Transgender Algae Shows how Males and Females Evolved Separately

Using a certain type of "transgender" green algae, scientists have discovered the genetic origin of male and female sexes, showing how they evolved from a more primitive mating system into a single-celled relative.

Led by James Umen of the Enterprise Institute for Renewable Fuels at the Danforth Plant Science Center, the research team identified the master regulatory gene for sex determination in the multicellular alga Volvox carteri.

They discovered that it has acquired new functions compared to a related gene in its close relative, the unicellular alga Chlamydomonas reinhardtii, which does not have physically distinguishable sexes.

The findings are published in the journal PLOS Biology and may provide a possible blueprint for how sexes in other multicellular organisms like plants and animals may have originated.

For plants and animals, having distinguishable male and female reproductive cells, or gametes, is the norm. Male gametes are small motile sperm or pollen, while female gametes are large egg cells.

However, the evolutionary origin of these separate sexes is muddled because the distant unicellular relatives of plants, animals and other multicellular species ordinarily do not have distinct sexes. Instead, they have mating types - a system in which gametes of one mating type can only fuse with those with a different mating type, but the cells of each mating type are indistinguishable from each other in size and morphology.

Typically these unicellular ancestors are very distantly related to today's plants and animals, but in Volvox males and females evolved relatively recently.

During a previous study, Umen and his colleagues had identified a gene in Volvox males called MID, whose counterpart in Chlamydomonas was known to control differentiation of its two mating types.

By forcing genetically female Volvox to express MID, and conversely blocking MID expression in genetic males, the researchers were able to convert what would have been egg cells into packets of functional sperm cells in females, and vice versa for males.

They were even able to successful mate some of the genetically altered males and females.

The discovery of the master regulatory gene for sexes and mating types in this group of green algae shows that these two forms of reproduction share a common genetic origin, and hint that a similar evolutionary scenario may underlie the origin of sexes in animals, plants and other multicellular organisms.

"The identification of a conserved regulatory gene that controls sex and mating in the algae may lead to clues about how sex is controlled in other related groups of algae that are used for biotechnological applications," Umen said in a statement.