Date Published: July 02, 2013
Publisher: Blackwell Publishing Ltd
Author(s): Wynn K Meyer, Sidi Zhang, Sachiko Hayakawa, Hiroo Imai, Molly Przeworski.
How many distinct molecular paths lead to the same phenotype? One approach to this question has been to examine the genetic basis of convergent traits, which likely evolved repeatedly under a shared selective pressure. We investigated the convergent phenotype of blue iris pigmentation, which has arisen independently in four primate lineages: humans, blue-eyed black lemurs, Japanese macaques, and spider monkeys. Characterizing the phenotype across these species, we found that the variation within the blue-eyed subsets of each species occupies strongly overlapping regions of CIE L*a*b* color space. Yet whereas Japanese macaques and humans display continuous variation, the phenotypes of blue-eyed black lemurs and their sister species (whose irises are brown) occupy more clustered subspaces. Variation in an enhancer of OCA2 is primarily responsible for the phenotypic difference between humans with blue and brown irises. In the orthologous region, we found no variant that distinguishes the two lemur species or associates with quantitative phenotypic variation in Japanese macaques. Given the high similarity between the blue iris phenotypes in these species and that in humans, this finding implies that evolution has used different molecular paths to reach the same end. Am J Phys Anthropol 151:398–407, 2013.© 2013 Wiley Periodicals, Inc.
Our quantitative analysis of iris color variation in the four primate species known to have blue irises revealed the blue iris phenotype to be highly similar across species (Fig. 1). Yet our sequencing results indicated that no variant within the region homologous to the causal regulatory region in humans is solely responsible for blue iris pigmentation in either blue-eyed black lemurs or Japanese macaques. Regardless of the genetic architecture underlying the phenotypic variation in these other organisms, the genotype-phenotype map clearly differs from that in humans. Given that the derived phenotype displays marked similarity among species, this implies some level of flexibility in the evolution of this trait.