The colors of mice

Science this week features an article by Elizabeth Pennisi about the research of evolutionary biologist Hopi Hoekstra. She studies pigment variations in wild mice.

Pigment clines have become an interesting model field study, because we now understand the molecular pathway of melanin production. With a dozen or so genes influencing natural pigment variations in mammals, it’s a complex enough system that selection on color will lead to different genetic outcomes. That means we can look at parallelism in many natural cases to understand the evolutionary dynamics.

Hoekstra and her team are part of a genomics explosion in natural history studies. "This is an example of work ... merging the green and white side of biology, in which we learn about trait evolution from the biochemical levels within cells to how those traits are selected for or against in natural populations," says Hans Ellegren, an evolutionary biologist at Uppsala University in Sweden. Mark McKone, a biologist at Carleton College in Northfield, Minnesota, agrees: The work "could be a model for how to approach evolution in the postgenomic period," when genetic information and tools are more readily available.

A couple of weeks ago, Hoekstra’s lab had a research paper illuminating pigment evolution among the deer mice of the Nebraska Sand Hills: “On the origin and spread of an adaptive allele in deer mice.” I like this example a lot, because the color variation in Sand Hills is clearly postglacial – this was not an agreeable Like the evolution of stickleback varieties in British Columbia, it’s a good example of rapid selection on new colonists.

Multiple lines of evidence suggest that the wideband allele arose de novo and was not a preexisting allele. First, the haplotype carrying the deletion has greatly reduced variation relative to the wild type (Fig. 4A), which is inconsistent with a model in which the causative mutation was neutrally segregating in the population before any selective pressure (38). Second, the U-shaped SFS [site frequency spectrum] is most consistent with a model in which selection acts on a newly arising mutation (fig. S3). If the beneficial mutation existed on multiple haplotypes before the selective pressure, we would expect to see an excess of intermediate frequency mutations, which was not observed (38). Finally, the posterior probability density of the allele age falls entirely within the estimated age of the Sand Hills (Fig. 4B).
Taken together, our results demonstrate that variation at the Agouti locus is responsible for adaptive coloration in deer mice living on the Nebraska Sand Hills.

There are still details to be worked out in this example – at the biochemical level, how does this allele cause lighter color? Can we say more about the spatial dynamics of the allele after it originated? But what I really like is that it shows obvious parallels to adaptive variations in humans that have recently been selected. It stands out in mice because Hoekstra is out there looking for pigment variations. Postglacial environments are one example in nature where recent environmental changes have generated new selection pressures. Of course, humans have induced new pressures on ourselves by means of massive cultural change.

Anyway, I thought it was worth pointing out the articles could go together as a set. I’ll follow up with a second post on Hoekstra’s take on developmental biology.

References:

Linnen CR, Kingsley EP, Jensen JD, Hoekstra HE. 2009. On the origin and spread of an adaptive allele in deer mice. Science 325:1095-1098. doi:10.1126/science.1175826

Pennisi E. 2009. How beach life favors blond mice. Science 325:1330-1333. doi:10.1126/science.325_1330