Abstract
Recently, it was pointed out that classic models for the evolution of anisogamy do not take into account the possibility of parthenogenetic reproduction, even though sex is facultative in many relevant taxa (e.g., algae) that harbour both anisogamous and isogamous species. Here, we complement this recent analysis with an approach where we assume that the relationship between progeny size and its survival may differ between parthenogenetically and sexually produced progeny, favouring either the former or the latter. We show that previous findings that parthenogenesis can stabilise isogamy relative to the obligate sex case, extend to our scenarios. We additionally investigate two different ways for one mating type to take over the entire population. First, parthenogenesis can lead to biased sex ratios that are sufficiently extreme that one type can displace the other, leading to de facto asexuality for the remaining type that now lacks partners to fuse with. This process involves positive feedback: microgametes, being numerous, lack opportunities for syngamy, and should they proliferate parthenogenetically, the next generation makes this asexual route even more prominent for microgametes. Second, we consider mutations to strict asexuality in producers of micro- or macrogametes, and show that the prospects of asexual invasion depend strongly on the mating type in which the mutation arises. Perhaps most interestingly, we also find scenarios in which parthenogens have an intrinsic survival advantage yet facultatively sexual isogamous populations are robust to the invasion of asexuals, despite us assuming no genetic benefits of recombination. Here, equal contribution from both mating types to zygotes that are sufficiently well provisioned can outweigh the additional costs associated with syngamy.
Highlights
Explaining the origin of gamete size differences is a much celebrated success story in evolutionary ecology [1,2,3,4,5]
The coevolution of gamete sizes when there are two mating types differs from the simplest settings, as there is a game-theoretic aspect to the problem: Given that sexual reproduction involves syngamy where two gametes fuse, the door is open to the evolution of microgametes that ‘bypass’ the tradeoff by fusing with a macrogamete, which guarantees that the resulting zygote is large even though the microgamete’s own contribution to size remains meagre
We focus on relaxing one simplification made by early anisogamy models: that sexual reproduction is obligate, i.e., that gametes can only participate in syngamy — or die
Summary
Explaining the origin of gamete size differences is a much celebrated success story in evolutionary ecology [1,2,3,4,5]. Under a wide range of conditions [1,6,7,8], isogamy (equal gamete sizes) ceases to be stable and evolves into anisogamy (unequal gamete sizes) via disruptive selection that has its origins in what is essentially a quality–quantity tradeoff. In contexts other than gamete size evolution, ecological situations with a quality–quantity tradeoff (or size-number tradeoff) typically lead to an optimal solution that reflects the best compromise between the two conflicting demands on reproductive success [9,10,11]. Evolution of microgametes (sperm) can be a successful strategy even though it comes with a clear cost: if microgamete producers (males) and macrogamete producers (females) are approximately abundant, and do not differ greatly in their budget for gamete production, sperm will vastly outnumber fertilisation opportunities, and most of the produced microgametes are a wasted investment [7]
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