“Thanks to the new methods, we can now examine very long stretches of DNA comprising up to 150,000 base pairs.” “Up until recently, high-throughput sequencing technologies could only sequence segments of DNA on the order of 100 bp in length, which is not long enough to capture large-scale structural mutations”, Professor Wolf said. Such undetected differences in previously hidden regions of the genome are known as structural variations and could include deletions, insertions or inversions of large blocks of genomic DNA. To follow up on earlier studies, the research team addressed the same question but used a much more rigorous and technically demanding method to find differences in vast stretches of DNA. “Even when we find an association between a single-base variant and plumage color, the mutation actually responsible for the color change might be located thousands of base-pairs away.”Ĭlearly, a more exhaustive (and much more expensive) effort was required to better understand what they were seeing. “However, we have never been able to directly determine the functional effects of such single-base variations on plumage color”, said the lead author of the recent study, Dr Weissensteiner, who now is a postdoctoral researcher at Pennsylvania State University. They published an earlier study that necessarily ‘skimmed the surface’ by identifying single-base pair (bp) differences at sites within the two species’ genomes. “The fact that this zone is so clearly defined implies that hybrid progeny are subjected to negative selection.” New technologies illuminate small details of genomesĮver since it was first discovered that the two crows were genetically the same, Professor Wolf and his collaborators have been working on gaining a clearer understanding for the genetic basis for reproductive isolation in these birds. In this situation, both crow species are demonstrating strong assortative mating on the basis of plumage color, which makes it extremely difficult for hybrids to find a mate. A modification in a major effect gene can rapidly and profoundly alter the trajectory subsequent evolution. Major effect genes are those that have fundamental control over a particular trait. “Only two major effect genes, which together encode the feather color, differ sharply on either side of the hybrid zone - the gray alleles are not found to the west of the zone and the black allele is absent in the eastern region”, Professor Wolf said. (Credit: Cruithne9 / CC BY-SA 4.0) Cruithne9 via a Creative Commons license contact zone (white line) separating the two species. But paradoxically, the two species cannot be distinguished based on their genetics.Ī map of Europe indicating the distribution of the carrion and hooded crows on either side of a. However, after careful observations, experts concluded in 2002 these two crows really are different species because they very rarely hybridized and the hybrid offspring were less fit than either parent ( ref) - both hallmarks of speciation. The progeny of such crosses have plumage of an intermediate color”, Professor Wolf explained.ĭespite their dramatic differences in plumage color, these crows still have similar habits and morphologies, so ornithologists long considered them to be geographic races of just one species. “Within this narrow zone, there is a low incidence of interbreeding. This stark boundary between the two crows suggests that individuals of both color forms carefully choose life partners that look just like themselves, a practice known as assortative mating. (Credit: Leander Khil) Leander KhilĪs hybrid zones go, this one is remarkably stable and narrow: it’s only 20 to 50 km (12-31 miles) wide and, in Germany, it basically follows the River Elbe. Two hybrid carrion x hooded crows (Corvus (corone) corone x C (c) cornix) show the color variation.
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