Parthenogenesis and the Evolution of Anisogamy

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.

The evolution of parthenogen fertilization rates in switching environments: from facultative cell-fusion to oogamy

“…when cell masses are zero). This feature is common in simplified models of anisogamy evolution [16]; we comment on these further in the Results, Section 3.…”

“…gametophytes), or developing as diploid adults (e.g. sporophytes) for the next generation [49]; assuming no fitness differences arising from ploidy, these interpretstions are mathematically equivalent up to a generational delay [16]. Adults then produce haploid gametes, completing the generational cycle.…”

“…Traditionally, models assume obligate sexual reproduction (unfertilized gametes die at the end of each generation) and a fixed fertilization rate (the rate at which gametes encounter one another and fuse to form zygotes) [5, 6, 7], with the further evolution of oogamy (motile microgametes and sessile macrogametes) possible when factors such as speed-mass relationships [8], costly motility [9] or internal fertilization are accounted for [10]. Recently, however, inspired by the life-histories of green and brown algae such as Blidingia minima (isogamous [11]), Urospora neglecta (anisogamous [12]), and Saccharina japonica (oogamous [13]), whose gametes can develop parthenogenically should they fail to find a mate (see [11, 12, 14], respectively), the first of these assumptions was relaxed in two theoretical papers [15, 16]. In [15], extra survival costs were incurred by gametes developing parthenogenically, while in [16] extra survival costs on either the parthenogenic or the sexual reproductive route were considered.…”

“…Recently, however, inspired by the life-histories of green and brown algae such as Blidingia minima (isogamous [11]), Urospora neglecta (anisogamous [12]), and Saccharina japonica (oogamous [13]), whose gametes can develop parthenogenically should they fail to find a mate (see [11, 12, 14], respectively), the first of these assumptions was relaxed in two theoretical papers [15, 16]. In [15], extra survival costs were incurred by gametes developing parthenogenically, while in [16] extra survival costs on either the parthenogenic or the sexual reproductive route were considered. These studies lead to a natural question; if the sexual route (via fertilization) and asexual route (via parthenogenesis) carry different survival costs, how should the fertilization rate evolve to account for this?…”

Novel Approaches for Species Concepts and Delimitation in Polyploids and Hybrids

“…Asexual lineages are still under the compatibility regime as long as they retain some components of meiosis (automixis in animals, autogamy in plants, and facultative apomixis belong to these "semi"-sexual forms; syngamy can be more easily skipped than meiosis [159,175]). Agamic polyploids with a mix of facultative sexuality and clonality cannot form species-like lineages, just big clusters of many local clones may appear with a reticulate pangenome-like structure as in prokaryotes [36]; Figure 1b.…”