Evolution of Anisogamy in Organisms with Parthenogenetic Gametes

Abstract

The two sexes are defined by the sizes of the gametes they produce, anisogamy being the state with two differing gamete sizes (hence, females and males). The origin of this divergence has received much research interest, both theoretically and empirically. The gamete dynamics (GD) theory is a widely accepted theoretical explanation for anisogamy, and green algae have been an important empirical testing ground for the theory. However, some green and brown algae produce parthenogenetic gametes (gametes that can develop without fusing with another gamete), in contrast to an assumption in GD theory that unfused gametes do not develop. Here, we construct a GD model accounting for parthenogenetic gametes. We find that under conditions of panmixia and highly efficient fertilization (i.e., conditions of classical GD models from 1972 onward), the results remain largely unaltered by parthenogametes. However, under gamete-limited conditions anisogamy evolves less easily in the new model, and a novel result emerges: whereas previous models typically predict the evolution of either anisogamy or small isogamy, the current model shows that large isogamy can evolve when parthenogenetic gametes evolve under conditions of inefficient fertilization. Our analyses uncover unexplored complications relating to sex ratios under this relatively uncharted gametic system. We discuss limitations these complications impose on our models and suggest avenues for future research. We compare model results to algae with parthenogenetic gametes in nature.

Parthenogenesis and the Evolution of Anisogamy

“…Taken together, this suggests that at least in principle, the propensity of either sex to reproduce parthenogenically may be more labile than commonly assumed. Categorical descriptions of such possibilities for parthenogenic reproduction in isogamous and anisogamous species across the brown algae can be found in [27,33], while a thorough discussion of empirical results relevant for the following theoretical treatment is provided in [25] for both brown and green algae.…”

“…In contrast to previous approaches, but in line with [25], we assume that unfertilised gametes (those that remain in the gamete pool at the end of the fertilisation period) can become a haploid parthenosporophyte and develop into adults [40]. The probability of this succeeding is modelled with an equivalent function as Equation ( 2)) but now with a parameter β p ; S p (β p , m i ) = e −β p /m i .…”

“…The 'parthenogenetic disadvantage' case has β p > β z , which implies that for any juvenile size, it is more challenging for a parthenogenetically produced juvenile to recruit into the adult population than for a sexually produced zygote to do so. Although [25] model this in a different way (a fixed survival reduction for parthenogens), they consider the parthenogenetic disadvantage scenario the more likely one, and restrict their analysis to this case. However, we additionally analyse the 'parthenogenetic advantage' case (β p < β z ), where zygote formation is associated with intrinsic costs.…”

“…Here, we have chosen to examine one such route in detail, and by coincidence, it is near identical with that of [25], a paper that we only became aware of when finalising ours. Both their work and ours consider that gametes that did not participate in syngamy (sex) may survive to become adults (parthenogenesis).…”

“…Our treatment below differs somewhat from theirs, in that we model potential survival differences between sexually and parthenogenetically produced juveniles by modifying the size-survival relationships directly (in [25], parthenogenetic survival is a fixed fraction of sexually produced zygote survival). We also provide a complementary approach to theirs when tracking variation in adult sex ratios, including considering that a particular mating type may, via parthenogenesis, spread to be so common that sex (with the now outcompeted mating type) becomes impossible.…”