Why Does The Mere Presence Of Males Shorten The Lives Of Females In Some Animal Species?
Scientists have known for years that the presence of male worms and flies can shorten the lifespan of female or hermaphroditic counterparts, but they had no clue why. The theory has been that physical stress of mating caused their early death. However, a recent study out of the Stanford University School of Medicine and published in Science Express may put this, and other theories to rest.
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According to the Stanford researchers, males act out a calculated code at the molecular level to control the lives of females. specifically, male roundworms secrete signaling molecules that drastically curtails the life of the opposite sex. When the baby-making females have done their jobs, their utility is over. The common laboratory roundworm, known as Caenorhabditis elegans, or C. elegans, subjected to the study is a 1-millimeter-long, translucent worm with a life span of 20 days. The male population comprises about 0.01 to 0.1 percent, while the rest are hermaphrodites, with both male and female reproductive organs. The hermaphrodites can produce more offspring, if they mate with a male.
According to Dr. Anne Brunet, associate professor of genetics and senior author of the study, physical stress of copulation is a weak explanation for the male-induced demise, while a degree of active signaling is involved in the death of females. Premature demise of hermaphrodites was observed to take place, according to Brunet, after "just placing hermaphrodites on plates where males had previously been present."
The study will not probably be applicable to mammals or humans because mothers/parents are needed to rear the young. "In worms, once the male has mated and eggs are produced, the hermaphrodite mother can be discarded," Brunet said. "The C. elegans mother is not needed to care for the baby worms."
The life of female C. elegans worms was found shortened by over 20 percent in the continuous presence of young males, according to researchers. In addition, this effect was evident even when gendered commingling was prevented. The researchers used sterile hermaphrodites to create a control group, eliminating the possibility that stress of copulation or energy demands could be linked to the cause of death. The affected hermaphrodites showed symptoms of aging and structural decline, even without mating
Another finding that proves the point is that males deficient in pheromone production could not induce premature demise of hermaphrodites "and hermaphrodites that cannot sense pheromones are resistant to male-induced demise," according to Brunet. The researchers could observe large changes in hermaphrodite gene expression taking place only in the presence of males. The changes impacted genes involved in the expression of certain neurons and genes involved in neurodegenerative diseases. The male-induced demise could be impeded with blocking of one identified gene, an insulin-like peptide known as INS-11.
The researchers were able to observe male-induced demise in a domesticated as well as a wild strain of C. elegans, leading to the conclusion that the phenomenon has been conserved over 20 to 30 million years of evolution.
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