Convergent mutations produce convergent colors

Just a quick post on a  new paper in Proceedings of the National Academy of Sciences by Erica Bree Rosenblum and her colleagues.  It’s about the genetic basis of evolutionary convergence: the phenomenon in which different species, responding to the same environmental pressures, come to evolve similar traits. (I show examples of convergent marsupial and placental mammals in WEIT.)

In this case the trait is body color, and the species are three different lizards in three genera: the eastern fence lizard (Scleroporus undulatus), little striped whiptail (Aspidoscelis inornata) and lesser earless lizard (Holbrookia maculata).  Each lizard has independently evolved a white “race” on the white gypsum background at White Sands, in New Mexico (see below).  There’s little doubt that the white color is an adaptation to hide the lizards from predators; moreover, since the gypsum formation is no older than 6,000 years, this color change must have evolved fairly quickly.

Fig. 1 (from paper). “Fig. 1. Mutations associated with blanched coloration in White Sands lizards. (A) Blanched morphs from white sands on top and dark morphs in ancestral dark soil habitat on bottom. (B ) Amino acid schematic of the melanocortin-1receptor (Mc1r); replacements statistically associated with coloration in the focal taxa are shown in red.

Previous work has shown an association between color and amino-acid-replacement mutations in the same gene, Mc1r (Melanocortin 1 receptor).  Mc1r is a hormone receptor on the surface of melanocytes, and is involved in melanin synthesis.  In these species, the wild-type (active) form of Mc1r synthesizes mainly dark pigment, but its inactivation produces a lighter color, and not just in lizards.  Mc1r mutants, for example, produce coat-color changes in mice and horses, and produce red hair in humans.

The conclusion that convergence in the color of lizard skin is based on convergence at the genetic level — mutations in the same gene in three different species — is based on association studies based on wild-caught animals: when a lizard has the light color, it has a mutant form of Mc1r.  Formal genetics isn’t possible in these species, so a more rigorous conclusion was impossible.  In this study, however, functional assays of these mutants (and their wild-type alternatives) showed that, in two of the three species, the Mc1r protein had reduced activity.  In the third species (H. maculata), there was no difference in the biological activity of the mutant versus wild-type form of the protein.  This means that the “convergent” mutation they detected may be only a spurious association, not an amino-acid substitution affecting color, and so the genetic “convergence” involves two rather than three species.

Finally, genetic analysis of lizards of different colors showed that in the two species where Mc1r is involved in color changes, the mutant white-color alleles work in different ways.  In S. undulatus, heterozygotes are the same color as white individuals, so the white allele is dominant, while it’s recessive in A. inornanta.  The authors note that these dominance relationships make sense in light of the changes in the protein: the “dominant” mutant produces a protein that is able to prevent the wild-type form from integrating into membranes.  The different dominance relationships also lead to different geographic distributions of the alleles in the two species: there are a lot more copies of the “dark-color” allele in S. undulatus living on the white sands, because in that species the white allele is dominant, sheltering the alternative allele from selection in the white/nonwhite heterozygotes.  In A. inornata, on the other hand, you have to have two copies of the “white” allele to have a white color, so dark alleles are immediately removed by selection.

This is a nice example, in at least two species, of convergent mutations producing convergent phenotypes. (This is not completely novel: it’s been known for some time for insecticide resistance, where resistance to organophosphate insecticides involves changes at not only the same gene in different species, but identical amino acid changes). Moreover, functional tests tell us something about the biochemical/developmental basis for white color.  Finally, this is a case in which evolutionary change has occurred rapidly, and over a known stretch of time.

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Rosenblum, E. B. et al. 2010. Molecular and functional basis of phenotypic convergence in white lizards at White Sands.  Proc. Nat. Acad. Sci. USA, early edition (www.pnas.org/cgi/doi/10.1073/pnas.0911042107)