Animal sperm arose once

Gene for male reproductive cells traced back 600 million years

The seeds of human reproduction were sown some 600 million years ago, scientists have found. A sperm-producing gene that appeared at the dawn of animal evolution is present in nearly every animal, from sea anemones to people, suggesting that the male reproductive cell evolved from a common ancestor.

The finding may offer clues about male infertility, and could lead to male contraceptive drugs and even new pesticides.

Until now, scientists weren’t sure if sperm production evolved several times in different animal lineages, or if a single ancestor started the ball rolling. Most animals’ sperm go through strikingly similar developmental stages, suggesting a common origin. Yet no one had found a reproductive gene common to all animals.

Now a study published July 15 in PLoS Genetics suggests that a gene called BOULE arose 600 million years ago and has been crucial to sperm production ever since.

“This is the first human sperm gene that has been found functionally conserved in insects and in mammals,” says geneticist and study coauthor Eugene Xu of the Northwestern University Feinberg School of Medicine in Chicago.

Xu’s team knew that 10 to 15 percent of infertile men lack a gene called DAZ, and that two ancestral forms of that gene — BOULE and DAZL — are needed for fertility in a few other species. But the researchers didn’t know if they’d find similar genes in other animals, or whether those genes would also produce sperm.

So the team searched through a database containing genetic information on a broad spectrum of organisms, looking for key gene sequences that are common to BOULE, DAZL and DAZ. Fungi and plants lacked these genes, as did really ancient animals, such as sponges. But all other animal lineages from cnidarians and mollusks to fish, bird and primates had some form of the gene.

“This is strong evidence that the Boule gene is only required for sperm production in insects and in mammals, which diverged 400-600 million years ago,” Xu says.

A KEY GENE Mice with the intact BOULE gene have mature sperm in their reproductive tracts (left). But for mice with a disrupted BOULE gene, sperm development halts with the production of precursor cells (right). Courtesy of Eugene Xu

COMMON SOURCE Genes involved in the production of sperm in humans, mice and fruit flies all trace back to a common ancestor that existed about 600 million years ago, new research shows. Courtesy of Eugene Xu

Next, the researchers tested mice to be sure the gene was in charge of just sperm production, not more general cell processes. Sure enough, theprotein encoded by the gene was found only in mouse testes. And if the BOULE gene was disrupted, otherwise healthy mice didn’t produce sperm.

“It’s a significant finding in the field,” says evolutionary biologist Timothy Karr of the Arizona State University Biodesign Institute in Tempe. The next step is to confirm that the BOULE protein functions the same way in other animals, he says. “Proteins can assume different functions, and it’s tricky to assume that a gene in one lineage is doing the same thing in another.”

Xu says his team will next look in mice and fruit flies to see exactly where the BOULE protein shows up and what it does.

With more study, researchers might be able to target the DAZ protein with male birth control drugs, or kill pests by disrupting the insect form of the protein. And now that researchers know sperm genes are similar among animals, experts can study human infertility more easily in animal models.

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