Convergent melanism in populations of a Solomon Island flycatcher is mediated by unique genetic mechanisms

Parallel selection pressures in independent taxa can lead to the evolution of the same phenotype, but whether selection acts on the same molecular mechanisms may depend on the genetic biases of the convergent trait. For example, despite hundreds of genes known to regulate pigmentation, the melanocortin-1 receptor (MC1R) is repeatedly implicated in convergent colour evolution in vertebrates, suggesting MC1R is a biased target of colour change. Taking advantage of the variable Monarcha castaneiventris complex, we explore whether similarity in plumage colour among populations is due to convergence, and whether this trait is mediated by the same genetic changes. Populations of M. castaneiventris comprise taxa that have diversified in colour, including two subspecies, M. c. ugiensis and M. c. obscurior, having entirely black or melanic plumage. Our previous work suggests that two populations of M. c. ugiensis have evolved melanism independently through point mutations in MC1R or in Agouti Signalling Protein, the reverse agonist of MC1R. Under a phylogenetic framework, we find that melanism in M. c. obscurior and M. c. ugiensis evolved independently within the past 500 000 years. Further, a combination of candidate gene and reduced representation sequencing approaches failed to detect mutations that predict colour in M. c. obscurior; however, we find that the melanic mutations from M. c. ugiensis were absent in M. c. obscurior. Our results suggest that melanism can evolve rapidly among independent populations, and that convergent melanism can be mediated by unique mutations, including within young clades that share genetic architectures.