It might be as simple as flipping a genetic switch.
One of the most famous and well-studied avian hybrid zones runs across central Europe. Here, Carrion Crow (Cornix corone) and Hooded Crow (C. cornix) come into contact and interbreed. Hybridization is limited due to differences in plumage. The all-black Carrion Crows and the grey-coated Hooded Crows prefer to mate with birds of the same plumage type (i.e. they mate assortatively). Despite the clear differences in plumage, these species are genetically almost identical. Previous work uncovered just a few genomic regions that are differentiated, representing less than 1 percent of the genome. This begs the question: what do these genomic regions actually do? Two recent studies provide the first clues into this mystery.
To pinpoint the genes underlying a particular trait, biologists often resort to an approach called genome-wide association (GWA) mapping. This method indicates which genetic variants are most strongly associated with a certain trait, in this case plumage color. Based on a dataset of more than 400 individuals, Ulrich Knief, Christen Bossu and their colleagues found three genomic regions. A big region – spanning 2,800,000 base pairs – on chromosome 18 and two smaller regions on chromosomes 1 and 1A. Interestingly, these regions also popped up in previous work looking for differentiation.
The genetic variant on chromosome 1A lies within the gene LRP6, whereas the region on chromosome 1 holds two interesting genes: NDP and EFHC2. These names probably do not ring a bell. Although you might have heard of NDP, because I have written about it before. This gene is responsible for the plumage patterns in pigeons. And it might also explain the difference between a Carrion and a Hooded Crow. The big region on chromosome 18 contains 88 genes. It is thus difficult to pinpoint the exact genes responsible for the plumage differences.
And then there is the transcription factor MITF. For readers not familiar with molecular biology, a transcription factor is a protein that controls the activity patterns of other genes. A kind of genetic control switch. If MITF is important in the regulation of plumage color, you would expect it to show increased activity during the formation of feathers.
Previous work failed to find support for the idea that MITF is a crucial genetic switch. The reason for this could be that MITF has multiple roles during development and is active in multiple cell types. The solution to this issue is to quantify the activity of MITF on a single cell level. And that is exactly what Chi-Chih Wu and his colleagues did. They managed to quantify gene expression in melanocytes – the cells producing the pigment – and show that MITF is indeed activated in these cells.
So, MITF seems to be key regulator in the development of plumage color. A few weeks ago, I met Jochen Wolf in Uppsala. He has been the driving force behind unraveling the mechanisms at work in the crow hybrid zone. At one point during our chat, he said: “Turning a Carrion Crow into a Hooded Crow? It could be as simple as flipping a switch!”
Knief, U., Bossu, C.M., Saino, N., Hansson, B., Poelstra, J., Vijay, N., Weissensteiner, M. & Wolf, J.B.W. (2019) Epistatic mutations under divergent selection govern phenotypic variation in the crow hybrid zone. Nature Ecology & Evolution, 3:570-576.
Wu, C., Klaesson, A., Buskas, J., Ranefall, P., Mirzazadeh, R., Söderberg, O. & Wolf, J.B.W. (2019) In situ quantification of individual mRNA transcripts in melanocytes discloses gene regulation of relevance to speciation. The Journal of Experimental Biology, 222: jeb194431.
These papers have been added to the Corvidae page.