Three sex phenotypes in a haploid algal species give insights into the evolutionary transition to a self-compatible mating system

Reorganization of the ancestral sex-determining regions during the evolution of trioecy in Pleodorina starrii

The coexistence of three sexual phenotypes (male, female and bisexual) in a single species, 'trioecy', is rarely found in diploid organisms such as flowering plants and invertebrates. However, trioecy in haploid organisms has only recently been reported in a green algal species, Pleodorina starrii. Here, we generated whole-genome data of the three sex phenotypes of P. starrii to reveal a reorganization of the ancestral sex-determining regions (SDRs) in the sex chromosomes: the male and bisexual phenotypes had the same "male SDR" with paralogous gene expansions of the male-determining gene MID, whereas the female phenotype had a "female SDR" with transposition of the female-specific gene FUS1 to autosomal regions. Although the male and bisexual sex phenotypes had the identical male SDR and harbored autosomal FUS1, MID and FUS1 expression during sexual reproduction differed between them. Thus, the coexistence of three sex phenotypes in P. starrii is possible.

Digest: Three sexes from two loci in one genome: A haploid alga expands the diversity of trioecious species

Multicellular eukaryotes exhibit a remarkable diversity of sexual systems; however, trioecy, the coexistence of male, female, and cosexual or hermaphrodite individuals in a single species, is remarkably rare. Takahashi et al. (2021) report the first known instance of trioecy in a haploid organism. In contrast to other known cases of trioecy, the authors report evidence for genetic control of all three sexes by two loci. These results complicate models for sexual system turnover and expand the known diversity of trioecy species in several ways.