Scientists Explain Why Males’ Company Can Be Deadly For Females In Some Species

The mere presence of male roundworms can significantly shorten the lifespan of their female counterparts, say researchers at the Stanford University School of Medicine in California.

The researchers, who discovered this characteristic in laboratory roundworms, said if carried out after reproduction, this "male-induced demise" could be useful in conserving precious resources for a male's offspring or to decrease the supply of mates for male competitors.

It was known already that the company of some male worms and flies can be deadly for their female or hermaphroditic counterparts. While there has been no clear-cut explanation, some researchers have speculated that the physical stress of mating could cause their early death.

But the new study suggests that sex is not only to be blamed here. It seems males also carry out a calculated plan at the molecular level to kill off the baby-makers once they are done with their jobs.

“We've found that males induce the expression of a large number of genes involved in sensation and signaling in hermaphrodites,” Anne Brunet, the university associate professor of genetics and the study’s lead author, said in a statement. “This raises the possibility that the male-induced demise is not just due to the physical stress of copulation but instead involves some degree of active signaling. Indeed, we found that just placing hermaphrodites on plates where males had previously been present was sufficient to induce the premature demise of hermaphrodites.”

The researchers examined the common laboratory roundworm, known as Caenorhabditis elegans, which is one millimeter long and translucent. These creatures generally live for about 20 days, and their normal population consists of about 0.01 to 0.1 percent males with the remainder consisting of hermaphrodites that have both male and female reproductive organs.

The study, published Thursday in Science Express, showed that male worms can initiate the killing process even across distances. The continuous presence of young males can shorten the average lifespan of C. elegans hermaphrodites by more than 20 percent.

The researchers said the effect persisted even when the genders were prevented from co-mingling, or when the hermaphrodites were sterile. In addition, affected hermaphrodites also displayed premature symptoms of aging, such as slower movement, an increased incidence of paralysis, general decrepitude and structural decline.

The researchers placed hermaphrodites on laboratory dishes that had formerly contained male worms. They found that the hermaphrodites exhibited a shortened lifespan, indicating that the males had left behind some substance that was harming them.

“The observation that this male-induced demise is present in several species of worms and has also been shown in flies suggests that it could have some adaptive benefits,” Brunet said. “It will be interesting, of course, to determine whether males also affect the lifespan of females in other species, particularly mammals.”