A cool example of adaptive introgression in the seasonal camouflage of snowshoe hares.
If you have ever taken an ecology course, you should know the snowshoe hare (Lepus americanus). Together with its predator, the lynx (Lynx lynx), the snowshoe hare is a textbook example of predator-prey interactions: “When the population size of the hare increases, there is more food for the lynx to eat. As a result the lynx population also increases. The large lynx population will kill more hares so the hare population decreases. This will also cause the hare population to decrease.”
During winter, snowshoe hares are white, well-camouflaged against the snowy background. But when the snow melts, some hares molt into a brown fur, allowing them to blend in again. Keeping their white fur in a brownish landscape would make them an easy prey (nicely shown here). But the genetics of this seasonal camouflage were unknown. A recent paper in Science explores this uncharted territory and stumbles upon a fascinating result.
Finding the genes
To understand the genetics of fur color, you first need to know which genes underlie the trait. Therefore, Matthew Jones (University of Montana) and his colleagues sequenced the genomes of several snowshoe hares and tested which genetic markers associated with coat color. The researchers found a single region on chromosome 4, containing among others the pigmentation gene Agouti.
Further analyses, including a captive breeding experiment, showed that the uncovered genes acted like typical Mendelian traits. Hares that stay brown in winter have two recessive alleles (aa), while hares that turn white in winter have at least one dominant allele (Aa or AA). The textbook example of predator-prey dynamics also turned out to be a textbook example of Mendelian inheritance!
Enter the jackrabbits
Comparing white with brown snowshoe hares revealed that the region in chromosome 4 was markedly different. This could be due to long-term maintenance of this polymorphism or introgression from another species. The results pointed to the latter possibility: the brown winter coats of the snowshoe hare were likely acquired through hybridization with black-tailed jackrabbits (L. californicus). This introgressed gene helped them to better cope with snowless conditions. A very nice example of adaptive introgression.
Jones, M.R., Mills, L.S, Alves, P.C., Callahan, C.M., Alves, J.M., Lafferty, D.J.R., Jiggins, F.M., Jensen, J.D., Melo-Ferreira, J. & Good, J.M. (2018) Adaptive introgression underlies polymorphic seasonal camouflage in snowshoe hares. Science, 360:1355-1358.