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      A Novel Role for Mc1r in the Parallel Evolution of Depigmentation in Independent Populations of the Cavefish Astyanax mexicanus

      1 , 2 , 1 , *
      PLoS Genetics
      Public Library of Science

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          The evolution of degenerate characteristics remains a poorly understood phenomenon. Only recently has the identification of mutations underlying regressive phenotypes become accessible through the use of genetic analyses. Focusing on the Mexican cave tetra Astyanax mexicanus, we describe, here, an analysis of the brown mutation, which was first described in the literature nearly 40 years ago. This phenotype causes reduced melanin content, decreased melanophore number, and brownish eyes in convergent cave forms of A. mexicanus. Crosses demonstrate non-complementation of the brown phenotype in F 2 individuals derived from two independent cave populations: Pachón and the linked Yerbaniz and Japonés caves, indicating the same locus is responsible for reduced pigmentation in these fish. While the brown mutant phenotype arose prior to the fixation of albinism in Pachón cave individuals, it is unclear whether the brown mutation arose before or after the fixation of albinism in the linked Yerbaniz/Japonés caves. Using a QTL approach combined with sequence and functional analyses, we have discovered that two distinct genetic alterations in the coding sequence of the gene Mc1r cause reduced pigmentation associated with the brown mutant phenotype in these caves. Our analysis identifies a novel role for Mc1r in the evolution of degenerative phenotypes in blind Mexican cavefish. Further, the brown phenotype has arisen independently in geographically separate caves, mediated through different mutations of the same gene. This example of parallelism indicates that certain genes are frequent targets of mutation in the repeated evolution of regressive phenotypes in cave-adapted species.

          Author Summary

          As we approach the 150th year since publication of On the Origin of Species, understanding the genetic architecture underlying evolutionary change remains an important challenge. When an organism enters a completely new environment or ecological niche, certain traits are no longer necessary for survival, while other new traits become critical for maintaining fitness. An example of such a transition is provided by cave animals. Many disparate taxa (e.g., crustaceans, salamanders, fish) have colonized caves, presumably to escape predation or expand populations into an unexploited niche. Strikingly, similar traits evolve convergently despite significant phylogenetic distance between these organisms. Caves provide a unique environment including the absence of light, few predators, few sources of food, etc. Under these conditions, one observes striking changes in morphology including reduction in eyes, expansion of non-visual sensory systems, and a suite of metabolic and behavioral changes. To understand the genetic underpinnings of these shifts, we have established the blind Mexican cave tetra, A. mexicanus, as a genetic system. In this paper, we use this system to investigate a classic morphological feature in these animals, depigmentation. We identify the gene Mc1r as being responsible for reduction in melanin content in multiple caves.

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          A single amino acid mutation contributes to adaptive beach mouse color pattern.

          Natural populations of beach mice exhibit a characteristic color pattern, relative to their mainland conspecifics, driven by natural selection for crypsis. We identified a derived, charge-changing amino acid mutation in the melanocortin-1 receptor (Mc1r) in beach mice, which decreases receptor function. In genetic crosses, allelic variation at Mc1r explains 9.8% to 36.4% of the variation in seven pigmentation traits determining color pattern. The derived Mc1r allele is present in Florida's Gulf Coast beach mice but not in Atlantic coast mice with similar light coloration, suggesting that different molecular mechanisms are responsible for convergent phenotypic evolution. Here, we link a single mutation in the coding region of a pigmentation gene to adaptive quantitative variation in the wild.
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            Genetic analysis of cavefish reveals molecular convergence in the evolution of albinism.

            The genetic basis of vertebrate morphological evolution has traditionally been very difficult to examine in naturally occurring populations. Here we describe the generation of a genome-wide linkage map to allow quantitative trait analysis of evolutionarily derived morphologies in the Mexican cave tetra, a species that has, in a series of independent caves, repeatedly evolved specialized characteristics adapted to a unique and well-studied ecological environment. We focused on the trait of albinism and discovered that it is linked to Oca2, a known pigmentation gene, in two cave populations. We found different deletions in Oca2 in each population and, using a cell-based assay, showed that both cause loss of function of the corresponding protein, OCA2. Thus, the two cave populations evolved albinism independently, through similar mutational events.
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              The G protein-coupled receptor repertoires of human and mouse.

              Diverse members of the G protein-coupled receptor (GPCR) superfamily participate in a variety of physiological functions and are major targets of pharmaceutical drugs. Here we report that the repertoire of GPCRs for endogenous ligands consists of 367 receptors in humans and 392 in mice. Included here are 26 human and 83 mouse GPCRs not previously identified. A direct comparison of GPCRs in the two species reveals an unexpected level of orthology. The evolutionary preservation of these molecules argues against functional redundancy among highly related receptors. Phylogenetic analyses cluster 60% of GPCRs according to ligand preference, allowing prediction of ligand types for dozens of orphan receptors. Expression profiling of 100 GPCRs demonstrates that most are expressed in multiple tissues and that individual tissues express multiple GPCRs. Over 90% of GPCRs are expressed in the brain. Strikingly, however, the profiles of most GPCRs are unique, yielding thousands of tissue- and cell-specific receptor combinations for the modulation of physiological processes.

                Author and article information

                Role: Editor
                PLoS Genet
                PLoS Genetics
                Public Library of Science (San Francisco, USA )
                January 2009
                January 2009
                2 January 2009
                : 5
                : 1
                : e1000326
                [1 ]Department of Genetics, Harvard Medical School, Boston, Massachusetts, United States of America
                [2 ]Cave Biology Research Group, Department of Biology, New York University, New York, New York, United States of America
                Stanford University School of Medicine, United States of America
                Author notes

                Conceived and designed the experiments: JBG RB CJT. Performed the experiments: JBG RB. Analyzed the data: JBG RB CJT. Contributed reagents/materials/analysis tools: JBG RB. Wrote the paper: JBG RB CJT.

                Gross et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
                : 16 July 2008
                : 2 December 2008
                Page count
                Pages: 14
                Research Article
                Developmental Biology/Developmental Evolution
                Ecology/Evolutionary Ecology
                Evolutionary Biology/Animal Genetics
                Genetics and Genomics



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