Evolutionary divergence of the sex-determining gene uncoupled from the transition to anisogamy in volvocine algae.

Volvocine algae constitute a unique comparative model for investigating the evolution of oogamy from isogamous mating types. The sex- or mating type-determining gene encodes a conserved RWP-RK transcription factor found in either the or male mating locus of dioecious volvocine species. We previously found that from the isogamous species () could not induce ectopic spermatogenesis when expressed heterologously in females, suggesting coevolution of Mid function with gamete dimorphism. Here we found that ectopic expression of from the anisogamous species () could efficiently induce spermatogenesis when expressed in females and, unexpectedly, that from the isogamous species was also able to induce spermatogenesis. Neither nor could complement a mutant, at least partly owing to instability of heterologous Mid proteins. Our data show that Mid divergence was not a major contributor to the transition between isogamy and anisogamy/oogamy in volvocine algae, and instead implicate changes in cis-regulatory interactions and/or trans-acting factors of the Mid network in the evolution of sexual dimorphism.