The sex genes are some of the fastest-changing throughout animal genomes. But one particular sex gene is so perfectly evolved that it hasn't changed in 600 million years: the sperm gene.
The gene, dubbed Boule, is found in everything from flies to roosters to sea anemones to trout to humans. As far as its discoverers can tell, it's the only gene that is exclusively required for sperm production throughout the animal kingdom.
Eugene Xu, assistant professor of obstetrics and gynecology* at the Northwestern medical school, explains what his team's findings mean:
"Our findings…show that humans, despite how complex we are, across the evolutionary lines all the way to flies, which are very simple, still have one fundamental element that's shared. It's really surprising because sperm production gets pounded by natural selection. It tends to change due to strong selective pressures for sperm-specific genes to evolve. There is extra pressure to be a super male to improve reproductive success. This is the one sex-specific element that didn't change across species. This must be so important that it can't change."
Boule is almost certainly the oldest human sperm-specific ever found or ever will be found. The confirmation of its existence across all animals settles the debate as to how sperm evolved in different animals. Although sperm in different animals seems nearly identical, that in itself wasn't proof of a common origin – for instance, birds and insects both fly, but their evolutionary mechanisms that allow them to do so are completely unrelated.
Because the gene is exclusively involved in sperm production, it's a great target for the development of new male contraceptives. It might also help treat male infertility. And Xu sees benefits beyond the world of human sex:
"We now have one strong candidate to target for controlling [mosquito and parasite] breeding. Our work suggests that disrupting the function of Boule in animals most likely will disrupt their breeding and put the threatening parasites or germs under control. This could represent a new direction in our future development of pesticides or medicine against infectious parasites or carriers of germs."
In order to prove the universality of Boule, Xu had to go to some pretty extreme lengths. Though it was easy to get rooster and sea urchin sperm, rainbow trout proved elusive. The fish he bought at a Chicago market came pre-gutted, leaving it without the crucial parts Xu needed. He says he yelled, "I need the testicles!" to the fishmonger and decided he would just have to catch his own trout. Fortunately, his fishing expedition was not in vain – he found Boule in the trout, and indeed in every other animal from the primitive sea anemone to the slightly less primitive human male.
*Yes, we realize it doesn't technically make sense that an obstetrician/gynecologist would spend all his time looking at sperm. That's the wonder and the mystery of science for you.