Extensive hybridization in contact zones leads to mitonuclear discordance.
Different genes tend to tell different stories. This phenomenon – known as gene tree discordance – can be particularly obvious when comparing mitochondrial DNA with nuclear markers. Mismatches between these molecular markers can be the outcome of different processes, such as hybridization, incomplete lineage sorting, sex-biased dispersal or natural selection. Regardless of the underlying mechanism, mitonuclear discordance can be problematic in species delimitation, especially when morphological information is insufficient to delineate species. How can you discriminate between two or more species when different molecular markers give you different answers? This issue is relevant for the Canada Jay (Perisoreus canadensis) which consists of three morphotypes that might even interbreed. A recent study in the Biological Journal of the Linnean Society attempted to unravel this complex situation.
The most widespread morphotype of the Canada Jay – the Boreal morphotype – can be found from Alaska to Newfoundland. The other two morphotypes are more restricted: the Pacific form resides along the coast from California to British Columbia, and the Rocky Mountains form occurs at high elevation in the Rocky Mountains from New Mexico to British Columbia. Apart from their distinct – but overlapping – distributions, these morphotypes show variation in their plumage patterns. Specifically, the Pacific birds have a darker crown patch compared to the Boreal birds, whereas the Rocky Mountain birds have a conspicuous white head. Despite these subtle differences, some intermediate birds have been observed, suggesting the occurrence of hybridization.
Between 1990 and 2018, Brendan Graham and his colleagues collected blood samples from over 600 individuals. Based on several morphological traits, each of these individuals was assigned to one morph. However, 15 individuals showed traits of at least two morphotypes and were classified as intermediates. Next, the researchers took a closer look at the genetic make-up of all birds, using 12 microsatellites and the mitochondrial control region. Both marker types were reasonable successful in discriminating between the three morphotypes. The geographic distribution of the morphotypes was mirrored by the mitochondrial lineages, although some populations contained individuals from multiple lineages. And analyses of the nuclear data pointed to three distinct clusters, corresponding to the three morphotypes. However, in regions of overlap, a large number of individuals (27%) were assigned to a genetic cluster that did not match their morphology.
Moreover, no less than 222 individuals showed mismatches between the mitochondrial and nuclear assignments. Mitonuclear discordance was most prominent in sympatric areas, such as southern British Columbia and northern Washington, Wyoming and Utah. The most likely explanation for these patterns seems extensive hybridization after a period of isolation.
The mismatches between morphology and the genetic markers in this study suggest that the plumage patterns in the morphotypes are controlled by a few genetic loci. Similar patterns have been uncovered in several other bird groups, such as wagtails, woodpeckers and warblers. The authors note that “analysis with next generation sequencing techniques may help uncover the genes associated with plumage variation in Canada Jays.” And when they manage to perform these analyses, you will definitely read about it on the Avian Hybrids blog.
Graham, B. A., Cicero, C., Strickland, D., Woods, J. G., Coneybeare, H., Dohms, K. M., Szabo, I. & Burg, T. M. (2021). Cryptic genetic diversity and cytonuclear discordance characterize contact among Canada jay (Perisoreus canadensis) morphotypes in western North America. Biological Journal of the Linnean Society, 132(4), 725-740.
Featured image: Canada Jay (Perisoreus canadensis) © Mdf | Wikimedia Commons