Hybrid speciation in birds: a fresh perspective on an old case and a new addition to the list.
Let me start with a joke. What do you call a cross between en bulldog and a shitzu? Wait for it… a bullshit! Hilarious as this may be, hybridization between two closely related species can lead to something completely different, occasionally even a new species. This phenomenon – hybrid speciation – is quite controversial in birds. Many putative hybrid species have failed to pass the test. Some well-documented cases include the Italian Sparrow (Passer italiae) and the Audubon’s Warbler (Setophaga auduboni). Recently, two studies on hybrid speciation appeared. One providing a genomic perspective on an old case, the other presenting a new hybrid species in the Amazon basin. Let’s dive right in (the papers, not the Amazon…).
Big Bird Revisited
In 2009, Peter and Rosemary Grant presented a peculiar case of putative hybrid speciation. They reported that a male hybrid between Medium Ground Finch (Geospiza fortis) and Common Cactus Finch (G. scandens) arrived on the Galapagos Island of Daphne Major. This bird – which they nicknamed ‘Big Bird’ – mated with a female Medium Ground Finch. The offspring of this couple started interbreeding amongst each other for several generations, resulting in a hybrid lineage that is reproductively isolated from its parental species.
Now, Sangeet Lamichhaney and his colleagues (including the Grants) revisited this hybrid lineage using genomic data. They showed that the Grants were partly wrong. The male that ended up on Daphne Major was not a hybrid, but a Large Cactus Finch (G. conirostris) from Espanola, an island more than 100 km from Daphne Major. Nevertheless, the offspring of this male and his Medium Ground Finch partner did interbreed amongst each other, which culminated in an increasing inbreeding coefficient.
Further analyses of the beak morphology of this lineage indicated that it is ecologically distinct from its parental species. And in 2009, the Grants already established that it is reproductively isolated from the Medium Ground Finch (if it is also isolated from the other parental species, which lives on another island, has not been tested yet). All in all, strong evidence for hybrid speciation.
A Memorable Manakin
The Golden-crowned Manakin (Lepidopthrix vilasboasi) had not been seen since the 1950’s until it was rediscovered in 2002. Alfredro Barrera-Guzman and his colleagues took advantage of this rediscovery and sequenced several individuals from an isolated population between the Tapajós and the Jamanxim rivers. When they compared the DNA sequences with two closely related species – Snow-capped Manakin (L. nattereri) and Opal-crowned (L. iris), they were in for a surprise. The Golden-crowned Manakin turned out to be a hybrid species! Detailed genetic analyses (including coalescent modelling) reinforced this finding.
There is, however, one intriguing observation. Recent hybrids between Snow-capped and Opal-crowned Manakin still occur in the Amazon Basin. But these hybrids do not have the striking yellow crown patches of the Golden-Crowned Manakin. Were the authors wrong? Did they jump to conclusions too quickly? No, further investigations into the nanostructure of these crown patches provided some clues.
The researchers studied the keratin matrix of the crown feathers using electron microscopy. This revealed that the matrix of the hybrids is intermediate between the parental species (no surprise there). But the same goes for the Golden-crowned Manakin. So where does the yellow color come from? In the hybrid species – which originated about 260,000 years ago – the crown patch has been thinkened by carotenoids, which explains the yellow color. The accumulation of carotenoids probably compansated for the loss of brightness (a feature that the parental species use to attract females). Apparently, female Golden-crowned Manakins have a weak spot for yellow. And who could blame them?
Barrera-Guzmán, A. O., Aleixo, A., Shawkey, M. D. & Weir, J. T. (2017). Hybrid speciation leads to novel male secondary sexual ornamentation of an Amazonian bird. Proceedings of the National Academy of Sciences, 201717319.
Lamichhaney S, Han F, Webster MT, Andersson L, Grant BR, Grant PR. (2017). Rapid hybrid speciation in Darwin’s finches. Science:eaao4593.