Molecular and functional basis of phenotypic convergence in white lizards at White Sands

Abstract

There are many striking examples of phenotypic convergence in nature, in some cases associated with changes in the same genes. But even mutations in the same gene may have different biochemical properties and thus different evolutionary consequences. Here we dissect the molecular mechanism of convergent evolution in three lizard species with blanched coloration on the gypsum dunes of White Sands, New Mexico. These White Sands forms have rapidly evolved cryptic coloration in the last few thousand years, presumably to avoid predation. We use cell-based assays to demonstrate that independent mutations in the same gene underlie the convergent blanched phenotypes in two of the three species. Although the same gene contributes to light phenotypes in these White Sands populations, the specific molecular mechanisms leading to reduced melanin production are different. In one case, mutations affect receptor signaling and in the other, the ability of the receptor to integrate into the melanocyte membrane. These functional differences have important ramifications at the organismal level. Derived alleles in the two species show opposite dominance patterns, which in turn affect their visibility to selection and the spatial distribution of alleles across habitats. Our results demonstrate that even when the same gene is responsible for phenotypic convergence, differences in molecular mechanism can have dramatic consequences on trait expression and ultimately the adaptive trajectory.

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-1 receptor (Mc1r); replacements statistically associated with coloration in the focal taxa are shown in red.

Fig. 2.

Functional assays for Mc1r in White Sands lizards. (A) Cell-based assays identifying partial loss-of-function Mc1r alleles for blanched S. undulatus and A. inornata. Derived and wild-type alleles in a mammalian expression vector were tested for basal and agonist-induced cAMP accumulation in response to increasing concentrations of α-MSH. Green fluorescent protein (GFP) plasmid-transfected cells served as controls. (B) ELISAs identifying reduced cell-surface expression for the derived S. undulatus Mc1r allele. Specific optical density (OD) readings (OD value of HA-tagged construct minus OD value of control-transfected cells) are given for each species as a percentage of the wild-type variant for total and cell-surface expression.

Fig. 3.

Dominance relationships of Mc1r alleles. Dorsal coloration (mean and standard deviation for area under the spectral curve) for Mc1r genotypes showing the derived allele is dominant in S. undulatus and recessive in A. inornata. n, number of alleles sampled; “light” and “dark” refer to statistically distinguishable groups.

Fig. 4.

Spatial distribution of Mc1r alleles in the wild for S. undulatus (derived Mc1r allele dominant) and A. inornata (derived Mc1r allele recessive). Proportion of wild-type (black) and derived (white) alleles across dark soil, ecotone, and white sand habitat. n, number of alleles sampled.