Origin of species

Origin of species

Study of origin of species enters the molecular age

November 20, 1998

Nothing brings two people closer together than sex; but for closely related species of fruit flies, it may be what keeps them apart. Researchers at the University of Chicago recently discovered a gene that appears to play a crucial role in causing one species to split into two--and stay that way. The gene causes the male progeny of two recently separated species to be sterile, a condition known as hybrid male sterility.

"How speciation occurs is one of the central questions in evolutionary biology," says Chung-I Wu, PhD, chairman and professor of the department of ecology and evolution at the University of Chicago, and senior author of the paper in the November 20 issue of Science. "Geographic isolation and changes in the environment are only a part of what drives speciation. There are also changes at the genetic level that are driven by sexual selection. As a result, two newly formed species can't mix back into one."

Many species of insects and mammals exhibit male sterility in inter-species hybrids at the early stages of speciation. For example, the male progeny of the yak-cow cross are sterile. Male "zonkeys," the offspring of a zebra and a donkey, are also sterile. Wu speculated that hybrid male sterility must be an early indicator of speciation.

Chau-Ti Ting, a postdoctoral fellow in Wu's lab pinpointed a gene (there may be many) involved in producing sterility in hybrid males. It does this by evolving so rapidly that male offspring produced by two newly divergent or "sibling" species are so different from their parents that they are unable to make sperm.

The gene Ting thinks contributes to hybrid male sterility is a homeobox gene. Homeobox genes, usually involved in development and cell differentiation, are some of the slowest evolving genes in all of nature. This means that they are almost identical in organisms as diverse as worms and monkeys. But when Wu looked at a certain homeobox gene in two sibling species of fruit fly, D. simulans and D. mauritiana, they were very different.

"Homeobox genes usually don't differ by more than a few base pairs--even when you compare them in humans and invertebrates. But when we looked at this homeobox gene in two very closely related species of fruit fly, they were extremely different--suggesting that this gene is evolving at a highly accelerated rate," says Wu.

Wu thinks that the reason the homeobox gene is on the evolutionary fast track--it evolves 100 to 1,000 times faster than any other homeobox gene studied--is because it controls male sexual function.

Genes pertaining to male reproduction evolve quickly because they are under extreme pressure due to the intense competition among males to fertilize eggs. "The idea is that something might pop up that gives one male an edge in the race to reproduce," says Wu. "The advantageous gene will be selected for and passed on to male progeny."

Usually sexual advantages in males have to do with sperm production, or the composition of the ejaculate, "anything that makes it easier for their sperm to get to the egg," Wu explains. The rapidly changing homeobox gene makes sibling species incompatible and halts the gene flow between them. The species can then evolve separately without their unique innovations being lost by blending.