Evolution of sex and mating loci: An expanded view from Volvocine algae

Sexual reproduction in Volvocine algae coevolved with the acquisition of multicellularity. Unicellular genera such as Chlamydomonas and small colonial genera from this group have classical mating types with equal-sized gametes, while larger multicellular genera such as Volvox have differentiated males and females that produce sperm and eggs respectively. Newly available sequence from the Volvox and Chlamydomonas genomes and mating loci open up the potential to investigate how sex-determining regions co-evolve with major changes in development and sexual reproduction. The expanded size and sequence divergence between the male and female haplotypes of the Volvox mating locus (MT) not only provide insights into how the colonial Volvocine algae might have evolved sexual dimorphism, but also raise questions about why the putative ancestral-like MT locus in Chlamydomonas shows less divergence between haplotypes than expected.

► Volvocine algae include unicellular Chlamydomonas and multicellular Volvox. ► Chlamydomonas is isogamous while Volvox is oogamous. ► The Volvox mating locus (MT) is expanded relative to Chlamydomonas MT. ► A model for MT expansion driven by sexual selection is presented. ► Models for unexpectedly low divergence of Chlamydomonas MT haplotypes are presented.