New Life for Old Theory
Authors: Davenport R. John 
Published: August 2003

Evolutionary theory holds that mothers who delay reproduction live longer because they focus their resources on survival rather than procreation. But how their offspring fare remains mysterious. New research resurrects a decades-old theory that old mothers produce less-fit offspring. The study could help scientists zero in on the genes involved and figure out whether the same holds true for humans.

In the 1940s, Albert Lansing's research on small, multicellular organisms called rotifers suggested that older mothers bear shorter lived offspring than do younger mothers. But because the experiments included a small number of organisms that die easily and because other studies generated contradictory data, biologists questioned whether the so-called Lansing effect existed. Evolutionary theories of aging developed in the 1960s predicted that old mothers should instead produce persistent offspring, because of fitness genes they inherit from mom. Experiments bore out that idea. When scientists bred flies for many generations, choosing offspring from only the oldest mothers to populate the subsequent generation, average life-span increased. "The evolutionary folks really wrote [Lansing] off," says evolutionary biologist Daniel Promislow of the University of Georgia in Athens, a co-author of the paper. But such selection experiments don't rule out the Lansing effect, because they monitor the influence of genetic changes over many generations rather than the consequence of parental age in a single generation.

In the new work, Priest and colleagues designed a fly-breeding experiment to test for the Lansing effect. They took female
Drosophila melanogaster
of different ages, bred them to males of a single age, and tallied the life-span of the first-generation offspring. In five of the six genetic lines tested, female offspring died younger when they came from older mothers, confirming the Lansing effect; male flies didn't show a clear trend. In the strains that exhibited the Lansing effect, maternal age shortened life-span to different degrees. The variability suggests that genetic differences could alter the magnitude of the maternal age effect, says Promislow, and natural selection acting on these differences might steer the evolution of life-span.

Although maternal age altered the longevity of offspring, paternal maturity had little impact. The difference between the sexes suggests that accumulation of mutations isn't to blame for the increase in mortality with maternal age, says evolutionary geneticist Kim Hughes of the University of Illinois, Urbana-Champaign, because both males and females would amass those changes. Rather, she suggests that old mothers might lose the ability to nurture eggs.

Because not all the strains followed the pattern, evolutionary biologist Michael Rose of the University of California, Irvine, questions whether the study reveals a general maternal-age effect. Other evolutionists are encouraged, however. The work "brings the [Lansing effect] back into the mainstream," says Hughes, a shift that other recent studies also support. Promislow says he'd like to pinpoint genes that enhance or weaken the maternal age effect in flies and then look for those genes in humans. Such a quest could reveal whether maternal age influences human longevity. Regardless of which child mothers prefer, mother nature favors the firstborn.

--R. John Davenport; suggested by Donna Holmes

N. K. Priest, B. Mackowiak, D. E. Promislow, The role of parental age effects on the evolution of aging.
Evolution
56
, 927-935 (2002).
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