Volvox are green algae that can form large and amazingly beautiful colonies of up to 50,000 cells, and have been widely studied as a model for the evolution of multicellularity, but they also have a huge diversity of mating systems. Matthew Herron has a great post over on his blog, Fierce Roller (which focuses on the the biology of volvocine algae and evolutionary mathematical models amongst other things), about the unusual and complicated world of Volvox sex, direct from the the Volvox 2015 conference. For those of us who work mostly on model organisms, it’s good to be reminded that these organisms are just the tip of the proverbial evolutionary iceberg, and that there are many great practical and conceptual discoveries awaiting all over the tree of life.
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.