Evolution 2013: The Genomics Of Anole Pigmentation

Figure from Nicholson et al. (2007) showing variation in dewlap color among various species of anoles.

Figure from Nicholson et al. (2007) showing variation in dewlap color among various species of anoles.

As Nick Crawford, recent Ph.D. of Boston University, points out, the genomics era allows scientists unprecedented access to understanding the genetic basis of adaptation and, by extension, the genetics of speciation. For his doctoral thesis, Nick focused on understanding the genetics of colorful adaptation in Anolis lizards, which is genome-enabled. Adaptive radiations provide lots of variation among closely related organisms, making anoles a great system for studying the genetics of adaptation.

One feature of anoles that really stands out is how colorful they are. Just a casual glance at some of the color variation in dewlaps among species reveals that color is likely an important component of species diversification in anoles. Nick focused on Anolis marmoratus, a colorful anole from the Caribbean island of Guadeloupe. Anolis marmoratus is an excellent choice for studying the genetics underlying color. This species exhibits strong geographic variation in coloration and, as I discussed in my talk a few days ago, lacks a strong signal of genetic structure. In this case, searching for the genes underlying local adaptation can be conducted without the confounding effects of population structure.

One of Nick's slides showing the ranges of A. m. marmoratus (orange color) and A. m. speciosus (blue) on the islands of Basse Terre (left) and Grande Terre (right) in Guadeloupe.

One of Nick’s slides showing the ranges of A. m. marmoratus (orange color) and A. m. speciosus (blue) on the islands of Basse Terre (left) and Grande Terre (right) in Guadeloupe.

Nick focused on A. m. marmoratus, which has red marbling on its head, and A. m. speciosus, which has a blue head and, oftentimes, a blue body and tail. These two species are clinally distributed along the eastern side of Basse Terre and A. m. speciosus ranges into the nearby island of Grande Terre (see Figure 1). Rather than use RAD tags, Nick sequenced the genomes for 20 individuals (10 each per subspecies). For every 5 kb along the genome, Nick measured divergence using various metrics of structure and assessed sequence divergence.

Overall, Nick found that about 2% of the genome falls within divergent regions for these two subspecies. Importantly, he found divergence in two genes involved in carotenoid pigmentation and one gene involved in melanosome transport. Divergence in the two carotenoid genes could very well underlie the color divergence in A. m. marmoratus, which has distinct red marbling on its head. These genes fall in regions containing several fixed single nucleotide polymorphisms (SNPs) in a row. Nick suggests that these are likely single haplotypes that are being selected in different environments. Finally, he found no evidence of coding sequence changes, and so he posits that the modifications are probably cis-regulatory in nature. For many years we have been waiting to find out how divergence in coloration occurs in anoles. After seeing Nick’s work, it appears we are closer than ever before to understanding local color adaptation at a genomic level, so stayed tuned to his work for more to come.

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2 Comments

  1. Daniel Scantlebury

    Is there any role for epigenetics in this system?
    http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010326

    • It certainly seems possible. However, my results don’t suggest that it plays a strong role – at least relative to divergence in nucleotide sequence.

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