Evolutionary analysis of MID homologs during the transition from homothallic species to heterothallic species in Volvox sect. Volvox (Chlorophyceae)

The diversity and the evolution of mating systems have been studied in various eukaryotes. Volvox sect. Volvox of the volvocine green algae is suitable for evolutionary studies of the transition from homothallic to heterothallic mating systems because this transition is recognized in distal lineages within Volvox sect. Volvox. MID homologs are important in determining the mating type minus and male in heterothallic species of the volvocine algae. However, MID homologs have not been studied within the entire Volvox sect. Volvox. Here, we analyzed MID homologs from almost all taxa of Volvox sect. Volvox and found no significant differences in MID sequences between homothallic species and males of heterothallic species. Furthermore, MID of the male genotype of the heterothallic Volvox rousseletii showed an expression pattern that was almost identical to that of a closely related homothallic species, Volvox ferrisii. Therefore, the function of MID might have been conserved within the ancestral homothallic lineage of Volvox sect. Volvox and during the evolutionary transition of the heterothallic male genotype from the ancestral homothallic species, possibly related to male gametogenesis.