∗ The Department of Genetics, The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem 9190401, Israel

ABSTRACT Ever since Darwin first addressed it, sexual reproduction reigns as the ‘queen’ of evolutionary questions. Multiple theories tried to explain how this apparently costly and cumbersome method has become the universal mode of eukaryote reproduction. Most theories stress the adaptive advantages of sex by generating variation, they fail however to explain the ubiquitous persistence of sexual reproduction also where adaptation is not an issue. I argue that the obstacle for comprehending the role of sex stems from the conceptual entanglement of two distinct processes – gamete production by meiosis and gamete fusion by mating (mixis). Meiosis is an ancient, highly rigid and evolutionary conserved process identical and ubiquitous in all eukaryotes. Mating, by contrast, shows tremendous evolutionary variability even in closely related clades and exhibits wonderful ecological adaptability. To appreciate the respective roles of these two processes, which are normally linked and alternating, we require cases where one takes place without the other. Such cases are rather common. The heteromorphic sex chromosomes Y and W, that do not undergo meiotic recombination are an evolutionary test case for demonstrating the role of meiosis. Substantial recent genomic evidence highlights the accelerated rates of change and attrition these chromosomes undergo in comparison to those of recombining autosomes. I thus propose that the most basic role of meiosis is conserving integrity of the genome. A reciprocal case of meiosis without bi-parental mating, is presented by self-fertilization, which is fairly common in flowering plants, as well as most types of apomixis. I argue that deconstructing sex into these two distinct processes – meiosis and mating – will greatly facilitate their analysis and promote our understanding of sexual reproduction. Key words: meiosis, mating, sex, heteromorphic sex chromosomes, hermaphrodites, self-pollination. CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 (1) The problem with sexual reproduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 (2) Quests for an explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 II. A proposition to unscramble sex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 III. Sex without meiotic recombination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 IV. Not all mating is equal – isogamy versus anisogamy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 V. Meiosis without bi-parental mating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 VI. Asexual is not necessarily (and rarely) ameiotic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 VII. Wild sex – everything goes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 VIII. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 IX. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 X. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 XI. Supporting Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

* Address for correspondence (Tel: +972-2-6585123; E-mail: [email protected]). Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

2 I. INTRODUCTION (1) The problem with sexual reproduction Sexual reproduction, the joining of the genomes of two parents to produce progeny that contain re-assorted portions of the parental genomes (Bernstein et al., 1984), remains the most fascinating mystery of evolution (Hadany & Comeron, 2008). Sex is omnipresent in most eukaryotes and has apparently been so since the dawn of eukaryotic existence (Speijer, Lukes & Elias, 2015). The cost of sexual reproduction is considered prohibitive (Bell, 1982; Maynard Smith, 1978): the vast resources devoted to the production of male and female gametes, as well as to primary and secondary sexual organs and behaviour for acquiring a mate, the breaking up of successful combinations in an absolute lottery and the requirement of a second parent (males in animals). However, in spite of these apparent disadvantages, species that opted for exclusive asexual reproduction are rare and generally considered evolutionary dead ends. Only a few genera have managed to survive and flourish for tens of millions of years without sex (Judson & Normark, 1996). The recently published genome of a member of such a group, the asexual bdeloid rotifer Adineta vaga, revealed, as will be discussed below, amazing mechanisms that apparently enabled and are required for such long-term asexual survival (Flot et al., 2013). (2) Quests for an explanation Charles Darwin was the first to address the paradoxical nature of sex (Darwin, 1876). He was duly followed by many of the most prominent students of evolution – who thoroughly and creatively focused on this issue over the past 150 years. The most dominant group of theories stresses the importance of sex for generating variability to improve adaptation (Maynard Smith, 1978). By contrast, Muller (1964) and Kondrashov (1988) proposed that the role of sex is to eliminate deleterious mutations. Finally Bernstein, Hopf & Michod (1987), H¨orandl (2009) and H¨orandl & Hadacek (2013) suggested that meiosis is an essential mechanism to repair DNA damage. Given the general prevalence of sex and its ancient origin (Speijer et al., 2015), the requirement for it should be universal and basic. I argue that theories that stress the role of sex in promoting variability do not satisfy these fundamental criteria. Maynard Smith (1971), p. 319), for example, postulated that ‘the most important advantage of sex arises when two genetically different populations migrate into a new environment, in which the best adapted genotype is a combination of genes from the two invading populations’’. These are rare events and if taken literally we should expect that once formed and settled, the most successful form should stop having sex and preserve its superior genetic combination by clonal reproduction. Even if such cases could be demonstrated experimentally (Lachapelle & Bell, 2012; Morran, Parmenter & Phillips, 2009) they could not apply for the majority of species and are rather rare in the Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

Michael Brandeis field. Sex persists also in environments that change very little over time and in living fossils like the horseshoe crabs (Limulidae), which have hardly evolved for hundreds of millions of years, and yet have a lively sex life (https://www .youtube.com/watch?v=Hos1ms0O9mM). Moreover, sex is not necessarily required for speciation; bdeloid rotifers, one of the few clades to reproduce asexually for a very long time, underwent speciation like sexual clades (Fontaneto et al., 2007). Sexual reproduction could indeed be useful for co-evolution and protection from parasites as proposed by Hamilton’s red queen model (Hamilton, 1980; Lively, 2010) and demonstrated experimentally (Morran et al., 2011). However, this seems to work only under rather specific circumstances and fails to explain the widespread prevalence of sex (Otto & Nuismer, 2004). As none of these theories seem to be all inclusive it was recently suggested that sex has to be explained by a combination of theories (Zimmer, 2009). This proposition is both messy and logically flawed. If the persistence of sex has different explanations under different circumstances, we would expect situations where none is appropriate and where sex would disappear. Such cases are however exceedingly rare. We thus continue to seek a plausible and all-inclusive explanation for the advantage and widespread prevalence of sexual reproduction.

II. A PROPOSITION TO UNSCRAMBLE SEX I argue here that our problem in understanding sexual reproduction is caused by the bundling of two different processes. Sexual reproduction in eukaryotes is comprised of alternation of meiosis, that generates haploid cells from diploid cells to serve as gametes, and of their fusion to form the next generation of diploid offspring. On the face of it, this could be viewed as two parts of the same process but in fact these are distinct events that should be evaluated separately. As mating is obvious and meiosis is hidden from the eye the general misconception is that meiosis is the servant of mating, as suggested by its name which means ‘reduction division’. This is an atypical chicken and egg conundrum. Atypical as meiosis pre-dated mating, as we know it, by a billion or more years (Wilkins & Holliday, 2009). Meiosis, in all its details is one of the most conserved and rigid processes and is the same in all eukaryotes from yeast to the blue whale (Wilkins & Holliday, 2009). By contrast everything that has to do with mating is extremely variable and two species of the same genus can present vastly different types of behaviour.

III. SEX WITHOUT MEIOTIC RECOMBINATION Biology is an experimental science and fully to appreciate the respective roles of meiosis and mating it should be essential to separate them experimentally. Fortunately, this is not required, as nature has provided such a

Separating meiosis from mating quasi-experiment in most animals and even in a few plants. Heteromorphic sex chromosomes (see glossary in Table 1), where one sex has two different types of sex chromosomes can be viewed as such a built-in ‘experiment’. The Y chromosome, present only in males, undergoes de facto asexual transmission from father to son without recombination in many different animals, as well as in some plants. The W chromosome, in birds and some other animals, is present only in females and undergoes a similar transmission from mother to daughter. The other sex chromosomes X (complementing Y) and Z (complementing W) do undergo normal recombination in females (X) and males (W), although only half as often as autosomes. I argue that what happens to the heteromorphic Y and W chromosomes over the generations would have happened to all chromosomes in the absence of meiosis. Comparing the fate of the non-recombining heteromorphic sex chromosomes (Y,W) to those of recombining homomorphic ones (X,Z), as well as autosomes, should thus reveal the importance of meiosis. Recent years have seen the publication of the sequences of Y chromosomes of various organisms. The dominant emerging feature of these studies is the exceedingly rapid rate of Y chromosome evolution and degradation (Bachtrog, 2013; Johnson & Lachance, 2012). The differences between human and chimpanzee autosomes suggest an evolutionary distance of about 6 million years. Their respective Y chromosomes are 50-fold more different than their autosomes, as if they were 300 million years apart (Hughes et al., 2010), suggesting that Y chromosomes undergo a fifty-fold higher rate of evolution! A rapid rate of evolution is common to all heteromorphic sex chromosomes from the ancient mammalian Y chromosome to the young Silene latifolia (a plant) Y chromosome (Muyle et al., 2012) to the very young Drosophila miranda (an insect) neo-Y chromosome (Bachtrog et al., 2008). A similar expedited rate of evolution was further identified in a recently evolved Y-like social chromosome of the Solenopsis invicta fire ant (Wang et al., 2013). These data strongly suggest that meiosis is essential for maintaining a genuine diploid state. It is important however to stress that meiosis must have originally evolved in simple eukaryotes who spent most of their life cycle in a haplotonic state. These organisms often restore diploidy only for the sake of meiosis. Indeed, many such simple haplotonic eukaryotes like algae, fungi, bryophytes and others survive to this day side by side with the more complex diplotonic eukaryotes that dominate our current flora and fauna. Once a barrier of recombination appears between two homologous autosomes they rapidly drift apart and become very different from each other (Wang et al., 2013). With just under 500 species bdelloid rotifers represent the highest metazoan taxonomic rank that has given up both meiosis and sexual reproduction millions of years ago, amounting to an ‘evolutionary scandal’. The recent sequencing and analysis of the bdelloid rotifer Adineta vaga highlights the results, as well as the consequences, of asexuality. The genome of A. vaga was duplicated to become tetraploid and subsequently the four alleles diversified and specialized resulting in almost

3 50000 protein coding genes! Moreover, their allelic regions became re-arranged and sometimes are even located on the same chromosome, rendering it incompatible with meiotic pairing. Instead A. vaga applies widespread gene conversion and has supplemented its genome with a large number (8%) of genes acquired by horizontal gene transfer from other eukaryotes and prokaryotes (Flot et al., 2013). It is possible that the lack of meiosis enables these horizontally transferred genes to become incorporated into the genome and remain there. Meiotic recombination could thus be the reason that such genes are much less common in sexual organisms. The A. vaga genome is particularly rich in genes involved in cellular and DNA damage, suggesting that loss of meiosis sensitizes organisms to such damage. Interestingly these patterns are reminiscent of polyploidy in prokaryotes that lack meiosis (Markov & Kaznacheev, 2016). To conclude, the remarkable and unique genome of A. vaga, that is so different from that of diploid sexual organisms, shows the requirements for success at asexuality as well as its long-term consequences. I thus propose that meiosis is necessary for preserving genomic integrity in the short, medium and long term. The role of sex as an error-repair mechanism has been advocated in the past, in particular by Bernstein et al. (1987), Long & Michod (1995) and H¨orandl (2009). The origins of this idea can be traced back to Weismann, Thomson & Thomson (1904) who suggested, long before the molecular basis of meiosis was elucidated, that sex might rejuvenate the genome. Archetti (2010) calculated that the price of preventing loss of heterozygosity outweighs the price of sexual reproduction even in the short run. In an excellent perspective, Wilkins & Holliday (2009) described how meiosis evolved from mitosis. They propose that the primary role of meiosis was to prevent recombination-generated damage. This is done by the perfect alignment achieved in meiosis that is problematic and therefore suppressed during mitosis. Indeed, many of the mutations observed in the non-recombining Y chromosome (Rozen et al., 2003; Skaletsky et al., 2003) and the genome of A. vaga (Flot et al., 2013) are such recombination-induced duplications. Recombination itself pre-dates meiosis and is highly prevalent in prokaryotes that have the entire enzymatic toolkit required for its execution. It therefore did not have to be invented for the purpose of meiosis where it is essential for the evolution of meiotic chiasmata that hold together homologous chromosomes.

IV. NOT ALL MATING IS EQUAL – ISOGAMY VERSUS ANISOGAMY Mating involves, as a rule, the fusion of two gametes (syngamy) see glossary in Table 1 and their nuclei (karyogamy). The gametes can be either of similar size and properties (isogamous), as is the rule in unicellular organisms, or of very different size and properties as is the rule in multicellular organisms. Exceptions to all these rules exist, in particular in fungi (Lee et al., 2010) and some other Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

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Table 1. Glossary Anisogamy - Syngamy of two specialized gametes different in size and structure. Characteristic of multicellular eukaryotes. The difference between the two gametes is usually defined as ‘sex’: male and female. The female gamete, the oocyte, is large and contains organelles, maternal RNA and protein. The small paternal gamete, the spermatid in animals or pollen in plants, is motile or mobile Apomixis – Forms of asexual parthenogenetic reproduction, mainly restricted to plants Automixis - Reproductive processes by which an offspring is derived from a product or products of a single meiotically dividing cell Gonochorism – Separation of males and females into two different organisms Hermaphrodite – An organism that produces both male and female gametes Isogamy - Syngamy of two gametes identical in size and structure. Characteristic of unicellular eukaryotes. The difference between the two gametes is usually defined as ‘mating type’, rather than ‘sex’ (or gender). As, apart from their mating type, gametes are similar there is no intrinsic restriction to the number of such types. Many unicellular organisms have several or many different mating types Parthenogenesis – Form of asexual reproduction by which an offspring develops from an unfertilized egg without any paternal genetic contribution. Some parthenogenetic species lack males altogether while others are facultatively parthenogenetic. Sex chromosomes – A pair of chromosomes that originated from a homologous pair of autosomes that diverged over evolutionary time. Homology between sex chromosomes is lost over time and can be absent altogether. In most organisms with genetic sex determination these chromosomes are called X and Y. In males, the heteromorphic sex chromosome Y is present together with a single copy of the X chromosome, which is the homomorphic sex chromosome (XY). X is present in two copies in females (XX). Birds as well as other animals have a reciprocal system where females carry the heteromorphic sex chromosome W: females are WZ and males ZZ Syngamy - A form of reproduction in which two (usually haploid) gametes fuse and form (a usually diploid) offspring. Sexual reproduction is syngamous by definition

unicellular algea (Lehtonen, Kokko & Parker, 2016) but will not be discussed here. Isogamous gametes differ only in mating type, which is not analogous to sexes. Billiard et al. (2011) for example suggested that ‘sexes’ should be used for gametes based on morphological differences such as size and motility while ‘mating types’ are defined by molecular mechanisms that define compatibility. While two is the most common number of mating types, certain species, such as some Basidiomycota, have up to thousands of different ones (Casselton, 2002). Mating types do not ensure outcrossing as for example, in the budding yeast Saccharomyces cerevisiae (Fig. 1; see online Supporting information, Movie S1) where each meiosis gives rise to both mating types that can re-mate. A single mating type would greatly facilitate the finding of a potential mating partner; it is yet unresolved why at least two, which is a less-efficient alternative in terms of finding a partner, are required and are so common. A possible answer is that mating requires cellular signaling which can take place efficiently only between different types (Hadjivasiliou & Pomiankowski, 2016; Hoekstra, 1982). I further speculate that two mating types are the most effective approach to ensure binary fusion of gametes. A single, and to a lesser extent multiple, mating type, can easily result in fusion of multiple gametes as suggested previously (Haag, 2007). However numerous other explanations exist for the prevalence of two mating types (Billiard et al., 2011). I hypothesize that isogamous mating first and foremost serves meiosis by restoring cellular diploidy, but might also have additional, context-specific, benefits. In most complex multicellular eukaryotic organisms, except in some fungi, the gametes differ massively in size, motility and content. The female gamete is huge, non-motile and carries organelles, maternal RNA, proteins Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

and nutrients. The tiny male gametes by contrast are motile, at least passively, and usually carry little except their haploid genome. The appearance and fixation of anisogamy, also called oogamy, is a fascinating evolutionary advance for which an explanation is far from obvious. A hint for one such transition from isogamous mating types in unicellular green algae to anisogamous sexes in a closely related multicellular green alga was discovered by Nozaki et al. (2006). This transition happened independently multiple times during evolution as it is found in all land plants, in all Metazoa, and in several other groups, whose last common ancestors were isogamous unicellular organisms (Kirk, 2006). In multicellular organisms, gametes are some of the most specialized and sophisticated cells. This is particularly obvious in primitive organisms with relatively unspecialized somatic cells. Oocytes are the largest cells in the body while male gametes are the only independent ‘free swimmers’. I thus speculate that anisogamy was a precondition to the evolution of complex organisms. All multicellular organisms, such as fungi, that are isogamous are relatively simple in terms of cellular differentiation. The evolution of specialized oocytes can be considered as the advent of parenting and parental investment. It is important to stress that the twofold cost of sex, i.e. the requirement for two parents, is restricted to anisogamous types and stems from the production of males (Lehtonen, Jennions & Kokko, 2012; Maynard Smith, 1978). The rather obsessive, if justified, preoccupation with the cost of males overshadows the much higher cost of maternal investment. In animals this investment ranges from generation of eggs, some of them huge, to maternal care. In plants this investment includes seed and fruit production and adaptations involving defence, dispersal and establishment.

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Fig. 1. Basic isogamous ‘mating’ between two yeast cells. Two haploid budding yeast Saccharomyces cerevisiae cells of opposite mating types fuse and form a diploid offspring. Time is indicated in hours. See also online Supporting information, Movie S1.

Every female, or hermaphrodite (Charnov, 1979), could in theory reduce investment per offspring in order to increase their number (McGinley & Charnov, 1988). Fitness is however ultimately determined by optimization of offspring quality versus quantity. In most cases mothers also make a higher genetic contribution to offspring in the form of organelle DNA.

V. MEIOSIS WITHOUT BI-PARENTAL MATING As sex is by definition considered to entail bi-parental mating (Bernstein et al., 1984), self-fertilization can be viewed as meiosis without mating. About 95% of the more than 300000 species of flowering plants (Christenhusz & Byng, 2016) are hermaphrodites (Table 1). About 60% of these species have self-incompatibility mechanisms that prevent self-fertilization. However up to 40% of plant species also can and do undergo self-pollination which may lead to various levels of self-fertilization (Richards, 1997). It is estimated that self-fertilization is the major mode of fertilization in about 10–15% of plant species (Wright, Kalisz & Slotte, 2013). On the face of it selfing has several compelling benefits – efficient purging of lethal mutations or those with large negative fitness effects, conservation of advantageous genetic combinations and the elimination of the cost of males. Moreover in non-motile organisms like plants where outcrossing is limited by the availability of mates and pollinators (Ghiselin, 1969), selfing enables a significant increase in the number of progeny. This could be particularly useful for ensuring fertilization when colonizing new habitats as stated by Baker’s ‘law’ (Baker, 1955, 1966; Cheptou, 2012; Pannell et al., 2015). Consistently with this rule,

the range of self-pollinating plants is larger than of their close cross-fertilizing relatives (Grossenbacher et al., 2015). A different hypothesis for the advantages of selfing, called Allard’s argument (Allard, 1975), highlights its role in adaptation of specific selfing populations to specific habitats. This rule was supported both by studies in plants (Allard et al., 1992) and in hermaphrodite snails (Selander & Kaufman, 1973). Darwin (1876) observed that inbreeding is disadvantageous compared to outcrossing both in animals and in plants. In addition to the meticulous experiments he performed on 57 different plant species (Darwin, 1876), the advantage of outcrossing has since been observed in many species both experimentally and in some wild populations (Charlesworth & Willis, 2009; Kardos et al., 2016). However, by far the most convincing indication for the importance of outcrossing is manifested by the prevalence of self-incompatibility mechanisms that prevent self-fertilization. Even most of those organisms that do undergo selfing to various degrees invest considerable resources to achieve outcrossing. Interestingly while it is widely accepted that inbreeding can often lead to so-called ‘inbreeding depression’ the precise reasons for this depression are far from obvious (Charlesworth & Willis, 2009; Hedrick & Garcia-Dorado, 2016). The breakdown of self-incompatibility that enables selfing is one the most frequent evolutionary transitions in flowering plants (Sicard & Lenhard, 2011). It has been hypothesized that such a breakdown will eventually lead to extinction and is an evolutionary dead-end (Stebbins, 1957). However whether this is true, and in particular why, is still unclear (Wright et al., 2013). Recent analysis has shown that the mixed self and outcrossing strategy of self-compatible plants could be a stable and viable approach rather than an Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

6 intermediate stage on the way to high rates of selfing (Goodwillie, Kalisz & Eckert, 2005; Winn et al., 2011). An additional point to consider is that even plants that cross-pollinate will do so predominantly with their nearest neighbours. As these neighbours are likely to be genetically very close to them such mating can be considered as almost inbreeding.

VI. ASEXUAL IS NOT NECESSARILY (AND RARELY) AMEIOTIC Parthenogenesis, found in a very wide variety of organisms, is a form of reproduction in which offspring develop from unfertilized eggs (Engelst¨adter, 2008). If self-pollination could be considered as a by-product of cross-fertilization, parthenogenesis either lacks mating altogether or the offspring lack any paternal genetic contribution. Nevertheless, the unfertilized eggs that give rise to most types of parthenogenetic offspring do undergo at least partial meiosis (Mirzaghaderi & H¨orandl, 2016). In parthenogenetic vertebrates (Lampert, 2008) and invertebrates (Engelst¨adter, 2008) ploidy is in most cases restored by various mechanisms of automixis (Table 1) or maintained by premeiotic genome duplication. In some groups, like ants and bees, parthenogenetic offspring are viable haploid males. The rather complex mechanisms to restore diploidy are another testimony to the importance of the meiotic process per se. Only relatively few obligate parthenogenetic species, like A. vaga reproduce by parthenogenesis where eggs do not undergo meiosis. Apomixis (Table 1), a widespread form of parthenogenesis in plants, can take place by a bewildering variety of mechanisms, some of which are devoid of meiosis (Bicknell & Koltunow, 2004). Apomixis in flowering plants usually takes place in parallel to normal sexual reproduction even in self-incompatible species and might be much more widespread than originally considered (Plitmann, 2002). Many ferns (Fig. 2I) are asexual and obligately apomictic. In most cases they duplicate their genome prior to normal meiosis. So again, as in animals, very few asexual plants reproduce without at least a partial meiotic stage. An overview of the various mechanisms by which meiosis has been retained in asexual reproduction is shown in Fig. 3 and is detailed in an excellent review by Mirzaghaderi & H¨orandl (2016).

VII. WILD SEX – EVERYTHING GOES Once we separate the rigid meiotic process from the adaptable process of mating it is clear that sexual reproduction is one of the most varied and rich facets of eukaryotic life (Fig. 2). It is not surprising that no single theory can embrace a level of variety that is adapted to any ecological and evolutionary niche. A recent collection of papers termed ‘Weird sex’’ (Aanen, Beekman & Kokko, 2016) Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

Michael Brandeis highlights the huge variability of mating strategies. Here I focus mainly on a limited number of aspects in flowering plants and animals as models. It is however important to remember that other plants and fungi present a rich array of additional mating strategies. Let us consider first the issue of sex (gender) separation. About 95% of all flowering plants are hermaphrodites, mostly with ‘perfect’ flowers that produce both male and female gametes. A minority of these plants have separate male and female flowers on the same plant. Dioecy, separation of sexes, appears in about 5% of species. Just under half of these belong to clades that are entirely dioecious. The others are dispersed in many mixed families. This distribution leads to the striking conclusion that the transition to dioecy has happened independently anywhere between 1000 and 5000 times (Renner, 2014). Dioecy is considerably more common in plants that have a long life cycle like trees and that are wind pollinated. In plants separation of the sexes is initiated by a mutation that silences the male or female function in a hermaphrodite. It was originally proposed that at least two such mutations are required and that they must be linked on the same chromosome (Charlesworth & Charlesworth, 1978). More recent evidence suggests that a single sterility mutation can be sufficient to generate dioecy (Renner, 2016). As transitions to dioecy have happened independently so many times, and as many different mutations can cause male or female sterility, it is likely that these transitions took place by a considerable variety of mechanisms (Diggle et al., 2011). In rare cases chromosomes that carry these mutations gradually evolve to create distinct sex chromosomes (Ming, Bendahmane & Renner, 2011). Only 44 plant species are currently known to have such sex chromosomes. The intermediate stages of heteromorphic sex chromosomes that have been identified in these plants present a striking example of how heteromorphic sex chromosomes evolve from autosomes (Charlesworth, 2002). While it is conceivable that many more dioecious plants have sex chromosomes, particularly young ones that cannot be distinguished cytologicaly, the majority of dioecious plants seem not to have such chromosomes. Plants started to colonize the land more than 450 million years ago and eventually formed huge and species-rich forests that transformed our planet. The appearance, less than 200 million years ago, of the complex sex organ of flowers led to their explosive diversification. Much of this diversification can be attributed to the astounding evolutionary flexibility of sexual strategies, manifested among others by: (i) exceedingly frequent transitions from self-incompatibility to self-pollination as well as flexible optimized mixed strategies; (ii) a large number of transitions from hermaphroditism to dioecy; (iii) the evolution of apomixis; and (iv) a huge variety of strategies to entice pollinators and dispersers. Flower evolution and co-evolution with most groups of terrestrial animals had an enormous impact on life on earth (Fig. 2A, B and E). Animals are mostly gonochorists (Table 1), separated into males and females, and only 5% of all species are

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(B)

(A)

(D)

(C)

(E)

(G)

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Fig. 2. (A–C) Co-evolution between flowers and insects is a major evolutionary driving force for both groups. (A) A fly covered with pollen on a bulbous dandelion (Leontodon sp.). (B) Natal acraea butterfly (Acraea natalica) on a foxglove (Digitalis sp.). (C) Golden orb-web spider (Nephila sp.) demonstrating extreme sexual dimorphism. The tiny male (top) cautiously approaches the huge female in order to mate with her, taking advantage of the opportunity provided while she is busy devouring a beetle. (D) During oestrus lions (Panthera leo) mate dozens of times a day. The lioness prompts the lion to mount her for a brief copulation lasting less than a minute. The barbed penis of the lion induces ovulation and its retraction hurts the lioness, which responds by angrily lashing at him. (E) Copulation of the Campylotes histrionicus moths can last for hours thus preventing other males from mating. The extremely rich colours of butterflies serve both in intra- and inter-species communication signalling to potential mates and predators, respectively. Butterflies have highly sophisticated spectral vision with up to 15 different photoreceptors (Chen et al., 2016). (F) Fungi are the only multicellular organisms that are isogamous. (G) Most snails are hermaphrodites. (H) The tiny flower of the self-pollinating spring whitlow (Erophila verna) is a mere 2 mm across. (I) Ferns reproduce sexually without flowers or the assistance of insects. Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

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Fig. 3. Overview of eukaryotic sex. Meiosis is obligatory in most groups except for apomictic parthenogenetic animals and apomictic plants (light green), where some of the facultative apomictic/parthenogenetic species (light yellow) and especially the obligate ones (very light yellow) in some cases avoid meiosis or parts of it (dashed line).

hermaphrodites. However, once insects, which are the most numerous clade, and but for unknown reasons has no hermaphrodite species, are subtracted, about one-third of animals are hermaphrodites (Jarne & Auld, 2006). Animal hermaphrodite species are therefore neither particularly rare nor on the decline (Grober & Rodgers, 2008). Moreover, evolution from a hermaphrodite to a gonochorist state is not unidirectional and several groups seem to have crossed back and forth several times (Jarne & Auld, 2006). On the face of it hermaphroditism has considerable advantages. Finding a mate is easier as every individual is a potential mate and the cost of males is eliminated. In addition, as no sex determination is required, heteromorphic sex chromosomes are absent together with their problematic non-recombining legacy. Interestingly the selfing rate in animal hermaphrodites seems to be considerable and follows similar patterns to those in hermaphrodite plants (Jarne & Auld, 2006). This conclusion is however based on the only 0.2% of hermaphrodite species for which data are available. Hermaphroditism and gonochorism (Table 1) are not the only options. There are for example animals, mainly fish, Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

that switch from male to female or vice versa as they grow. A further option is dwarf males, where males are tiny compared to females thereby reducing their cost. Such a phenomenon evolved separately in spiders (Fig. 2C), fish and other clades. A second issue which demonstrates considerable variability is sex determination. In animals this is genetically controlled by sex chromosomes which have evolved from autosomes (Ellegren, 2011). Due to their inherent instability these tend to get occasionally lost and renewed. Sex determination can also be environmental (ESD), using cues like temperature as is common in reptiles, or social as in some fish. ESD is not a particularly robust method to maintain an optimal male:female ratio. Indeed, it has been speculated that dinosaurs went extinct due to a temperature shift that sharply skewed their sex ratio (Miller, Summers & Silber, 2004). On the other hand, the option to vary the sex ratio from the canonical 1:1 ratio imposed by sex chromosomes could have adaptive advantages. Compared with the enormous sexual flexibility of flowering plants, animal systems are considerably more rigid and conserved. To compensate for this genetic and

Separating meiosis from mating physiological rigidity animals demonstrate considerable behavioral flexibility. Such variability among otherwise closely related species, can be seen in big cats (Fig. 2D), primates and many other animal families. The wide prevalence, and multiple appearances of many phenotypic, genetic and behavioural alternatives strongly suggests that these are advantageous in the appropriate ecological conditions. Moreover, the variability of all these characteristics among members of closely related clades indicates that they are highly flexible and can be altered with relative ease.

VIII. CONCLUSIONS (1) The lack of a satisfactory all-inclusive theory that can explain the ancient phenomenon of sexual reproduction is highly frustrating. Marco Archetti wrote that the late John Maynard Smith stated ‘we have the answers, we just can’t agree on them’ (Archetti, 2010). Conceptually untying the Gordian knot of meiosis and mating strategies could promote such agreement. This approach enables us to attribute to meiosis an all-important role in protecting the genome. It also allows the development of theories to address the exceedingly rich repertoire of mating strategies and behaviours into which both animals and plants invest extravagantly (Fig. 3). This will enable us to embrace this richness rather than trying to squeeze it into a one-size-fits-all theory. (2) To understand the role of sex, meiosis and mating must be analysed separately. (3) Meiosis is highly conserved and rigid, mating is extremely varied and flexible. (4) Non-recombining heteromorphic sex chromosomes represent mating without meiosis. (5) Self-pollination and most cases of automixis exemplify meiosis without mating.

IX. ACKNOWLEDGEMENTS I am grateful to Avi Shmida for introducing me to the magic world of plants. The ideas described herein are the result of extensive fruitful discussions with him over the years. I would like also to thank Dan Cohen, Giora Simchen, Amos Panet, Ariel Darvasi, Uzi Motro, Raphael Falk and Liran Carmel and, in particular, Uzi Plitmann, for reading the manuscript and making illuminating remarks.

X. REFERENCES Aanen, D., Beekman, M. & Kokko, H. (2016). Weird sex: the underappreciated diversity of sexual reproduction. Philosophical Transactions of the Royal Society, B: Biological Sciences 371, 20160262. Allard, R. W. (1975). The mating system and microevolution. Genetics 79, 115–126. Allard, R. W., Zhang, Q., Maroof, M. A. & Muona, O. M. (1992). Evolution of multilocus genetic structure in an experimental barley population. Genetics 131, 957–969.

9 Archetti, M. (2010). Complementation, genetic conflict, and the evolution of sex and recombination. Journal of Heredity 101(1), S21–S33. Bachtrog, D. (2013). Y-chromosome evolution: emerging insights into processes of Y-chromosome degeneration. Nature Reviews Genetics 14, 113–124. Bachtrog, D., Hom, E., Wong, K. M., Maside, X. & de Jong, P. (2008). Genomic degradation of a young Y chromosome in Drosophila miranda. Genome Biology 9, R30. Baker, H. G. (1955). Self-compatibility and establishment after ‘‘long-distance’’ dispersal. Evolution 9, 347–348. Baker, H. G. (1966). The evolution, functioning and breakdown of heteromorphic incompatibility systems. I. The plumbaginaceae. Evolution 20, 349–368. Bell, G. (1982). The Masterpiece of Nature: The Evolution and Genetics of Sexuality. University of California Press, Berkeley. Bernstein, H., Byerly, H. C., Hopf, F. A. & Michod, R. E. (1984). Origin of sex. Journal of Theoretical Biology 110, 323–351. Bernstein, H., Hopf, F. A. & Michod, R. E. (1987). The molecular basis of the evolution of sex. Advances in Genetics 24, 323–370. Bicknell, R. A. & Koltunow, A. M. (2004). Understanding apomixis: recent advances and remaining conundrums. Plant Cell 16(Suppl), S228–S245. Billiard, S., Lopez-Villavicencio, M., Devier, B., Hood, M. E., Fairhead, C. & Giraud, T. (2011). Having sex, yes, but with whom? Inferences from fungi on the evolution of anisogamy and mating types. Biological Reviews of the Cambridge Philosophical Society 86, 421–442. Casselton, L. A. (2002). Mate recognition in fungi. Heredity 88, 142–147. Charlesworth, B. & Charlesworth, D. (1978). Model for evolution of dioecy and gynodioecy. American Naturalist 112, 975–997. Charlesworth, D. (2002). Plant sex determination and sex chromosomes. Heredity (Edinb) 88, 94–101. Charlesworth, D. & Willis, J. H. (2009). The genetics of inbreeding depression. Nature Reviews Genetics 10, 783–796. Charnov, E. L. (1979). Simultaneous hermaphroditism and sexual selection. Proceedings of the National Academy of Sciences of the United States of America 76, 2480–2484. Chen, P. J., Awata, H., Matsushita, A., Yang, E. C. & Arikawa, K. (2016). Extreme spectral richness in the eye of the Common Bluebottle butterfly, Graphium Sarpedon. Frontiers in Ecology 4, 1–12. Cheptou, P. O. (2012). Clarifying Baker’s law. Annals of Botany 109, 633–641. Christenhusz, M. J. M. & Byng, J. W. (2016). The number of known plants species in the world and its annual increase. Phytotaxa 261, 201–217. Darwin, C. R. (1876). The Effects of Cross and Self Fertilization in the Vegetable Kingdom. John Murray, London. Diggle, P. K., Di Stilio, V. S., Gschwend, A. R., Golenberg, E. M., Moore, R. C., Russell, J. R. & Sinclair, J. P. (2011). Multiple developmental processes underlie sex differentiation in angiosperms. Trends in Genetics 27, 368–376. Ellegren, H. (2011). Sex-chromosome evolution: recent progress and the influence of male and female heterogamety. Nature Reviews Genetics 12, 157–166. Engelst¨adter, J. (2008). Constraints on the evolution of asexual reproduction. BioEssays 30, 1138–1150. Flot, J. F., Hespeels, B., Li, X., Noel, B., Arkhipova, I., Danchin, E. G., Hejnol, A., Henrissat, B., Koszul, R., Aury, J. M., Barbe, V., Barthelemy, R. M., Bast, J., Bazykin, G. A., Chabrol, O., Couloux, A., et al. (2013). Genomic evidence for ameiotic evolution in the bdelloid rotifer Adineta vaga. Nature 500, 453–457. Fontaneto, D., Herniou, E. A., Boschetti, C., Caprioli, M., Melone, G., Ricci, C. & Barraclough, T. G. (2007). Independently evolving species in asexual bdelloid rotifers. PLoS Biology 5, e87. Ghiselin, M. T. (1969). The evolution of hermaphroditism among animals. The Quarterly Review of Biology 44, 189–208. Goodwillie, C., Kalisz, S. & Eckert, C. G. (2005). The evolutionary enigma of mixed mating systems in plants: occurrence, theoretical explanations, and empirical evidence. Annual Review of Ecology, Evolution, and Systematics 36, 47–79. Grober, M. S. & Rodgers, E. W. (2008). The evolution of hermaphroditism. Journal of Theoretical Biology 251, 190–192. Grossenbacher, D., Briscoe Runquist, R., Goldberg, E. E. & Brandvain, Y. (2015). Geographic range size is predicted by plant mating system. Ecology Letters 18, 706–713. Haag, E. S. (2007). Why two sexes? Sex determination in multicellular organisms and protistan mating types. Seminars in Cell & Developmental Biology 18, 348–349. Hadany, L. & Comeron, J. M. (2008). Why are sex and recombination so common? Annals of the New York Academy of Sciences 1133, 26–43. Hadjivasiliou, Z. & Pomiankowski, A. (2016). Gamete signalling underlies the evolution of mating types and their number. Philosophical Transactions of the Royal Society, B: Biological Sciences 371, 20150531. Hamilton, W. D. (1980). Sex versus non-sex versus parasite. Oikos 35, 282–290. Hedrick, P. W. & Garcia-Dorado, A. (2016). Understanding inbreeding depression, purging, and genetic rescue. Trends in Ecology & Evolution 31, 940–952. Hoekstra, R. F. (1982). On the asymmetry of sex: evolution of mating types in isogamous populations. Journal of Theoretical Biology 98, 427–451.

Biological Reviews (2017) 000–000 © 2017 Cambridge Philosophical Society

Michael Brandeis

10 ¨ Horandl, E. (2009). A combinational theory for maintenance of sex. Heredity (Edinb) 103, 445–457. ¨ Horandl, E. & Hadacek, F. (2013). The oxidative damage initiation hypothesis for meiosis. Plant Reproduction 26, 351–367. Hughes, J. F., Skaletsky, H., Pyntikova, T., Graves, T. A., van Daalen, S. K., Minx, P. J., Fulton, R. S., McGrath, S. D., Locke, D. P., Friedman, C., Trask, B. J., Mardis, E. R., Warren, W. C., Repping, S., Rozen, S., Wilson, R. K. & Page, D. C. (2010). Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content. Nature 463, 536–539. Jarne, P. & Auld, J. R. (2006). Animals mix it up too: the distribution of selffertilization among hermaphroditic animals. Evolution 60, 1816–1824. Johnson, N. A. & Lachance, J. (2012). The genetics of sex chromosomes: evolution and implications for hybrid incompatibility. Annals of the New York Academy of Sciences 1256, E1–E22. Judson, O. P. & Normark, B. B. (1996). Ancient asexual scandals. Trends in Ecology & Evolution 11, 41–46. Kardos, M., Taylor, H. R., Ellegren, H., Luikart, G. & Allendorf, F. W. (2016). Genomics advances the study of inbreeding depression in the wild. Evolutionary Applications 9, 1205–1218. Kirk, D. L. (2006). Oogamy: inventing the sexes. Current Biology 16, R1028–R1030. Kondrashov, A. S. (1988). Deleterious mutations and the evolution of sexual reproduction. Nature 336, 435–440. Lachapelle, J. & Bell, G. (2012). Evolutionary rescue of sexual and asexual populations in a deteriorating environment. Evolution 66, 3508–3518. Lampert, K. P. (2008). Facultative parthenogenesis in vertebrates: reproductive error or chance? Sexual Development 2, 290–301. Lee, S. C., Ni, M., Li, W., Shertz, C. & Heitman, J. (2010). The evolution of sex: a perspective from the fungal kingdom. Microbiology and Molecular Biology Reviews 74, 298–340. Lehtonen, J., Jennions, M. D. & Kokko, H. (2012). The many costs of sex. Trends in Ecology & Evolution 27, 172–178. Lehtonen, J., Kokko, H. & Parker, G. A. (2016). What do isogamous organisms teach us about sex and the two sexes? Philosophical Transactions of the Royal Society, B: Biological Sciences 371, 20150532. Lively, C. M. (2010). A review of Red Queen models for the persistence of obligate sexual reproduction. Journal of Heredity 101(Suppl. 1), S13–S20. Long, A. & Michod, R. E. (1995). Origin of sex for error repair. I. Sex, diploidy, and haploidy. Theoretical Population Biology 47, 18–55. Markov, A. V. & Kaznacheev, I. S. (2016). Evolutionary consequences of polyploidy in prokaryotes and the origin of mitosis and meiosis. Biology Direct 11, 28. Maynard Smith, J. (1971). What use is sex? Journal of Theoretical Biology 30, 319–335. Maynard Smith, J. (1978). The Evolution of Sex. Cambridge University Press, Cambridge; New York. McGinley, M. A. & Charnov, E. L. (1988). Multiple resources and the optimal balance between size and number of offspring. Evolutionary Ecology 2, 77–84. Miller, D., Summers, J. & Silber, S. (2004). Environmental versus genetic sex determination: a possible factor in dinosaur extinction? Fertility & Sterility 81, 954–964. Ming, R., Bendahmane, A. & Renner, S. S. (2011). Sex chromosomes in land plants. Annual Review of Plant Biology 62, 485–514. ¨ Mirzaghaderi, G. & Horandl, E. (2016). The evolution of meiotic sex and its alternatives. Proceedings of the Royal Society B: Biological Sciences 283, 20161221. Morran, L. T., Parmenter, M. D. & Phillips, P. C. (2009). Mutation load and rapid adaptation favour outcrossing over self-fertilization. Nature 462, 350–352. Morran, L. T., Schmidt, O. G., Gelarden, I. A., Parrish, R. C. II & Lively, C. M. (2011). Running with the Red Queen: host-parasite coevolution selects for biparental sex. Science 333, 216–218. Muller, H. J. (1964). The relation of recombination to mutational advance. Mutation Research 106, 2–9. Muyle, A., Zemp, N., Deschamps, C., Mousset, S., Widmer, A. & Marais, G. A. (2012). Rapid de novo evolution of X chromosome dosage compensation in Silene latifolia, a plant with young sex chromosomes. PLoS Biology 10, e1001308. Nozaki, H., Mori, T., Misumi, O., Matsunaga, S. & Kuroiwa, T. (2006). Males evolved from the dominant isogametic mating type. Current Biology 16, R1018–R1020. Otto, S. P. & Nuismer, S. L. (2004). Species interactions and the evolution of sex. Science 304, 1018–1020.

Pannell, J. R., Auld, J. R., Brandvain, Y., Burd, M., Busch, J. W., Cheptou, P. O., Conner, J. K., Goldberg, E. E., Grant, A. G., Grossenbacher, D. L., Hovick, S. M., Igic, B., Kalisz, S., Petanidou, T., Randle, A. M., et al. (2015). The scope of Baker’s law. New Phytology 208, 656–667. Plitmann, U. (2002). Agamospermy is much more common than conceived: a hypothesis. Israel Journal of Plant Sciences 50, S111–S117. Renner, S. S. (2014). The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database. American Journal of Botany 101, 1588–1596. Renner, S. S. (2016). Pathways for making unisexual flowers and unisexual plants:moving beyond the ‘‘two mutations linked on one chromosome’’ model. American Journal of Botany 103, 587–589. Richards, A. J. (1997). Plant Breeding Systems, Second Edition (Chapman and Hall, London; Boston. Rozen, S., Skaletsky, H., Marszalek, J. D., Minx, P. J., Cordum, H. S., Waterston, R. H., Wilson, R. K. & Page, D. C. (2003). Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature 423, 873–876. Selander, R. K. & Kaufman, D. W. (1973). Self-fertilization and genetic population structure in a colonizing land snail. Proceedings of the National Academy of Sciences of the United States of America 70, 1186–1190. Sicard, A. & Lenhard, M. (2011). The selfing syndrome: a model for studying the genetic and evolutionary basis of morphological adaptation in plants. Annals of Botany 107, 1433–1443. Skaletsky, H., Kuroda-Kawaguchi, T., Minx, P. J., Cordum, H. S., Hillier, L., Brown, L. G., Repping, S., Pyntikova, T., Ali, J., Bieri, T., Chinwalla, A., Delehaunty, A., Delehaunty, K., Du, H., Fewell, G., et al. (2003). The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes. Nature 423, 825–837. Speijer, D., Lukes, J. & Elias, M. (2015). Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life. Proceedings of the National Academy of Sciences of the United States of America 112, 8827–8834. Stebbins, G. L. (1957). Self fertilization and population variability in higher plants. The American Naturalist 91, 337–354. Wang, J., Wurm, Y., Nipitwattanaphon, M., Riba-Grognuz, O., Huang, Y. C., Shoemaker, D. & Keller, L. (2013). A Y-like social chromosome causes alternative colony organization in fire ants. Nature 493, 664–668. Weismann, A., Thomson, J. A. & Thomson, M. R. (1904). The Evolution Theory. E. Arnold, London. Wilkins, A. S. & Holliday, R. (2009). The evolution of meiosis from mitosis. Genetics 181, 3–12. Winn, A. A., Elle, E., Kalisz, S., Cheptou, P. O., Eckert, C. G., Goodwillie, C., Johnston, M. O., Moeller, D. A., Ree, R. H., Sargent, R. D. & Vallejo-Marin, M. (2011). Analysis of inbreeding depression in mixed-mating plants provides evidence for selective interference and stable mixed mating. Evolution 65, 3339–3359. Wright, S. I., Kalisz, S. & Slotte, T. (2013). Evolutionary consequences of self-fertilization in plants. Proceedings of the Royal Society B: Biological Sciences 280, 20130133. Zimmer, C. (2009). Origins. On the origin of sexual reproduction. Science 324, 1254–1256.

XI. SUPPORTING INFORMATION Additional supporting information may be found in the online version of this article. Movie S1. Two haploid budding yeast Saccharomyces cerevisiae cells of opposite mating types fuse and form a diploid offspring that is subsequently budding.

(Received 10 May 2017; revised 17 August 2017; accepted 18 August 2017 )

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