The spectacled tetraka, Xanthomixis zosterops, is a small, yellow-green songbird from Madagascar, and it may represent a rare instance where multiple species merged back into one. The work, published in Ecology and Evolution, suggests that this tropical warbler had undergone speciation in reverse, or despeciation.
The merger of previously isolated vertebrate lineages is thought to occur under certain circumstances, such as human alteration of habitat. Preliminary sequencing of mitochondrial DNA (mtDNA) from these Malagasy songbirds has suggested a possible instance of merging lineages. So, a team led by Nicholas Block from Stonehill College analyzed DNA from the muscles and feathers of 117 spectacled tetrakas.
Their mtDNA varies up to 7.6% from one bird to the next, and that’s a huge amount for animals within the same species. Compare that to human mtDNA from anywhere on the planet, which varies by 1.5% at the most. This suggests that tetrakas in the same species had different ancestors. Their nuclear DNA, on the other hand, is extremely similar – which means that despite their varied evolutionary history, the separate species came back together.
“This is something that lots of people would have said couldn’t happen,” study co-author John Bates from the Field Museum said in a statement. “It’s a very unusual pattern of evolution, but this discovery illustrates that unusual things do happen.” The team found that the spectacled tetraka comprises four mtDNA clades that likely began diverging 3.6 million years ago.
Furthermore, the researchers examined the DNA of lice living on 18 of these birds. Ectoparasites like these typically evolve along with their host, so each bird species has its own distinct parasite. Turns out, spectacled tetrakas living in different parts of the island hosted different kinds of lice. That suggests different parasite species evolved back when the bird lineages were separate – though the lice maintained that separation even after the birds merged together.
During wet periods in Madagascar, forests can become linked; when it’s dry, forests become patchy and isolated. The team thinks that tetrakas ancestors became separate species when the climate was drier, and then came back together when the forests linked up again. In that case, the parent species likely became extinct.
[Via Field Museum]