A molecular genetic investigation into gene flow and secondary contact in island sister taxa

Abstract

Investigations of colonization patterns have contributed to fundamental advances in evolutionary and ecological theory. Once populations have differentiated, secondary contact can lead either to the reinforcement of species boundaries or interbreeding. Its effects on the maintenance of biodiversity are not well known. Archipelagos like the Philippines are useful for studying the relationships of secondary contact because of their distinct geographic boundaries and simpler land vertebrate biota. A genus-level phylogeny of Philippine Tailorbirds (Passeriformes: Cisticolidae) inferred a deep split between two sister species on the island of Luzon. Some populations of Orthotomus chloronotus and O. derbianus co-occur parts of their range but they are phenotypically distinct. Because of that distinctiveness, these species were traditionally considered evolutionarily independent. Recently however, molecular and morphological evidence found evidence of gene flow between them (Sheldon et al. 2012). To investigate the extent of reproductive isolation between these species, molecular data was collected from populations across Luzon Island and three classes of DNA markers were sequenced: mitochondrial, autosomal, and sex-linked. The sex-linked chromosome was Green-backed Orthotomus chloronotus and gray-backed O. derbianus are paraphyletic at all loci; there is complete introgression in the sex chromosome, strong population structure in autosomal chromosomes, and strong phylogenetic structure in mitochondrial genes. These findings contradict taxonomic classification; allopatric populations of green- and gray-backed birds (respectively North and South Luzon) are genetically distinct from the sympatric population in Central Luzon. The southern population shows early signs of diverging from the rest of Luzon Orthotomus and coincides with the limits of the green-backed phenotype. The sympatric zone, Central Luzon, shows no signs of past or present genetic isolation, and this population is exchanging significant amounts of alleles with North Luzon green-backed birds. The dataset finds no evidence of genes segregating by plumage in Central Luzon. This leaves unanswered questions about the emergence of the derived phenotype, whether plumage is stochastic or under selective pressure, and the amount of data necessary to statistically evaluate complicated evolutionary hypotheses in natural populations.