Study details genetic origins of male-female differentiation

[Japan] How did male-female differentiation first evolve? A new study of a unique group of algae offers clues.

Genomic analysis of plants and animals has previously revealed a widening divergence among male and female sex chromosomes early during the evolution of multicellular organisms. But scientists have struggled to pinpoint exactly when distinct sperm and egg cell types first emerged on the evolutionary timeline.

In search of new insights, scientists led by the University of Tokyo’s Dr. Hisayoshi Nozaki investigated two species of volvocine green algae. Both Yamagishiella and Eudorina algal colonies feature 32 cells.

But Yamagishiella algae is isogamous, forming gametes of a similar shape and size. Eudorina algae forms differentiated gametes, a smaller male cell and a larger female cell.

Scientists sequenced the chromosomal regions that control gamete formation in each species and compared the results.

Researchers have previously hypothesized that an increase in genetic complexity would reveal the emergency of sex-differentiation, but the latest findings — detailed in the journal Communications Biology — showed the opposite.

Scientists found the chromosomal region responsible for sex-determination in Eudorina algae was the smallest and simplest found among volvocine algae species. The difference between the chromosomal regions in the two studied species wasn’t genetic complexity, researchers showed, but a single gene called MID.

“This new study punches a hole in the idea that increased genetic complexity of sex chromosomes accompanied the origin of sexes,” researcher James Umen said in a news release. “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.”


Photo: A colony of smaller male Eudorina algae cells has found a female Eudorina colony featuring 32 large egg cells. Photo by Hiroko Kawai-Toyooka/University of Tokyo.

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