Green algae show the genetic origin of the two sexes

A female Eudorina colony with 32 large egg cells (Image courtesy Hiroko Kawai-Toyooka, University of Tokyo. All rights reserved)
A female Eudorina colony with 32 large egg cells (Image courtesy Hiroko Kawai-Toyooka, University of Tokyo. All rights reserved)

An article published in the journal “Communications Biology” describes a research that offers new clues about the genetic origin of the two sexes. A team of researchers led by Dr. Hisayoshi Nozaki of the University of Tokyo studied the volvocine green algae, a group of algae that includes species that have been studied for years in the research on this topic. In this case the analyzes focused on species of the Yamagishiella and Eudorina genera, discovering a key gene in the differentiation between males and females.

Cells that merge together in the fertilization between sexed organisms are called gametes and there may be differences in size between male and female. The difference in size is called anisogamy and emerged in various lineages of eukaryotic organisms in which this difference was not there, a situation called isogamy. The genetic bases of that event are still unknown.

Various species of green algae have been studied for years to try to understand the genetic evolution of the two sexes, in this research two species were chosen, one belonging to the Yamagishiella genus characterized by isogamy and one belonging to the Eudorina genus characterized by anisogamy. The other characteristics of the two species studied are very similar, making them ideal for a comparison.

A genetic analysis of the DNA of the green algae studied provided surprising results. According to theoretical predictions, in the course of the evolution of the two sexes there was a growth in genetic complexity with a gene increase. Instead, Eudorina has the simplest sex-determining genes discovered so far among all the species of the volvocine group. The great difference between males and females is in fact determined by the presence or absence of a single gene called MID that resides in a tiny chromosomal region.

Probably the evolution of sex differentiation in those green algae began around that gene rather than with the acquisition of new genes. A greater complexity and therefore anisogamy emerged only later in some genera of green algae. For this reason, further investigations around this gene and the mechanisms that developed around it will offer new information on the evolution of mating in eukaryotes.

This type of research helps to shed light on the evolution of the two sexes but also has a very practical consequence. A better knowledge of the characteristics of these green algae helps to identify sexes in the species that can be bred as food, for the production of biofuels or for other biotechnology applications under development.

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