EVOLUTION is a complex beast, but it can operate in breathtakingly simple ways. Just a single pair of incompatible genes can help prevent closely related species from hybridising, experiments on a pair of fruit fly species have revealed.
Geneticist Daniel Barbash and colleagues at Cornell University in New York studied Drosophila melanogaster and Drosophila simulans, two species that although related are unable to fully hybridise. Male offspring produced by female D. melanogaster and male D. simulans die before hatching, and this barrier to genetic interchange helps keep the two species distinct.
By manipulating variants of different genes in each species, the researchers found that hybrid males die only if they carry fully functioning copies of two genes, one called Lhr from D. simulans, and the other Hmr from D. melanogaster (Science, vol 314, p 1292). Many biologists had thought that species were separated by the gradual accumulation of many small genetic differences, but Barbash鈥檚 experiments are the first to suggest that the mechanism can be much simpler. 鈥淚n at least some cases, evolutionary divergence is not a consequence of many dozens or hundreds of genes of very small effect,鈥� he says.
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鈥淢anipulating the two species showed that hybrids die only if they have fully functioning copies of two particular genes鈥�
The researchers also found that both Lhr and Hmr have changed more rapidly than one would expect by chance alone since D. melanogaster and D. simulans split into separate species. This suggests that natural selection operating on the genes鈥� normal functions has played a role in their divergence. The higher than usual number of changes that this produced has increased the chances of stumbling on a mutation that happened to be lethal to hybrids, Barbash says.
The species barrier may not be quite so simple, however. For one thing, when the researchers combined the two genes in non-hybrid D. melanogaster, male offspring developed normally. This implies that some other feature of the hybrid genome also plays a role in separating the species. In other words, the gene pair is necessary, but not sufficient, to create the barrier.
Moreover, D. melanogaster and D. simulans are fairly distinct genetically 鈥� about as different as humans are from macaques. More similar species may not have diverged enough to develop genes with such large effects on hybrids, says Chung-I Wu, an evolutionary geneticist at the University of Chicago.
