Striking coat colour variation in tuco-tucos (Rodentia: Ctenomyidae): a role for the melanocortin-1 receptor?

Abstract

South American tuco-tucos (Ctenomys) are characterized by striking variation in coat colour. A range of phenotypes, from pale blonde to dark black, is observed across species, with some of them matching their local substrate colour. Moreover, phenotypic convergence is evident in some taxa that occupy similar habitats. The present study investigated a role for the melanocortin-1 receptor (Mc1r) in determining coat-colour variation in a wide range of Ctenomys species. We sequenced 1250 bp, including the entire Mc1r coding region and a portion of the adjacent 5′ and 3′ untranslated regions, in 21 species. In total, 20 amino acid replacements were identified in Mc1r. However, our findings suggest that these changes have not contributed to coat-colour differences among tuco-tucos because no amino acid replacement was associated with pigmentation phenotype in a simple way. Levels of Mc1r expression were measured in skin samples from dorsal, flank, and ventral body regions in pale, brown, and melanic individuals. We did not observe any significant difference in transcript abundance among phenotypes, although we identified a significant reduction of expression level from the dorsal to ventral region in both pale and brown morphs but not in the completely melanic form. Thus, a role for Mc1r regulation in tuco-tucos colour pattern cannot be completely ruled out, although further functional assays are needed. Finally, selection analysis suggests that Mc1r, in a majority of lineages, has evolved under purifying selection but with relaxation in functional constraint in some regions, especially in the fourth transmembrane domain. In summary, the results obtained in the present study suggest that this trait may have a complex basis, and that other pigmentation genes are involved in generating the dramatic diversity in coat-colour phenotypes observed among Ctenomys species.