Evolution of an Expanded Sex-Determining Locus in Volvox

Revealing Volvox Female and male gametes of the green alga, Volvox, significantly differ in size. Those of Chlamydomonas, another green algae from a lineage that separated from Volvox some 200 million years ago, are the same size. We know sex in Chlamydomonas is governed by a sex-determining locus called MT. In a detailed comparison of the MT loci of Volvox and Chlamydomonas, Ferris et al. (p. 351) found that although MT has retained some similarity in gene order, its composition has greatly changed between the two species. In Volvox, new genes have been coopted into this locus and show sex-specific expression. Mating loci among green algae show conserved gene order, but also have many unique features that may explain gamete size differences. Although dimorphic sexes have evolved repeatedly in multicellular eukaryotes, their origins are unknown. The mating locus (MT) of the sexually dimorphic multicellular green alga Volvox carteri specifies the production of eggs and sperm and has undergone a remarkable expansion and divergence relative to MT from Chlamydomonas reinhardtii, which is a closely related unicellular species that has equal-sized gametes. Transcriptome analysis revealed a rewired gametic expression program for Volvox MT genes relative to Chlamydomonas and identified multiple gender-specific and sex-regulated transcripts. The retinoblastoma tumor suppressor homolog MAT3 is a Volvox MT gene that displays sexually regulated alternative splicing and evidence of gender-specific selection, both of which are indicative of cooption into the sexual cycle. Thus, sex-determining loci affect the evolution of both sex-related and non–sex-related genes.