I believe that sex is one of the most beautiful, natural, wholesome things that money can buy.

–Steve Martin

Volvox, and the volvocine algae in general, are well known as a model system for the evolution of multicellularity and cellular differentiation, but they are also an outstanding model for the evolution of sex and mating types. Volvocine algae are facultatively sexual, with haploid vegetative colonies reproducing asexually through mitosis but occasionally entering a sexual cycle that usually results in a diploid, desiccation-resistant zygote or ‘spore.’ Most of the small colonial species and unicellular relatives are isogamous, that is, the gametes are of equal size. Nevertheless, each species has two self-incompatible mating types, usually designated as ‘plus’ and ‘minus.’ In some of the larger species, the gametes have diverged into a small, motile form that we call sperm and a large, often immotile form that we call eggs. Across the eukaryotic domain, it is gamete size, not form of genitalia, fancy plumage, or receding hairline, that define males and females.

The volvocine algae span a wide range of mating systems, making them a useful (and I think underutilized) system for comparative studies of the evolution of sex. As I’ve already mentioned, both isogamous (equal-sized gametes) and oogamous (sperm and eggs) species exist, and there is good reason to suspect that oogamy has evolved independently in two separate lineages:

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