“Here today, up and off to somewhere else tomorrow! Travel, change, interest, excitement! The whole world before you, and a horizon that’s always changing!”
– Kenneth Grahame (Wind in the Willows)
Migration is one of the most fascinating phenomena in ornithology. How do birds know which way to fly and how do they find their way back? It is well established that migratory behavior has a strong genetic component. Surprisingly, little is known about the exact genes that underlie migration. In principle, you could identify these ‘migration genes’ by comparing the genomes of closely related bird populations that follow different migration routes. And that is exactly what Max Lundberg and his colleagues did.
In their study, published in Evolution Letters, they focus on two subspecies of the Willow Warbler (Phylloscopus trochilus) in Europe. One subspecies (P. t. trochilus) migrates to the southwest to wintering areas in West Africa, whereas the other subspecies (P. t. acredula) migrates in a southeastern direction to winter in Eastern and Southern Africa. The two populations overlap and interbreed in Sweden giving rise to a particular kind of hybrid zone, a so-called migratory divide (you can read more about this situation here).
A Lot of Data
The researchers performed extensive genetic analyses. They created a de novo assembly of the Willow Warbler genome (i.e. they assembled it from scratch), re-sequenced the whole genomes of nine samples from each population, and designed a molecular marker set (comprised of 6000 Single Nucleotide Polymorphisms or SNPs) for 1152 samples. This impressive dataset revealed that … there was almost no genetic differentiation between the populations. This suggests that these subspecies are at an early stage of divergence.
Nonetheless, three genomic regions – on chromosomes 1, 3 and 5 – were highly differentiated (see the figure below). The genetic markers on chromosome 3 correlated with breeding altitude and latitude, while the regions on chromosomes 1 and 5 perfectly matched the differences in migration route. Zooming in on these regions revealed several genes that are involved in the synthesis of fatty acids. This seems logical given that long-distance migrants mostly use fat as energy. Furthermore, the subspecies differ significantly in the distance they cover during migration. The authors admit that ‘it is tempting to speculate that these differences represent adaptations in fueling to their different routes.’
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. http://onlinelibrary.wiley.com/doi/10.1002/evl3.15/abstract
The paper has been added to the Phylloscopidae page.