The genetic basis of phenotypic convergence in beach mice: similar pigment patterns but different genes

Convergent evolution is a widespread phenomenon seen in diverseorganisms inhabiting similar selective environments. However,it is unclear if similar phenotypes are produced by the sameor different genes and mutations. Here we analyze the molecularmechanisms underlying convergent pigment pattern among subspeciesof the beach mouse (Peromyscus polionotus) inhabiting the Gulfand Atlantic coasts of Florida. In these two geographic regions,separated by more than 300 km, "beach mice" have lighter coloredcoats than do their mainland counterparts, produced by naturalselection for camouflage against the pale coastal sand dunes.We measured color pattern in eight beach mouse subspecies andshowed that three of the Gulf Coast subspecies are more phenotypicallysimilar to an Atlantic coast subspecies than to their Gulf Coastneighbors. However, light-colored beach mice do not form a monophyleticgroup. Previous results implicated a single derived amino acidchange in the melanocortin-1 receptor (Mc1r) as a major contributorto pigment pattern in the Gulf Coast beach mice; despite phenotypicsimilarities, the derived Mc1r allele was not found in the Atlanticcoast beach mouse populations. Here we show that Atlantic coastbeach mice have high levels of Mc1r polymorphism but they lackunique alleles. Functional assays revealed that single aminoacid mutations segregating in Atlantic coast beach mice do notcause any change in Mc1r activity compared with the activityof Mc1r from dark-colored mice. These joint results show thatconvergent pigment patterns in recently diverged beach mousesubspecies—whose developmental constraints are presumablysimilar—have evolved through a diversity of genetic mechanisms.