A genetic study of Golden-winged and Blue-winged Warbler reveals the gene underlying their migration direction.
Every evolutionary biologist dreams of finding that one gene underlying a particular trait under selection. For example, extensive work on the Threespine Stickleback (Gasterosteus aculeatus) uncovered the genetic basis of armor-plating: marine forms of this fish are heavily armored while lake populations are not (probably because of the absence of predators). Stickleback in lakes have repeatedly lost pelvic hindfins, which is due to recurrent deletions of a pelvic enhancer of the Pitx1 gene. Mostly, however, things are not that straightforward. Human height, for instance, is determined by numerous genes of small effect instead one single candidate gene. A recent study in the journal PNAS attempted to find the genetic basis of another complex trait: the direction of bird migration.
Several studies have tried to pinpoint the genes underlying migratory behavior in birds. In the Swainson’s Thrush (Catharus ustulatus), researchers found a region on chromosome 4 that is strongly associated with the direction of migration. And a study on the Willow Warbler (Phylloscopus trochilus) uncovered three genomic regions – on chromosomes 1, 3 and 5 – that corresponded to different migratory strategies (see also this blog post). In both cases, large genomic regions, containing hundreds of genes, were identified. Certainly a step in the right direction, but there is still a long way to go.
David Toews and his colleagues focused on another study system: the Golden-winged (Vermivora chrysoptera) and the Blue-winged Warbler (V. cyanoptera). These species were chosen for a reason: due to extensive hybridization, their genomes are largely undifferentiated (mainly “plumage genes” seem to be different). This lack of genetic differentiation makes it easier to find genes related to migration. Compare it to finding a needle in a very small haystack.
Both species breed in North America and winter in two particular regions: South America (mainly Venezuela) and Central America (Panama to Guatemala). By linking these migration strategies to genomic sequences, the researchers attempted to find the genetic basis of migration. And they did: the analyses converged on a small region (120,000 base pairs) on the Z-chromosome. This region showed reduced genetic diversity in warblers migrating to South America and Tajima’s D (a statistic to identify selective processes) was also much lower in these birds.
This region contained only one gene: VPS13A. The function of this gene in birds is not known, but recent work showed that it is associated with mitochondria. The researchers speculate that “selection on VPS13A may enhance the capacity in SA wintering birds to more efficiently remove reactive oxygen species resulting from a prolonged migration.”
This study shows the importance of choosing the right study system to answer your research question. The undifferentiated genomes of these warblers provided the perfect background to find genes related to migration. And it also highlights the importance of hybrids: if Golden-winged and Blue-winged Warbler did not hybridize, their genomes might have been too diverged to easily find migration genes. Do not underestimate the power of hybridization!
Delmore, K. E., Toews, D. P., Germain, R. R., Owens, G. L., & Irwin, D. E. (2016). The genetics of seasonal migration and plumage color. Current Biology, 26(16), 2167-2173.
Lello, L., Avery, S. G., Tellier, L., Vazquez, A. I., de los Campos, G., & Hsu, S. D. (2018). Accurate genomic prediction of human height. Genetics, 210(2), 477-497.
Lundberg M, Liedvogel M, Larson K, Sigeman H, Grahn M, Wright A, Åkesson S, Bensch S 2017. Genetic differences between willow warbler migratory phenotypes are few and cluster in large haplotype blocks. Evolution Letters 1: 155-168.
Toews, D. P., Taylor, S. A., Streby, H. M., Kramer, G. R., & Lovette, I. J. (2019). Selection on VPS13A linked to migration in a songbird. Proceedings of the National Academy of Sciences, 116(37), 18272-18274.
Xie, K. T. et al. (2019). DNA fragility in the parallel evolution of pelvic reduction in stickleback fish. Science, 363(6422), 81-84.