Evolution of Asexuality in Hybrid Species

To investigate why asexuality might spread in a hybrid population, we consider a population dynamics system consisting of two parental populations and a sexual, "wild-type" hybrid population. Model's specification allow us to determine the wild-type hybrid population size. Within this population we then look at the condition for a mutation bringing in some degree of asexual reproduction to invade.

A partially asexual mutant can invade if and only it reproduces more than sexual wild-types. Why would that be the case? Hybrid species mix diverged parental genomes, which often results in some partial sterility. A classic case is total male sterility. Under this assumption, our model show that asexuality can indeed evolve, as it gains benefits by restoring births when hybrid females mate with hybrid males. One serious downfall of such a scenario, however, is that for the mutant to invade the mutation must be "big": an almost completely asexual mutant must appear for it to be able to outcompete wild-type hybrids.

In this work, we made the additional effort to separate the two moments of amphimixis (sexual reproduction): the fusion of the egg and the sperm (which triggers embryogenesis, the development pathways of a new individual from an egg cell) and the fusion of the egg's and sperm's genomes (which allows a haploid gamete to become a diploid offspring). We ask: what if partial hybrid sterility were to affect only one of these moments? What if gametes were able to fuse, but not their genomes?

In a first model, we consider a that hybrid sperm bears a deficiency that makes it unable to pass its genome over to the fertilized oocyte. In such a scenario, hybrid x hybrid matings are again unable to produce offsprings. However, contrarily to the previous model, here it is because hybrid oocyte enters embryogenesis, but as a haploid egg cell doomed to rapid death. Analytical and numerical work show that in this case asexuality evolves much more easily. Indeed, here a relatively small mutation affecting only the meiotic pathway of the egg can restore fertility. Mutant females don't need to be (almost) fully asexual here to gain substantial benefits; it is enough that they produce diploid eggs in sufficient proportion. In particular, they don't need to evolve a mechanism to prevent fusion wth sperm in order to avoid triploid offsprings: hybrid x hybrid matings produce diploid offsprings as the sperm does not transmit its genome. We show that such a mechanism can evolve afterwards, as a way for female to also produce viable diploid offspring when mating with paternal males, which are able to transmit their genomes.

In a second model, we consider that the egg now bears the deficiency, and cannot "receive" paternal genomes from any sperm (hybrid or parental). We show that in that case asexuality evolves even more easily, as it restores fertility not only when hybrid females mate with hybrid males, but also when they mate with parental males.

For the first time, we propose a pathway that decouples the two necessary components of asexual reproduction: diploid egg production and prevention of fusion between egg and sperm. We argue that hybrid species may exhibit genomic incompatibilities that hampers their fertility; for this reason, they are a particularly fertile ground for the evolution of asexuality as a way to rescue fertility. We show that this should be particularly true under particular forms of sterility, as they allow to evolve asexuality one trait at a time, a much simpler evolutionary process.