The molecular basis of the plumage colour polymorphism in the Tahiti reed-warbler Acrocephalus caffer

Th e endemic Tahiti reed-warbler Acrocephalus caff er occurs in two distinct morphs, a typical or ‘ yellow ’ morph and a melanic or ‘ dark ’ morph, which are found together in the valleys of the eastern and central parts of the island of Tahiti (Society Islands, French Polynesia). We investigated the molecular basis of the plumage colour polymorphism in this species using sequences of the melanocortin-1 receptor (MC1R), a gene often found associated to melanism in birds. We found that the MC1R genotype was perfectly associated with plumage colour in the Tahiti reed-warbler, with the same nonsynonymous substitution that showed a correlation with phenotype in the Caribbean bananaquit Coereba fl aveola . An heterozygous reed-warbler at this site presented a melanic phenotype, suggesting that the melanic allele is dominant. All other Polynesian reed-warbler species, which do not have a melanic morph, shared the ‘ yellow ’ nucleotide at this position. Th ese results suggested that the same mutation point was linked to a melanic polymorphism in two unrelated passerine birds. Polymorphism in colour is a widespread phenomenon in birds and in other vertebrates: it is generally defi ned by the presence within a species of individuals of the same sex and age that have diff erent genetically determined coloration (Buckley 1987). Th e term ‘ morph ’ was fi rst proposed by Huxley (1955) and is generally used for the discrete categories of plumage. In their review of avian polymorphism, Galeotti et al. (2003) showed that changes in the distribution of the melanic pigments, with the defi nition of a ‘ dark ’ or melanic morph, is the most frequent polymorphism