Origin of Female/Male Gender as Deduced by the Mating-Type Loci of the Colonial Volvocalean Greens

Abstract

Colonial Volvocales (green algae) are a model lineage for the study of the evolution of sexual reproduction because isogamy, anisogamy, and oogamy are recognized within the closely related group, and several mating type (sex)-specific genes were identified in the closely related unicellular Chlamydomonas reinhardtii during the past century. In 2006, we first identified a sex-specific gene within the colonial Volvocales using the anisogamous colonial volvocalean alga Pleodorina starrii, namely, a male-specific gene called “OTOKOGI,” which is a homologue of the minus mating type-determining gene MID of the isogamous C. reinhardtii. Thus, it was speculated that the derived or minus mating type of C. reinhardtii is homologous to the male in the anisogamous/oogamous members of the colonial Volvocales. The discovery of the male-specific gene facilitated comparative studies of the mating-type locus (MT) (primitive sex chromosomal region) because it must be localized in MT. Recently, our international research group determined the genome sequence of MT in the oogamous Volvox carteri. V. carteri MT shows remarkable expansion and divergence relative to that from C. reinhardtii. Five new female-limited “HIBOTAN” genes and ten male-limited genes (including “OTOKOGI”) were identified in V. carteri MT. These observations suggest that the origins of femaleness and maleness are principally affected by the evolution of MT, which has undergone a remarkable expansion and gain of new male- and female-limited genes. Our recent results regarding the evolution of the volvocalean MT gene MAT3/RB are also discussed in relationship to the evolution of male–female sexual dimorphism.