Discovery of new gene reveals sex determination mechanism in green algae

What is it about?

We identified a new gene, named VSR1, that plays a vital role in the activation of genes specific to the development of female and male reproductive cells. This discovery enables us for the first time to model how sex is determined in green algae, Volvocines are a group of green algae which include two well-studied members: Chlamydomonas, a single-celled species, and Volvox a multicellular species. All volvocines, and most other green algae, can reproduce asexually; but under specific conditions they undergo sexual development and differentiate as either plus and minus gametes for Chlamydomonas, or eggs and sperm for Volvox. Until this study only part of the sexual differentiation process was understood. Much like the Y chromosome gene SRY acts as a male determinant in humans and other mammals, a sex-linked gene called MID in volvocine algae causes sperm development in Volvox or minus gamete differentiation in Chlamydomonas. What has remained mysterious until now was how the default pattern of plus or female gamete differentiation occurs when MID is absent. Using phylo-transcriptomics—a method to simultaneously compare evolutionary relationships and gene expression—the team discovered a new gene called Volvocine Sex Regulator 1 (VSR1) that activates the development of plus gametes in Chlamydomonas or oogenesis (egg formation) in Volvox. VSR1 is also important for minus gamete and male development. When MID is present it interacts with VSR1 and modifies it to become an activator of minus or male gamete development.

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Why is it important?

Besides the breakthrough for understanding sex determination in volvocine algae, the new findings may extend to other groups of green algae where lack of information about sexual cycles has precluded selective breeding as a mechanism of strain improvement. Remarkably, in every other green algal group where there is some piece of information about sexual differentiation, genes in the same family as VSR1 and MID are implicated, and this even holds true in plants which share a common ancestor with the green algae. The results in this study help tie together an emerging story where VSR1- and MID-like genes may have been involved in sexual differentiation at the very base of the lineage that gave rise to plants and algae, and searching for similar gene pairs in other species may help unveil their sex determination mechanisms as well.

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