Genes that help one creature live longer could hasten the death of another. A new study shows that genetic variations that confer longevity in some fruit flies often turn out to have adverse effects on flies of the opposite sex or those in a different environment.
Researchers have found mutant forms of genes in flies and worms that lengthen the animals' lifespans, and hope to identify similar life-extending genes in people. But whether certain genes can consistently lengthen lives is a matter of debate. Studies in several organisms have shown that gender and environment affect thebehaviour of genes.
In an attempt to gauge the extent of these effects, Trudy Mackay at North Carolina State University in Raleigh and her colleagues recorded the lifespans of inbred male and female flies raised on different diets at different temperatures. Using genetic mapping techniques, they then searched for genetic variations that correlated with long lifespans.
Each of the 17 variations that Mackay's team found increased longevity only in certain environments, or in a sex-dependent manner, or both. Ten variations that made flies live longer under some conditions made flies die sooner under others. One variation, for example, made females live longer if they experienced a hot spell early in life but die sooner if they lived at constant room temperature, with no effect on males.
"What's really exciting about the study is its relevance to the evolution of ageing," says John Tower, an expert on fruit fly ageing at the University of Southern California. It highlights the fact that mutations that decrease lifespan in some situations are nonetheless selected during evolution because they prove beneficial under other circumstances.
James Curtsinger at the University of Minnesota in St Paul recently performed similar studies, however, and found that environment or gender had virtually no effect on longevity-associated genetic variations. Although he describes the Mackay team as masters of fruit fly genome mapping, he believes their inbred flies, derived from genetically unrelated stocks, might have genomes that are unusually sensitive to the environment. "I think there's a serious limitation to this study they do not fully appreciate," he says.
Mackay concedes that her lab-bred flies are genetically different to those found in the wild, and that she needs to analyse other flies. But she points out that studies of other animals, including humans, back the idea that environment and gender alter the effects of genes. For example, geneticists at the University of Calabria in Italy have discovered a genetic variation that seems to make men, but not women, live longer.
If Mackay's studies prove generally applicable, they might help explain other contradictory genetic findings. "Many association studies have been done for candidate genes affecting characters like alcoholism and Alzheimer's disease," says Mackay. "Different researchers get different answers. One might wonder if the environment accounts for some of these disparities."
Author: Marina Chicurel
New Scientist issue 29th January 2000
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Source: Genetics (vol 154, p 213)