On the evolution of variation in sexual reproduction through the prism of eukaryotic microbes

Almost all eukaryotes undergo sexual reproduction to generate diversity and select for fitness in their population pools. Interestingly, the systems by which sex is defined are highly diverse and can even differ between evolutionarily closely related species. While the most commonly known form of sex determination involves males and females in animals, eukaryotic microbes can have as many as thousands of different mating types for the same species. Furthermore, some species have found alternatives to sexual reproduction and prefer to grow clonally and yet undergo infrequent facultative sex-ual reproduction. These organisms are mainly invertebrates and microbes, but several examples are also present among vertebrates suggesting that alternative modes of sexual reproduction evolved multiple times throughout evolution. In this review, we summa-rize the sex-determination modes and variants of sexual reproduction found across the eukaryotic tree of life and suggest that eukaryotic microbes provide unique opportunities to study these processes in detail. We propose that understanding variations in modes of sexual reproduction can serve as a foundation to study the evolution of sex and why and how it evolved in the first place.

Automated summary

Almost all eukaryotes exhibit sexual reproduction, but the systems by which sex is determined vary greatly even among closely related species. While animals typically have males and females, eukaryotic microbes can have thousands of different mating types. Some species have also developed alternative modes of reproduction, with occasional facultative sexual reproduction. In this review, we summarize the different modes of sex determination and reproductive variants across the eukaryotic tree of life, with a focus on the unique opportunities provided by eukaryotic microbes. We propose that studying the variations in sexual reproduction modes can serve as a foundation to understand the evolution of sex.