Sci. Aging Knowl. Environ., 5 March 2003
Something for Almost Nothing
Longevity's price is negotiable
Key Words: tradeoffs fecundity
When it comes to longevity-conferring mutations, no one rides for free--but researchers have found a cheap ticket. In new work, they extend fruit fly life span without undermining health--as long as the flies' food keeps coming. The study's approach of challenging the animals with taxing conditions is uncommon among researchers seeking mutations that prolong life. The results raise the possibility that humans, like flies, might eventually be able to live extra-long with minimal sacrifices.
In the arena of natural selection, living longer doesn't matter, only having the ability to pass along DNA. According to evolutionary biologists, mutations that lengthen life suppress reproduction; if they didn't, they would spread through the natural population (see "Get Wild"). But the all-you-can-eat laboratory lifestyle can mask negative effects. Three years ago, molecular geneticist Gordon Lithgow, now at the Buck Institute for Age Research in Novato, California, and colleagues showed that long-lived, apparently healthy mutant worms stood their ground against normal animals--under comfortable conditions. When the food supply fluctuated, however, the mutants perished. The results suggested that when food abounds, longevity can be uncoupled from traits that compromise evolutionary fitness.
The new work offers a second example of this phenomenon. Stephen Helfand, a molecular geneticist at the University of Connecticut Health Center in Farmington, and colleagues previously identified a gene, Indy, that nearly doubles fruit fly life span. Now they've found that the mutants match normal insects in tests of flight speed, resting metabolic rate, and number of eggs laid as long as chow is plentiful. Indy flies on a regime of diluted food, however, produced fewer than one-fifth the usual number of eggs, whereas normal flies managed to eke out up to half as many eggs as they typically laid. "We show that there is some set of physiological conditions where you really can 'have it all' in terms of long life at a high quality," says evolutionary biologist and co-author James Marden of Pennsylvania State University, University Park.
Steven Austad, an evolutionary biologist at the University of Idaho in Moscow, agrees that a favorable environment can eliminate the costs of longevity, but he adds that forcing the long-lived mutants to compete with normal flies would better establish how Indy flies compare. "As it is, you only know about the fitness components ... that they happened to test," he says. Even so, the study's design draws praise for pushing beyond cushy laboratory conditions. "Those few people who look for costs [of longevity mutations] don't usually find them because they're looking in benign environments," says Daniel Promislow, an evolutionary biologist at the University of Georgia in Athens.
In many places, people, like lab animals, enjoy abundant food. However, the paper reinforces the notion that "we don't know what the costs are going to be in the real world," Promislow says, making genetic enhancement of human life span risky. Marden says that he'd like to precisely define the conditions under which the long-lived mutants flourish. In the meantime, the work shows that controlling one's environment might slash the price of a ticket to longer life.
March 5, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150