The comparison of two Crow hybrid zones reveals that history does not always repeats itself. Genomically, that is.
How species originate and adapt to new environments are some of the central questions in evolutionary biology. With the advent of genomic data, it has become possible to unravel the genomic basis of speciation and adaptation. And hybrid zones can serve as the ideal laboratories for testing new ideas.
One of the most widely studied hybrid zones concerns the contact zone between Hooded Crow (Corvus cornix) and Carion Crow (C. corone) in Europe (you can read more about it here). A recent study revealed that several genomic regions were not exchanged between these hybridizing species (Poelstra et al. 2014). One of these regions harbored genes related to pigmentation and visual perception. Given that Hooded Crow and Carrion Crow differ in their color patterns, this genomic region could be crucial in the (probably still ongoing) speciation process.
Thus, the European crow hybrid zone provided important insights into the genomic basis of speciation. But it gets ever better! Did you know that there is another crow hybrid zone in Siberia? In eastern Russia, Hooded Crow is hybridizing with the all-black Eastern Carrion Crow (C. orientalis). An almost perfect replicate of the hybrid zone in Europe!
This set-up, two hybrids zones between species at different stages of speciation, provides an excellent opportunity to check whether the same genomic regions are involved. So, Nagarjun Vijay (Uppsala University) and colleagues sequenced and compared 124 genomes of different crow populations. They found that different genomic regions are under selection in the different hybrid zones. In the figure below, you can see the genomic comparison between Hooded Crow and Carrion Crow (above) and between Hooded Crow and Eastern Carrion Crow (below). Pay attention to the red dots, which represent genomic regions that are different between the species. If you move across the genomes, you will see that (with some exceptions) different genomic regions stand out in the two hybrid zones. Clearly, speciation does not repeat itself here.
Poelstra, J. W., Vijay, N., Bossu, C. M., Lantz, H., Ryll, B., Muller, I., Baglione, V., Unneberg, P., Wikelski, M., Grabherr, M. G. & Wolf, J. B. W. (2014). The genomic landscape underlying phenotypic integrity in the face of gene flow in crows. Science 344, 1410-1414.
Vijay, N., C. M. Bossu, J. W. Poelstra, M. H. Weissensteiner, A. Suh, A. P. Kryukov and J. B. Wolf (2016). Evolution of heterogeneous genome differentiation across multiple contact zones in a crow species complex. Nature communications 7: 13195.
This paper has been added to the Corvidae page