An extensive study on Eurasian blackcaps indicates that there are multiple genetic ways for birds to become migratory.
Bird migration has a genetic basis. Ornithologists reached this conclusion by studying European blackcaps (Sylvia atricapilla) with some clever experiments. Blackcaps orient their migration either southwest or southeast, depending on their migration routes. Hybrids between birds that use different migratory strategies direct their migration intermediate, namely south. This simple set-up suggests a genetic basis for bird migration. But which genes underlie this complex trait that integrates morphology, physiology and behavior? Several researchers have searched the genomes of various bird species for these “migration genes”. Different studies found different genomic regions related to migration (among others in thrushes, willow warblers and Vermivora warblers). The discrepancies between these species indicates that there are multiple genetic ways to a migratory lifestyle. A recent study in the journal eLife returned to the bird species that started it all – the Eurasian blackcap – and continued the search for migration genes.
Kira Delmore and her colleagues assembled the whole genome for this iconic species. This genome provided the backbone for genomic sequences that were generated for 110 individual birds. This huge dataset covered the entire spectrum of migratory behavior, from exclusively migratory populations in the north to short distance and partially migratory populations in the Mediterranean, and including non-migratory, or resident, populations from the European continent (Iberian Peninsula) and the Atlantic islands.
Comparing the genomes of all the populations revealed that migratory and resident populations went their separate ways about 30,000 years ago. This divergence led to some genetic differentiation between the distinct migration strategies. The perfect set-up to find migration genes. Indeed, the researchers did not hide their enthusiasm in the paper: “Evidence for limited population differentiation combined with dramatic differences in the migratory behaviour of blackcaps is ideal for identifying genomic regions that are associated with this focal trait.”
The researchers ran several analyses to pinpoint putative migration genes under selection. The search resulted in a short list of candidate genes, including SDC1. This gene codes for a transmembrane protein in the Wnt-pathway which is involved embryonic development. Perhaps a change in this pathway affects morphological traits important in migration, such as the shape of the wings? At the moment, we can only speculate about the possible roles of candidate genes. More research is obviously needed here.
Interestingly, none of the genes under selection in blackcaps overlapped with genomic regions in other migratory species, such as a region on chromosome 4 in Swainson’s thrushes (Catharus ustulatus) and regions on chromosomes 1 and 5 in willow warblers (Phylloscopus trochilus). As I mentioned in the beginning, there seem to be multiple genetic ways to a migratory lifestyle.
Apart from scanning the genomes for migration genes, the researchers also investigated the transition from migratory to resident. These analyses uncovered strong selection on a few genomic regions. A closer look at these regions showed that the genetic variants under selection were located in non-coding sections of the DNA. This suggests that changes in gene regulation underlie the transition to a resident lifestyle. Whether this finding can be extrapolated to other species remains to be determined, but it might indicate that a shift in migratory behavior can occur relatively quickly. In fact, a recent study on tyrannid flycatchers showed that a migratory strategy was lost multiple times during the evolution of this bird family. Could it all be regulatory changes?
Delmore, K., Illera, J. C., Pérez-Tris, J., Segelbacher, G., Ramos, J. S. L., Durieux, G., Ishigohoka, J. & Liedvogel, M. (2020). The evolutionary history and genomics of European blackcap migration. Elife, 9, e54462.