Towards understanding the mechanistic basis of a sex-limited color polymorphism

The presence of multiple discrete color patterns within a species has captivated evolutionary biologists for more than a century, especially when such polymorphism is confined to one sex. The brown anole Anolis sagrei exhibits a female-limited polymorphism in dorsal patterning, which is controlled by allelic variation at the autosomal gene CCDC170. Here, we present and test a threshold model that can explain why the polymorphism is female-limited. We hypothesize that allelic variation at the CCDC170 locus affects only female color pattern because this gene is co-expressed with its neighboring gene ESR1, highly expressed in female, but not male, embryos. By manipulating embryonic estradiol levels, we show that genetic males can be induced to express the polymorphism according to allelic variation at the CCDC170 locus, which is naturally masked by low expression levels of this gene. Inversely, treating genetic females with fadrozole, which depletes estradiol, leads to monomorphic patterns irrespective of genotype, as for natural males. Using RT-qPCR, we demonstrate that these effects are accompanied by a direct influence of estradiol and fadrozole on gene expression levels of CCDC170 and ESR1, thereby validating the threshold model. Our results suggest that the CCDC170-ESR1-locus is part of a mechanistic link between the morph-determining and the sex differentiation systems and provide a causal explanation for the developmental origin of a sex-limited color polymorphism.