BUTTERFLIES in Central America are providing evidence for inter-species breeding as an evolutionary force. Contrary to existing theories, the resulting hybrids can be highly fertile and viable, a study of two members of the Heliconius genus suggests.
James Mallet, an evolutionary biologist at University College London, set out to study the flow of genes between the two genetically and physically distinct butterfly species Heliconius cydno and H. melpomene (see below) whose ranges overlap in Panama. H. cydno, which is found mainly to the north, is coloured black, white and blue, sometimes with iridescent yellow markings. This pattern mimics other poisonous butterflies in its range. H. melpomene has distinctive red markings that mimic poisonous species prevalent in the Andes. The two species are thought to have diverged 1.5 million years ago.
Mallet and colleagues compared the structure of five different genes in the two species and found that one, which codes for an enzyme Mpi, has the same structure in butterflies of both species captured in Panama. Mpi mutates quickly, making it unlikely that the shared sequence is a remnant of the genome that existed before the species diverged. Statistical tests also indicate that the two species most likely shared the gene by breeding with one another, Mallet鈥檚 team reports in BMC Biology (DOI: 10.1186/1741-7007-4-11).
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The clincher comes from analysis of H. melpomene living further afield, in French Guiana. In these butterflies, the gene sequence is quite different, suggesting that the gene in Panamanian butterflies is only found where the two Heliconius species overlap. 鈥淕ene flow is there, but it鈥檚 difficult to find,鈥� Mallet says. 鈥淚f we look harder by checking lots of genes instead of one, then I think we鈥檒l find hybridisation everywhere.鈥�
He is in little doubt about the significance of the finding. 鈥淲e tend to believe that evolution is down to mutation or slow selection of certain traits, but now we鈥檙e realising that evolution can come from the acquisition of genes from other species.鈥� Last year researchers reported that a similar phenomenon may have occurred in flies (快猫短视频, 30 July 2005, p 12).
