@ARTICLE{TreeBASE2Ref31142, author = {Kayoko Yamamoto and Takashi Hamaji and Hiroko Kawai-Toyooka and Ryo Matsuzaki and Fumio Takahashi and Yoshiki Nishimura and Masanobu Kawachi and Hideki Noguchi and Yohei Minakuchi and James Umen and Atsushi Toyoda and Hisayoshi Nozaki}, title = {Three genomes in the algal genus Volvox reveal the fate of a haploid sex-determining region after a transition to homothallism}, year = {2020}, keywords = {Volvox, MT, homothallism}, doi = {10.1073/pnas.2100712118}, url = {}, pmid = {}, journal = {Proceedings of the National Academy of Sciences, U.S.A.}, volume = {}, number = {}, pages = {}, abstract = {Transitions between separate sexes (dioecy) and other mating systems are common across eukaryotes. Here, we study a change in a haploid dioecious green algal species with male- and femaledetermining chromosomes (U and V). The genus Volvox is an oogamous (with large, immotile female gametes and small, motile male gametes) and includes both heterothallic species (with distinct male and female genotypes, associated with a mating-type system that prevents fusion of gametes of the same sex) and homothallic species (bisexual, with the ability to self-fertilize). We date the origin of an expanded sex-determining region (SDR) in Volvox to at least 75 Mya, suggesting that homothallism represents a breakdown of dioecy (heterothallism). We investigated the involvement of the SDR of the U and V chromosomes in this transition. Using de novo whole-genome sequences, we identified a heteromorphic SDR of ca 1 Mbp in male and female genotypes of the heterothallic species Volvox reticuliferus and a homologous region (SDLR) in the closely related homothallic species Volvox africanus, which retained several different hallmark features of an SDR. The V. africanus SDLR includes a large region resembling the female SDR of the presumptive heterothallic ancestor, whereas most genes from the male SDR are absent. However, we found a multicopy array of the male-determining gene, MID, in a different genomic location from the SDLR. Thus, in V. africanus, an ancestrally female genotype may have acquired MID and thereby gained male traits.} }

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