A tiny variety of algae may yield clues to how separate sexes evolved, showing how one single gene was involved in the origin of distinct female and male sexes, researchers say.
That single gene in multicellular green algae known as Volvox carteri apparently evolved out of a more primitive, "sexless" mating process in a single-celled algae relative, they say.
The regulatory gene that provides for a determination of sex in V. carteri has evolved functions not found in a similar gene in a closely related unicellular algae variety known as Chlamydomonas reinhardtii, which doesn't display physically distinguishable sexes, the researchers report in the journal PLOS Biology.
Sexual reproduction evolved in tandem with multicellular life, the researchers say; while single-celled organisms normally reproduce by just dividing into two new organisms, multicellular organisms are unable to do that and must reproduce by sexual means.
Biologists have long assumed evolution from modest cell division to the more complex processes of sexual reproduction must have taken a long and complicated route.
"Much to our surprise, one single gene had evolved the capacity to affect the male/female difference," says study lead author James Umen, a biologist at Washington University in St. Louis.
To test the effect of the single gene, known as the MID gene, the researchers experimented with adding or removing the gene in V. carteri algae.
In the multicellular variety of algae, males have a version of the MID gene, while females don't.
When the gene was inserted into female V. carteri, their eggs suddenly changed into sperm packets. Similarly, when the MID gene's expression in males was suppressed, their sperm packets turned into eggs.
"By manipulating this one gene we can essentially give Volvox a sex change," Umen says, demonstrating the sex determination effect of that single gene.
The results suggest the evolution of the sexes may have begun with just a small change in a single gene involved in reproduction, the researchers say, with it acting as the "master switch" to regulate all genes involved in sex differentiation.
Other genetic researchers said the Washington University study is a ground-breaking one.
"It was surprising that all of this could be controlled by one gene," says Matthew Herron, an expert on Volvox alae doing postdoctoral research at the University of Montana. "Their work was exceptionally thorough."
Determining how different sexes developed in the kinds of organisms in the study could lead to better understanding of the evolution path of distinct sexes taken throughout all life, the Washington University researchers said.
