Multicellular eukaryotes are, with a few exceptions, characterised by anisogamy, with two types of gamete, one larger than the other. By contrast, most unicellular eukaryotes are isogamous (typically with two mating types, for instance plus and minus, rather than distinct male and female gametes), reflecting the ancestral state. The transition to multicellularity thus appears intimately linked to the transition from isogamy to anisogamy (and indeed to oogamy, a form of anisogamy with a large, immotile egg and a motile sperm). Volvocine algae provide a helpful model for understanding this transition, with species crossing the spectrum of gametic differentiation and multicellular complexity. The volvocine Mid transcription factors have been shown to specify minus mating types and male gametes. Previous work from James Umen and colleagues further suggested that changes in MID genes between species might underlie the evolution of anisogamy. Now, they explicitly test this connection using heterologous expression experiments. Surprisingly, they find that the expression of MID genes from both anisogamous and isogamous multicellular species was able to induce spermatogenesis in female Volvox carteri, a complex oogamous species. MID genes from more complex species were unable to substitute for MID in a unicellular isogamous species, although this may reflect poor levels of expression of the constructs. The results show that changes in Mid proteins did not drive the transition to oogamy in volvocine algae, implying that these drivers are to be found elsewhere in the genome.
