Sci. Aging Knowl. Environ., 28 May 2003
Mother-eating offspring might shape sex differences in worm longevity
Key Words: dioecious androdioecious pleiotropy mutation accumulation
Talk about ungrateful children. Nematode youngsters often gobble their mother from within--a behavior known as "bagging" because all that remains of Mom is a husk. Such cannibalism might propel the evolution of sex differences in worm life span, according to a new study. Curtailing maternal longevity favors speedy aging in mothers, the authors conclude.
Men die on average 5 years before women, but the opposite is true in the nematode Caenorhabditis elegans. Males outlive hermaphrodites (the species lacks females) by about 20% in the common lab strain. Geneticist David Gems of University College London suspected that the disparity evolved because hermaphrodites produce limited numbers of sperm, which run out. However, males face no such limitation, and they can sire progeny (by mating with hermaphrodites) later in life. Because males retain their fertility longer, Gems hypothesized, natural selection would favor durable males, and they would age more slowly than hermaphrodites would. If so, the male survival advantage would hold only for hermaphroditic species such as C. elegans, not for species with the standard sexes, which gave Gems and colleague Diana McCulloch a way to test the idea.
They compared life span in eight worm species, four composed of males and females, and four composed of males and hermaphrodites. Defying expectations, males outlived the other sex in seven of the species.
The evolutionary theory of aging (see "Aging Research Grows Up") suggests an explanation for why females and hermaphrodites are feeble, Gems says: The sex with a higher rate of "extrinsic mortality"--death from nonaging causes such as predation--will age faster. Because the creatures usually die young from these extrinsic causes, natural selection doesn't favor the construction of a sturdy body. Females and hermaphrodites suffer high mortality because their ravenous offspring eat them alive. When food is scarce, bagging can kill all mothers in some species, the researchers determined. This high mortality rate drives the evolution of sex differences in longevity, Gems suggests. However, the story has another twist. The male survival advantage was larger in male-female species than in the male-hermaphrodite species. Gems reckons that the difference stems from the rarity of males in hermaphroditic species--they are usually less than 1% of the population. Thus, natural selection gets few chances to prune harmful genes that spur aging in males, he says: "The results suggest that very different forces can act on each sex to mold its pattern of aging."
"It's a big contribution because male-female comparisons have been neglected in aging biology," says demographer James Carey of the University of California, Davis. Evolutionary biologist Daniel Promislow of the University of Georgia, Athens, shares Carey's enthusiasm. However, he finds the argument that bagging drives female longevity weak, noting that the degree of cannibalism doesn't correlate with the size of the male life span advantage. Further studies need to probe worm reproduction in the wild before we can understand the origins of sex differences, they say. The results might reveal whether young worms are born ungrateful or are driven to it by their upbringing.
May 28, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150