Convergent mutations produce convergent colors

Just a quick post on a  new paper in Proceedings of the National Academy of Sciences by Erica Bree Rosenblum and her colleagues.  It’s about the genetic basis of evolutionary convergence: the phenomenon in which different species, responding to the same environmental pressures, come to evolve similar traits. (I show examples of convergent marsupial and placental mammals in WEIT.)

In this case the trait is body color, and the species are three different lizards in three genera: the eastern fence lizard (Scleroporus undulatus), little striped whiptail (Aspidoscelis inornata) and lesser earless lizard (Holbrookia maculata).  Each lizard has independently evolved a white “race” on the white gypsum background at White Sands, in New Mexico (see below).  There’s little doubt that the white color is an adaptation to hide the lizards from predators; moreover, since the gypsum formation is no older than 6,000 years, this color change must have evolved fairly quickly.

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

Previous work has shown an association between color and amino-acid-replacement mutations in the same gene, Mc1r (Melanocortin 1 receptor).  Mc1r is a hormone receptor on the surface of melanocytes, and is involved in melanin synthesis.  In these species, the wild-type (active) form of Mc1r synthesizes mainly dark pigment, but its inactivation produces a lighter color, and not just in lizards.  Mc1r mutants, for example, produce coat-color changes in mice and horses, and produce red hair in humans.

The conclusion that convergence in the color of lizard skin is based on convergence at the genetic level — mutations in the same gene in three different species — is based on association studies based on wild-caught animals: when a lizard has the light color, it has a mutant form of Mc1r.  Formal genetics isn’t possible in these species, so a more rigorous conclusion was impossible.  In this study, however, functional assays of these mutants (and their wild-type alternatives) showed that, in two of the three species, the Mc1r protein had reduced activity.  In the third species (H. maculata), there was no difference in the biological activity of the mutant versus wild-type form of the protein.  This means that the “convergent” mutation they detected may be only a spurious association, not an amino-acid substitution affecting color, and so the genetic “convergence” involves two rather than three species.

Finally, genetic analysis of lizards of different colors showed that in the two species where Mc1r is involved in color changes, the mutant white-color alleles work in different ways.  In S. undulatus, heterozygotes are the same color as white individuals, so the white allele is dominant, while it’s recessive in A. inornanta.  The authors note that these dominance relationships make sense in light of the changes in the protein: the “dominant” mutant produces a protein that is able to prevent the wild-type form from integrating into membranes.  The different dominance relationships also lead to different geographic distributions of the alleles in the two species: there are a lot more copies of the “dark-color” allele in S. undulatus living on the white sands, because in that species the white allele is dominant, sheltering the alternative allele from selection in the white/nonwhite heterozygotes.  In A. inornata, on the other hand, you have to have two copies of the “white” allele to have a white color, so dark alleles are immediately removed by selection.

This is a nice example, in at least two species, of convergent mutations producing convergent phenotypes. (This is not completely novel: it’s been known for some time for insecticide resistance, where resistance to organophosphate insecticides involves changes at not only the same gene in different species, but identical amino acid changes). Moreover, functional tests tell us something about the biochemical/developmental basis for white color.  Finally, this is a case in which evolutionary change has occurred rapidly, and over a known stretch of time.

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Rosenblum, E. B. et al. 2010. Molecular and functional basis of phenotypic convergence in white lizards at White Sands.  Proc. Nat. Acad. Sci. USA, early edition (www.pnas.org/cgi/doi/10.1073/pnas.0911042107)



10 Comments

  1. Torbjörn Larsson, OM
    Posted January 6, 2010 at 9:15 am | Permalink

    So “genetics recapitulates phylogeny” at times?

    This is one of those where you know that it is possible, but nevertheless you are surprised to see it happen. (At least as a layman.) Happily surprised, at that.

    • Torbjörn Larsson, OM
      Posted January 6, 2010 at 9:17 am | Permalink

      D’oh! I meant “genetics recapitulates phenotypes”, of course. The danger of having to look up old and erroneous clichés.

  2. Sven DiMilo
    Posted January 6, 2010 at 9:23 am | Permalink

    Those photos are STAGED its a HOXE

  3. Sven DiMilo
    Posted January 6, 2010 at 9:26 am | Permalink

    Come Spring there will be pet-trade poachers all over those dunes.

  4. Sven DiMilo
    Posted January 6, 2010 at 9:45 am | Permalink

    Mc1r is a hormone receptor on the surface of melanocytes, and is involved in melanin synthesis. Its wild-type (active) form synthesizes dark pigment

    Well, but the receptor itself doesn’t do the synthesis, right? It starts a cascading reaction pathway that ends up activating the enzymes that do. The receptor’s job is to receive the hormone signal on the exterior cell surface and start the process that subsequently transmits the signal to the inside of the cell.

    The authors note that these dominance relationships make sense in light of the changes in the protein: the “dominant” mutant produces a protein that is able to prevent the wild-type form from integrating into membranes.

    What? I’m not following here…how does this work? Direct binding between proteins?

    Cool stuff for sure, but it’s kind of too bad that the details don’t support the nice clean story you expect from the spin. The same point mutation knocks out receptor function in one species but not another? But is equally correlated with phenotype in both? What’s up with that?

    • Sven DiMilo
      Posted January 7, 2010 at 7:48 am | Permalink

      Ah, now I see it’s not the same point mutation. I R stoopid. Never mind.

  5. Sven DiMilo
    Posted January 6, 2010 at 10:13 am | Permalink

    Finally, why are there three residues colored in the figure?

    • whyevolutionistrue
      Posted January 6, 2010 at 10:16 am | Permalink

      Because there are THREE species, each having an amino acid substitution. I suggest you read the paper if you have access to it.

  6. Posted January 6, 2010 at 2:11 pm | Permalink

    I know it’s being lazy and I could check pubmed, but is anything known about the mechanism in the rock pocket mouse (Chaetodipus intermedius) where there are 10 mutations in the Mcr1 gene that can case a dark coat?

    My favorite example of convergent evolution is the antifreeze glycoproteins of the northern Arctic cods (superorder Paracanthopterygii) and the Antarctic Nototheneiods(superorder Acanthopterygii). They are almost identical (based on repeats of the amino acid sequence (threonine – alanine (occasionally proline)- alanine). The codon usage though is totally different Boreogadus saida (arctic) uses the codons ACA/ACT (45%/42% for Threonine)-GCA/GCG (51%/30% for the first Alanine and GCA/GCG (53%/37% for the second Alanine). Dissostichus mawsoni (antarctic)uses ACA (85% for Threonine)-GCG/GCT (39%/55% for the first Alanine) and GCA (95% for the second Alanine).
    The gene in D. mawsoni is probably derived from the trypsinogen gene.

  7. Sven DIMilo
    Posted January 6, 2010 at 3:09 pm | Permalink

    Because there are THREE species, each having an amino acid substitution. I suggest you read the paper if you have access to it.

    Excellent suggestion. I would if I did, but do not so can not.

    But this was a stupid question; I misread the figure. mea culpa


4 Trackbacks/Pingbacks

  1. […] have discussed the evolution of mammalian coat colour several times on this blog, including here, here and here. It still remains a mystery, mainly because although we know a lot about the […]

  2. […] However, most examples of convergent adaptation remain at the level of form or function, rather than the genes involved. It’s possible that the same genes are involved in shaping a dolphin and an icthyosaur, but it’s unlikely we’ll ever know. An exception is the recent discovery of convergent mutations in species of sand lizards with white skins, all of which affect the melacortonin-1 receptor. But in a way, that isn’t too surprising – the melacortonin-1 receptor controls skin colour in these animals, and only a restricted number of molecular changes would give rise to an advantageous white form. […]

  3. […] Lizards in white sand deserts evolving white skins…by different mutations of the same gene […]

  4. […] such as natural selection guide which mutations get passed along) and that nature often works out different solutions to the same problem; here is another […]

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