Studying hybrid zones can provide important insights into the genetic basis of plumage coloration in birds.
Why is a Blackbird (Turdus merula) black and a Yellow Warbler (Setophaga petechia) yellow? This might seem like a trivial question, but the genetic basis of plumage color in birds is an active field of research. In general, the color differences between bird species can be traced back to several pigments: melanins, caroteniods and porphyrins. For example, carotenoids are responsible for the bright red plumage of the Northern Cardinal (Cardinalis cardinalis). But which genes regulate how birds use these pigments to create the colorful variety we see in nature?
Studying Hybrid Zones
To investigate the genetic control of plumage colors, researchers make use of hybrid zones. In these areas, different bird species come into contact and interbreed. The resulting hybrids are often a mixture of both species. Some feathers are colored like one species, while other feathers have the color of the second species. By comparing these patterns with the genes of the hybrids, ornithologists can figure out which genes belong to which plumage pattern.
Two recent studies have applied this approach to disentangle the genetics of bird coloration. In the first study, published in Current Biology, David Toews and his colleagues focused on Golden-winged Warbler (Vermivora chrysoptera) and Blue-winged Warbler (V. cyanoptera), two species that hybridize in eastern North America. They found six small genomic regions that housed genes involved in feather development and pigmentation. One feature, throat coloration, was associated with the promotor region (the on/off-switch, if you will) of a gene called agouti.
Alan Brelsford and his colleagues turned their attention to the western part of North America and took a closer look at two other interbreeding passerines: Audubon’s Warbler (Setophaga coronata auduboni) and Myrtle’s Warbler (S. c. coronata). For several plumage traits, they found associations with genomic regions, which harboured genes related to keratin filaments that build feathers. Two melanin-based traits – eye line and eye spot – could even be traced back to a single genomic region.
These two studies provide a long list of candidate genes involved in bird coloration. Now, researchers can set out to unravel the biochemical pathways underlying these traits. In the end, I might be able to tell you precisely why a Blackbird is black and a Yellow Warbler yellow. In the meantime, let us just enjoy our colorful feathered friends.
Brelsford, A., D. P. L. Toews and D. E. Irwin (2017). Admixture mapping in a hybrid zone reveals loci associated with avian feather coloration. Proceedings of the Royal Society B: Biological Sciences 284(1866).
Toews, D. P., S. A. Taylor, R. Vallender, A. Brelsford, B. G. Butcher, P. W. Messer and I. J. Lovette (2016). Plumage genes and little else distinguish the genomes of hybridizing warblers. Current Biology 26(17): 2313-2318.
Both papers have been added to the Parulidae page.