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The Evolution of Anisogamy Does Not Always Lead to Male Competition

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The Evolution of Anisogamy Does Not Always Lead to Male Competition bioRxiv preprint doi: https://doi.org/10.1101/2020.12.18.423382; this version posted December 18, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. The evolution of anisogamy does not always lead to male competition Mattias Siljestam1 and Ivain Martinossi-Allibert1,2,* 1Department of Ecology and Genetics, Animal Ecology, Uppsala University, Norbyvägen 18D, 752 36 Uppsala, Sweden; 2Department of Organismal Biology, Systematics, Uppsala University, Norbyvägen 18D, 752 36 Uppsala, Sweden This manuscript was compiled on December 18, 2020 1 Anisogamy has evolved in a large proportion of sexually reproduc- tion and parental care, but it does not prove very insightful 21 2 ing multicellular organisms allowing the definition of the female when it comes to revealing the influence of anisogamy on these 22 3 and male sexes, producing large and small gametes, respectively. traits because of the multitude of confounding factors that are 23 4 Anisogamy is the initial sexual dimorphism: it has lead the sexes not entirely understood. On the other hand, the theoretical 24 5 to experience selection differently, which makes it a good starting approach on which we will focus relies on thought experiments 25 6 point to understand the evolution of further sexual dimorphisms. For and mathematical models that aim at understanding the evolu- 26 7 instance, it is generally accepted that anisogamy sets the stage for tionary origins of the male and female sexes, i.e. the evolution 27 8 more intense intrasexual competition in the male sex than in the fe- of anisogamy. This line of research has also endeavoured to gen- 28 9 male sex. However, we argue that this idea may rely on assump- erate predictions on further evolutionary changes that would 29 10 tions on the conditions under which anisogamy has evolved in the result from the state of anisogamy, sometimes referred to as 30 11 first place. We consider here two widely accepted scenarios for the the sexual cascade (Parker, 2014), where anisogamy ultimately 31 12 evolution of anisogamy: gamete competition or gamete limitation. leads to sexual dimorphisms in intrasexual competition and 32 13 We present a mechanistic mathematical model in which both gamete may also influence dimorphism in parental care. We believe 33 14 size and an intrasexual competition trait for fertilisation can coevolve this approach to be most useful to specifically understand the 34 15 in a population starting without dimorphism between its two mating influence of anisogamy on further sexual dimorphisms, because 35 16 types. Two different intrasexual competition traits are investigated, it starts in a context where anisogamy is the only difference 36 17 gamete motility and the ability of gametes to capture gametes of the between males and females, therefore automatically removing 37 18 opposite mating type. We show that gamete competition and ga- the influence of potentially confounding factors. 38 19 mete limitation can lead to greatly different outcomes in terms of Starting in 1948, Bateman (Bateman, 1948) conducted an 39 20 which sex competes most for fertilisation. Our results suggest that influential and later questioned and debated (Sutherland, 1985; 40 21 gamete competition is most likely to lead to stronger competition in Tang-Martinez and Ryder, 2005; Gowaty et al., 2012) study in 41 22 males. On the other hand, under gamete limitation, competition in Drosophila melanogaster in which he measured higher variance 42 23 form of motility can evolve in either sex while gamete capture mainly in reproductive success in males than in females. He suggested 43 24 evolves in females. This study suggests that anisogamy does not that this result may be explained by the state of anisogamy 44 25 per se lead to more intense male competition. The conditions un- in the following way: males produce small, cheap, numerous 45 26 der which anisogamy evolves matter, as well as the competition trait gametes and females fewer, larger, more energetically costly 46 27 considered. ones, therefore the number of gametes produced by males 47 would never be a limiting factor of their reproductive success, 48 anisogamy | mating types | motility | intrasexual competition | sexual while it is the case for females. This would allow males to 49 cascade | gamete limitation | gamete competition | adaptive dynamics reach higher potential reproductive success than females. As 50 a consequence, because each offspring has a male and female 51 1 ost sexually reproducing multicellular organisms have parent and the population sex-ratio is assumed to be balanced, 52 2 Mevolved mating types and anisogamy (dimorphism in the fact that some males reach a high reproductive success 53 3 gamete size)(Lessells et al., 2009). In systems with two mating implies that other males will have low or null reproductive 54 4 types, the one producing larger gametes is denoted the female success. At the same time, most females are expected to have a 55 5 sex while the one producing smallPREPRINT gametes is the male sex. Be- reproductive success that is close to average. This results in a 56 6 cause it is the earliest possible sexual dimorphism, researchers higher variance in reproductive success for males. In the early 57 7 interested in sexual selection have tried to understand how 1970s, Trivers (Trivers et al., 1972) followed up on Bateman’s 58 8 anisogamy may have influenced the evolution of sexual dimor- idea and suggested an explanation for the causal relationship 59 9 phisms or sex-biases that evolved later on, most notably in between anisogamy, intrasexual competition and parental in- 60 10 intrasexual competition for mating and in parental care. Two vestment. Trivers argued that initial energy investment in 61 11 main approaches have been used to this end: first the study of offspring should determine which sex competes and which sex 62 12 sex-biases in intrasexual competition and parental care in con- cares for offspring. The larger initial energy investment in the 63 13 temporary species through empirical observation, supported zygote by females should automatically trigger competition for 64 14 by meta-analysis and review (Clutton-Brock, 1991; Cox and mating opportunities in males and create a stronger incentive 65 15 Calsbeek, 2009; Janicke et al., 2016; Singh and Punzalan, 2018) 16 and second mathematical modelling (e.g. Kokko and Rankin, Both authors contributed equally to this work. Conceptualization: I.M.; Methodology: M.S.; Formal 17 2006; Kokko et al., 2012; Jennions and Fromhage, 2017). The analysis: M.S.; Visualization: M.S.; Writing: I.M and M.S.; Funding Acquisition: I.M. 18 empirical approach is most powerful to inform us on how eco- Authors declare no competing interests. 19 logical settings, evolutionary history, and particularities of the * 20 mating system may influence sexual dimorphisms in competi- To whom correspondence should be addressed. E-mail: [email protected] December 18, 2020 | vol. XXX | no. XX | 1–20 bioRxiv preprint doi: https://doi.org/10.1101/2020.12.18.423382; this version posted December 18, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 66 in females to provide further care in order to increase off- reproductive success is limited by female fecundity, while male 127 67 spring survival. Trivers’s ideas (Trivers et al., 1972) regarding reproductive success is limited by male competitive ability. We 128 68 parental investment have been critically examined early on and therefore predict, in agreement with previous work (Schärer 129 69 resulted in the production of a wealth of theoretical work (e.g. et al., 2012; Lehtonen et al., 2016), that if anisogamy evolves 130 70 Dawkins and Carlisle, 1976; Queller, 1997; Kokko and Jen- in a gamete competition context, it should favour the evolution 131 71 nions, 2008; Jennions and Fromhage, 2017), which have shown of competition traits in the male sex. 132 72 that although there may be a causal link between anisogamy In the gamete limitation framework, which originated with 133 73 and parental investment, it ought to be more complex than Kalmus (Kalmus, 1932) the evolution of anisogamy arises as a 134 74 Trivers had initially predicted (Knight, 2002). In contrast, the strategy to increase gamete encounter rates, in an environment 135 75 mechanism through which anisogamy should result in more where gamete density is very low. The evolution of anisogamy 136 76 intense intrasexual competition in males has been relatively in this framework is favoured by sperm limitation, i.e. the fact 137 77 neglected by theoreticians. Although initially a verbal model, that not all the larger gametes get fertilised and this suggests 138 78 it has not received much critical attention until recently (see that both sexes experience selection to increase fertilisation 139 79 Lehtonen et al., 2016), and instead has been supported by success. For that reason, we predict that the gamete limitation 140 80 verbal arguments (e.g. Schärer et al., 2012; Parker, 2014) or context should allow the evolution of competition traits in 141 81 correlational empirical data (Janicke et al., 2016). both sexes. 142 As demonstrated by Lehtonen and Kokko (Lehtonen and 143 82 Here, we re-examine the causal relationship between Kokko, 2011), gamete competition and gamete limitation 144 83 anisogamy and the evolution of intrasexual competition. Al- are not two mutually exclusive theories on the evolution of 145 84 though much work has been done on the evolution of anisogamy anisogamy, but rather two complementary ideas that speculate 146 85 since Trivers developed his ideas (to cite a few: Parker et al., on how anisogamy may have evolved in populations of high or 147 86 1972; Parker, 1978; Schuster and Sigmund, 1982; Smith, 1982; low gamete densities.
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