A new study published in the journal Communications Biology has shed light on the earliest stages in the evolution of male-female differentiation and sex chromosomes--and found the genetic origins of the two sexes to be unexpectedly modest.
The research team focused on two especially informative and closely-related multicellular volvocine species from the genera Yamagishiella and Eudorina which bracket the transition from isogamy to male/female sexes. While 32-celled Yamagishiella and Eudorina colonies look very similar to each other, the former is isogamous while the latter produces small male gametes and large female gametes. The team used high-throughput genome sequencing of the chromosomal regions that specify mating type in Yamagishiella and male-female differentiation in Eudorina, and then compared these regions.
While evolutionary theory predicted an expansion and/or increased genetic complexity of the sex determining region associated with the evolution of sexes in Eudorina, the results of the study showed the opposite, with Eudorina having the most diminutive and genetically least complex sex-determining region found to date found among all volvocine species. In essence, the major difference between males and females in Eudorina could be reduced to the presence or absence of a single gene called MID that resides in a tiny chromosomal region.
"This new study punches a hole in the idea that increased genetic complexity of sex chromosomes accompanied the origin of sexes," said Umen. "Moreover, the work also has practical implications since it expands our understanding of how to identify mating types and sexes in new species of algae that we might want to breed as crops for improved traits relating to biofuel or biotechnology applications."