A NEW species of insect may have arisen in an evolutionary blink of an eye as a result of cross-species mating. The discovery suggests that hybridisation 鈥� an important force in producing new species in plants 鈥� may also be widespread in animals. Until now, it had been assumed that new animal species almost always arise by gradually splitting off from an existing lineage.
The probable new species belongs to a group of flies known as fruit maggots, highly specialised fruit parasites of which each species infests one particular plant species. In 1997, Bruce McPheron and his colleagues at Pennsylvania State University in University Park noticed one such infestation on introduced Asian honeysuckle bushes in north-eastern Pennsylvania. They wondered how the plant had such specialised pests within only 250 years of its introduction into North America.
The team found that, genetically, the honeysuckle maggots look like a hybrid of two native fruit-maggot species, the blueberry maggot and the snowberry maggot (Nature, vol 436, p 546). And they found signs of hybridisation in geographically separated areas: this suggests hybridisation happened more than once, but with the same shift in host. 鈥淭here must be something to hybridisation that makes them colonise honeysuckle,鈥� says Dietmar Schwarz, the lead researcher.
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They are not first-generation hybrids, however, which would have exactly half their genes from each parent species. Instead, the maggots had a wide range of ratios of genes from each species, indicating they had been interbreeding for at least a hundred generations.
The adults mate on their host plant, so hybrids on a different host are unlikely to interbreed with the parent species. This host shift should therefore lead rapidly to a new species, though the honeysuckle maggot may not yet have completed the process, Schwarz says.
If hybridisation often leads to a new host preference, it could be much more important in animal evolution than anyone had suspected, says Schwarz. After all, many or even most animal species live in close association with a particular host. 鈥淚t makes one think hybridisation could be important in generating biodiversity,鈥� says Thomas Dowling, an evolutionary biologist based at Arizona State University in Tempe.
