High rates of male infertility are a source of frustration for obvious baby-making reasons, but the subject also confuses scientists because it makes little evolutionary sense. If up to 15 percent of couples are infertile, then natural selection should have ensured that the specific gene mutations behind infertility would eventually die. In other words, if the only fathers carrying this trait cannot pass it onto their offspring, relatively few men should be born infertile.
Unless moms are keeping these mutations alive. Indeed, one 2014 study suggested that women unwittingly harbor genetic mutations that can cause infertility in men but leave women unscathed. Now the same team behind that study has uncovered an additional cache of 6,500 genes that exist in both men and women, but with vastly different effects.
“The basic genome is nearly the same in all of us, but it is utilized differently across the body and among individuals,” coauthor Moran Gershoni of the Weizmann Institute of Science in Israel told the Weizmann Wonder Wander. “Thus, when it comes to the differences between the sexes, we see that evolution often works on the level of gene expression.”
For the study, Gershoni and colleagues analyzed the RNA sequences of 544 adults and mapped them across 8,555 tissue samples. They then identified about 6,500 genes that, in at least one tissue, were expressed differently in men than in women. Certain genes responsible for the growth of body hair, for instance, were present in all skin samples but more highly expressed in men. Other genes responsible for fat storage were ubiquitous but expressed most strongly in women. Perhaps predictably, the largest percentage of sex-differentiated genes were found in the mammary tissue samples—a finding that may explain why dude nipples are more decorative than functional.
Gershoni and colleagues also found that natural selection is less likely to weed out harmful mutations of sex-specific genes—and that sex-specific genes that express strongest in men were the most vulnerable to unchecked mutation. “The more a gene was specific to one sex, the less selection we saw on the gene,” Gershoni wrote in the press release. “This selection was even weaker with men.” This may help explain why male infertility genes carried by mothers continue to cause problems for their sons, despite the failsafes of natural selection.
As for why the same genes are more likely to be mutated in men than in women, Gershoni has another theory: “In many species, females can produce only a limited number of offspring while males can, theoretically, father many more,” Gershoni explained. “So the species’ survival will depend on more viable females in the population than males. Thus natural selection can be more ‘lax’ with the genes that are only harmful to males.” Tough break, guys.