A transposable element associated with migratory behavior of Willow Warblers

But how does it contribute to migration?

What genes determine the migratory strategies of birds? Several studies have tried to answer this question in a variety of study systems, such as Vermivora warblers, European Blackcaps (Sylvia atricapilla) and Willow Warblers (Pylloscopus trochilus). Interestingly, different candidate genes have popped up in different studies, suggesting that the genetic basis for migration varies between species. In the Willow Warbler, for example, researchers took advantage of the divergent migratory strategies of two subspecies: trochilus migrates to the southwest, whereas acredula follows a southeastern route. These migratory differences were associated with three genomic regions, located on chromosomes 1, 3 and 5. However, a previously identified genetic variant – using the older AFLP-technique – could not be assigned to a particular genomic region. Given that most avian genome assemblies are far from complete, it could be that this variant – known as WW2 – resides in a difficult-to-assemble section of the genome, such as a region with repetitive sequences. That is why Violeta Caballero-López and her colleagues used an updated version of the Willow Warbler genome to determine the location and identity of the WW2-variant. Their findings recently appeared in the journal Molecular Ecology.

Endogenous Retrovirus

The newest Willow Warbler genome was sequenced using a long-read technique which allows scientists to reconstruct highly repetitive sections of the genome. Within one of these section, the researchers found the WW2-variant. Additional analyses indicated that it concerns a transposable element, which is a selfish genetic element that “jumps” around the genome using either a copy-and-paste or a cut-and-paste mechanism. This particular transposable element turned out to be an endogenous retrovirus (ERV) that inserted itself into the genome of an ancestral songbird a long time ago. A similar variant is also present in the genome of the Zebra Finch (Taeniopygia guttata), which diverged from the Willow Warbler at least 20 million years ago.

A detailed look at the WW2-variant revealed additional evolutionary changes. Apart from the ancestral version shared with the Zebra Finch, the researchers also uncovered a derived version. The latter version probably originated after a duplication and an inversion event. The resulting sequence then accumulated mutations, leading to divergence from the ancestral state. Interestingly, the derived version was much more abundant in the acredula-subspecies (7 to 45 copies) compared to the trochilus-subspecies (0 to 6 copies).

A schematic overview of the probable evolution of the WW2-variant. The ancestral version (small green arrow) duplicated and became inverted (figures c and d). The resulting sequence accumulated mutations and diverged into the derived version (small yellow arrow). From: Caballero‐López et al. (2021).

Smelly Migration?

The different number of copies of the derived version in the two subspecies suggests that the transposable element might be involved in their migratory behavior. The genomic region surrounding the WW2-variant contains several olfactory receptors. It is tempting to speculate that olfaction might help Willow Warblers during their migration (as shown in homing pigeons), but the researchers warn that more analyses are needed to test this hypothesis. Alternatively, the WW2-variants might interact with other genomic regions – perhaps the ones on chromosomes 1, 3 and 5 – to influence migratory behavior. A similar mechanism has been described in Carrion Crow (Cornix c. corone) and Hooded Crow (C. c. cornix) where a transposable element might be involved in the regulation of plumage coloration. Clearly, there are many new exciting questions to investigate in the Willow Warbler. Just as transposable elements “jump” around the genome and explore new territories, scientists keep delving into knowledge gaps to uncover surprising new insights. You never know where the next analysis will take you…


Caballero‐López, V., Lundberg, M., Sokolovskis, K., & Bensch, S. (2022). Transposable elements mark a repeat‐rich region associated with migratory phenotypes of willow warblers (Phylloscopus trochilus). Molecular Ecology31(4), 1128-1141.

Featured image: Willow Warblers (Pylloscopus trochilus) © Chris Romeiks/vogelart.info | Wikimedia Commons

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