In several species, the presence of males decreases the lifespan of the opposite sex. Why this should be isn’t entirely clear. In the case of the common laboratory roundworm, known as Caenorhabditis elegans, several causes of “male-induced demise” have been considered. Simple crowding is a possibility, as is physical damage due to the rigors of mating. These abuses, however, may not account for the male roundworm’s ability to dispose of his opposite number, the roundworm hermaphrodite. In fact, recent research indicates that the male roundworm is a subtle killer.
At the Stanford University School of Medicine, researchers have discovered that males of the laboratory roundworm secrete signaling molecules that significantly shorten the lifespan of the opposite sex. The scientists speculate that, if carried out after reproduction, this male-induced demise could serve to conserve precious resources for a male’s offspring or to decrease the supply of mates for other males.
The researchers published their results November 28 in Science, in a paper entitled “Males Shorten the Life Span of C. elegans Hermaphrodites via Secreted Compounds.” In their paper, the authors write: “The male-induced demise could occur without mating and required only exposure of hermaphrodites to medium in which males were once present. Such communication through pheromones or other diffusible substances points to a nonindividual autonomous mode of aging regulation.”
The continuous presence of young males shortened the average lifespan of C. elegans hermaphrodites by more than 20%. This effect persisted even when the genders were prevented from co-mingling, or when the hermaphrodites were sterile. Affected hermaphrodites also displayed symptoms of aging including slower movement, an increased incidence of paralysis, general decrepitude, and structural decline.
These deleterious effects were less marked, however, in certain circumstances. “Males that are deficient in pheromone production no longer induce a strong premature demise of hermaphrodites,” observed the study’s senior author Anne Brunet, Ph.D. an associate professor of genetics. “And hermaphrodites that cannot sense pheromones are resistant to male-induced demise.”
The pheromones involved have yet to be identified, and it is still unclear how or when they are secreted. Nonetheless, pheromones do seem to be at play. Their effects became evident when the researchers investigated the gene expression profiles of the affected hermaphrodites.
In particular, the scientists observed large changes in hermaphrodite gene expression that occurred only in the presence of males. Many of these changes affected genes expressed in neurons or involved in neurodegenerative diseases. Blocking the expression of one gene in particular, an insulin-like peptide known as INS-11, specifically impeded male-induced demise.
“We’ve found that males induce the expression of a large number of genes involved in sensation and signaling in hermaphrodites,” said Dr. Brunet. “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.” The harm experienced by nonmating hermaphrodites could be due to seminal fluid secreted by males that attempting unsuccessfully to mate with other, nearby males.
Musing on the potential adaptive benefits of male-induced demise, Dr. Brunet remarked that it would be “interesting, of course, to determine whether males also affect the lifespan of females in other species, particularly mammals.”
