World Congress Of Herpetology 7: Anole Color Evolution

Wrapping up our coverage of the World Congress of Herpetology held in Vancouver last week, I have a report on Nick Crawford’s talk on the genetics of colorful pigmentation in Anolis. Nick began by talking about the basic types of pigments that contribute to anole coloration, which include both pteridines and carotenoids. Synthesis of pteridines is much better understood, thanks largely to work on zebra fish (reviewed in Braasch et al. 2007). Nick first showed preliminary evidence from rtPCR analyses suggesting that specific genes along the pteridine synthesis pathway differ in predictable ways among parts of anoles with different coloration (e.g., white venter, green dorsum, pink dewlap).

Crawford went on to note that pteridines may be less important to dewlap coloration than are carotenoids, but that the latter represent a bit of a black box genetically and developmentally. Crawford then discussed a project in which he uses a bulk segregant approach to ask if regions of the genome associated with color differentiation can be identified by examining genomic sequence data from species with polymorphic coloration. Crawford was particularly interested in the polymorphic Lesser Antillean Anolis marmoratus. He obtained sequence data from two phenotypically distinct populations of this species using the Illumina hiSeq platform. Fortunately this data could be aligned to the A. carolinensis genome, and showed a relatively high degree of synteny with this previously published genome. Analyses of the new A. marmoratus dataset are still in their early stages, but preliminary analyses recover 1,300 fixed SNPs (only 330 of which appear to be exonic) and suggest the presence of genomic islands of differentiation similar to those reported in many other recently diverged species and incipient species.

Note Added in Press:

One talk we failed to cover at WCH was by Chris Schneider on a similar topic. Here’s the Abstract:

Schneider, Christopher (Boston University); Crawford, Nicholas; McGreevy, TJ; Messana, Nick (Boston University, Canada)

The genetic basis of phenotypic variation and divergence in Anolis marmoratus

Understanding the process of population divergence and speciation is a central goal of evolutionary biology. Ultimately, understanding this process requires that we link phenotypic variation to underlying genetic variation. Traditionally, such analyses were restricted to model organisms. However, advances in genomic technology, and complete genome sequences of phylogenetically diverse vertebrates, have opened the opportunity to link phenotype to genotype in non-model organisms. Here we examine the genetic basis of phenotypic variation in Anolis marmoratus, a polytypic species complex from the Caribbean islands of Guadeloupe. We examine genetic variation on a genome-wide scale with RAD-tag and bulk-segregant analyses and identify loci that show signatures of divergent natural selection. We are able to relate variation at many of those loci to patterns of phenotypic variation among populations, providing candidate loci for studies of adaptive divergence. Interestingly, on a genome-wide scale, there is little evidence for restricted gene flow among populations and, while portions of the genome show signatures of adaptive divergence, most of the genome remains homogeneous. This suggests that the genome is porous, with segments of the genome (adaptive islands) maintained at alternate states by divergent selection while the rest of the genome is homogenized by gene flow and recombination. We discuss the implications of these results for understanding the genetics of adaptation and speciation in Anolis.