Goncalves G, Hoekstra HE, de Freitas. TRO.
Striking coat colour variation in tuco-tucos (Rodentia: Ctenomyidae): A role for the melanocortin-1 receptor?. Biological Journal of the Linnean Society 2011;105:665-80.
AbstractSouth 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. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 665–680.
PDF Manceau M, Domingues VS, Mallarino R, Hoekstra HE.
The developmental role of Agouti in color pattern evolution. Science 2011;331:1062-5.
AbstractAnimal color patterns can affect fitness in the wild; however, little is known about the mechanisms that control their formation and subsequent evolution. We took advantage of two locally camouflaged populations of Peromyscus mice to show that the negative regulator of adult pigmentation, Agouti, also plays a key developmental role in color pattern evolution. Genetic and functional analyses showed that ventral-specific embryonic expression of Agouti establishes a prepattern by delaying the terminal differentiation of ventral melanocytes. Moreover, a skin-specific increase in both the level and spatial domain of Agouti expression prevents melanocyte maturation in a regionalized manner, resulting in a novel and adaptive color pattern. Thus, natural selection favors late-acting, tissue-specific changes in embryonic Agouti expression to produce large changes in adult color pattern.
PDF Barrett RD, Hoekstra HE.
Molecular spandrels: Tests of adaptation at the genetic level. Nature Reviews Genetics 2011;12:767-80.
AbstractAlthough much progress has been made in identifying the genes (and, in rare cases, mutations) that contribute to phenotypic variation, less is known about the effects that these genes have on fitness. Nonetheless, genes are commonly labelled as 'adaptive' if an allele has been shown to affect a phenotype with known or suspected functional importance or if patterns of nucleotide variation at the locus are consistent with positive selection. In these cases, the 'adaptive' designation may be premature and may lead to incorrect conclusions about the relationships between gene function and fitness. Experiments to test targets and agents of natural selection within a genomic context are necessary for identifying the adaptive consequences of individual alleles.
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