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Sex Change: Algae May Point to Origin of Male-Female Split

An unassuming little alga may hold the secret to how the sexes evolved.

A single gene that determines male or female sex in multicellular algae evolved from a more primitive version found in a single-celled ancestor that doesn't have sexes, according to a new study.

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Many scientists think the sexes in multicellular organisms originated from two different "mating types" of unicellular organisms, said study co-author James Umen, an evolutionary biologist at the Donald Danforth Plant Science Center. But it's been hard to find the evidence. Animals and plants have been having sex, with distinct male and female sexes, for between 500 million and 1 billion years, so the traces of that first step are long gone.

Fortunately, there's a species of multicellular algae known as Volvox carteri that apparently evolved sexes from a single-celled ancestor, Chlamydomonas reinhardtii, in just the past few hundred million years. [Image Gallery: Stunning Dual-Sex Animals]

The team focused on a single gene, dubbed MID, which is present in the "plus" mating type of C. reinhardtii but absent in the "minus" mating type. In V. carteri, males have a version of the MID gene, but females don't. The two versions of the gene are similar. Based on subtle differences in the genetic sequence, the team concluded that the MID gene in V. carteri evolved from the one in C. reinhardtii.

Scientists inserted the MID gene into female V. carteri organisms and found that the females' eggs turned into sperm packets. Reducing the expression of the gene in males turned their sperm packets into eggs. "By manipulating this one gene, we can essentially give Volvox a sex change," Umen said.

The findings, published Tuesday in PLOS Biology, suggest that the evolution of the sexes may have started with a small change in a mating-type gene.

Update for 5:25 p.m. July 10: James Umen is an associate member at the Danforth Center's Enterprise Rent-a-Car Institute for Renewable Fuels, as well as an adjunct professor at the Washington University in St. Louis. His connection to the Danforth Center was not included in an earlier version of this report.

This is a condensed version of a report from LiveScience. Read the full report. Follow Tia Ghose on Twitter and Google+. Follow LiveScience on Twitter, Facebook andGoogle+.