Sexual isolation with a single mutation

1994

Sexual isolation in one easy step

A simple genetic change in flies' pheromones may be all that's needed to spin off a new species.

The powerful chemical attractants that help animals find a mate or pick a mate of the same species from a crowd of similar organisms can change very easily, causing the sudden appearance of new, distinct species through reproductive isolation, University of Chicago researchers have found.

To the untrained eye, all fruit flies look alike. Indeed, sometimes the flies themselves may be fooled, so they rely on chemical identification. Jerry Coyne, PhD, professor of ecology and evolution, and his co-workers analyzed and even swapped the mate-attracting fragrances among four closely related species of Drosophila and showed that these pheromones are crucial for male suitors to recognize females of the same species. They also found that the genetic basis of pheromone differences is very simple and easily changed, refuting a central tenet of evolutionary theory--that new species only arise through the accumulation of many genetic changes, each of small effect. The finding is reported in today's issue of the journal Science.

Biologists have long puzzled over how a species originates and keeps separate from similar organisms nearby. "Why is there no intermediate form of bird between a robin and a cardinal?" Coyne asks. "It's a problem Darwin failed to solve. Despite entitling his book The Origin of the Species, he failed to give any real insight into this problem and did not really know what a species was."

Some barriers to the melding of species arise only after fertilization, if the hybrids are inviable or sterile. But sexual isolation through mate discrimination may be what gives many animal species their unique identities, and accounts for the brilliant plumage and bizarre mating rituals seen in some vertebrates.

Female insects often attract mates with Contact pheromones--waxy substances that rub off their abdomens and stimulate chemoreceptors on the forelegs or mouth parts of eligible males to induce courtship.

Coyne's team analyzed the signature pheromones and breeding behavior of four species of Drosophila. In two species, males and females wear a unisex fragrance of tricosene, and males of either species will court females of the other. They will not, however, court females of the remaining two species, which wear a foreign, feminine dust of heptacosadiene. Males of these latter two species wear tricosene and will try to woo any breed of female.

Even males of the two more finicky species can be fooled, however, by perfume trickery. They will court hybrid females--which carry about half the normal amount of each fragrance--with roughly half the ardor with which they pursue their own species. Moreover, females of species that these males would normally shun can be perfumed by crowding them together with properly scented females of the males' species. If the perfume is right, the picky males find even dead females fetching, ruling out any overriding seductive behavior on the part of the female.

Genetic crosses and back-crosses to the parental generation using specially bred flies showed that the ratio of tricosene to heptacosadiene in hybrid flies was determined almost entirely by which species chromosome 3 was inherited from. The involvement of only one of Drosophila's four chromosomes suggests a simple genetic basis for the pheromone signature and may mean that the trait resides at a single genetic locus.

The finding that sexual isolation may be brought about by changes in at most a few genes argues against the evolution of new species in this manner by long-term runaway processes, as dictated by evolutionary dogma, Coyne said.

Other authors on the Science paper are research assistant Anne Crittenden, who performed the genetic crosses, and Katherine Mah, a student in the College who analyzed the pheromones for her senior Honors project. Mah is now a graduate student at the University of Michigan.