Expanded male sex-determining region conserved during the evolution of homothallism in the green alga Volvox

Male and female genotypes in heterothallic (self-incompatible) species of haploid organisms, such as algae and bryophytes, are generally determined by male and female sex-determining regions (SDRs) in the sex chromosomes. To resolve the molecular genetic basis for the evolution of homothallic (bisexual and self-compatible) species from a heterothallic ancestor, we compared whole-genome data from Thai and Japanese genotypes within the homothallic green alga Volvox africanus. The Thai and Japanese algae harbored expanded ances-tral male and female SDRs of -1 Mbp each, representing a direct heterothallic ancestor. Therefore, the expanded male and female ancestral SDRs may origi-nate from the ancient (-75 mya) heterothallic ancestor, and either might have been conserved during the evolution of each homothallic genotype. An expanded SDR-like region seems essential for homothallic sexual reproduction in V. africanus, irrespective of male or female origin. Our study stimulates future studies to elucidate the biological significance of such expanded genomic regions.

Automated summary

By comparing genome data from Thai and Japanese genotypes of the homothallic green alga Volvox africanus, it was found that expanded ancestral male and female sex-determining regions (SDRs) may have originated from an ancient heterothallic ancestor and were conserved during the evolution of homothallic genotypes. An expanded SDR-like region appears to be essential for homothallic sexual reproduction in V. africanus, regardless of its male or female origin. This study highlights the future possibility of elucidating the biological significance of these expanded genomic regions.