Posted by Hanna Laakkonen
Title: Identifying the genomic targets of natural selection on bill morphology in the Inaccessible Island finch Nesospiza acunhae
Abstract: Adaptation to food sources is a key element in divergence processes, and may promote ecological speciation in the face of gene flow. In birds, the size and shape of the bill is the target for natural selection on feeding efficiency, and it can also affect male song, a sexually selected trait. Here, we investigate the Inaccessible Island finch Nesospiza acunhae, part of a small radiation endemic to the South Atlantic islands of Tristan da Cunha. Inaccessible Island hosts two subspecies: a smallbilled generalist feeding mainly on grass seeds, and a large-billed specialist foraging on the seeds of Phylica trees. Notably, despite the extremely small geographic scale – Inaccessible Island is only 16 km2 – there is simultaneous occurrence of strong bill size-associated assortative mating, and parallel hybridization between the two subspecies in confined parts of the highland plateau. Here we report on the genomic landscape of bill morphology and detect two genomic regions with significantly elevated differentiation between small- and large-billed birds. The first region, on chromosome 1, contains 4–6 genes, of which one (KPNA3) is part of a large pathway where several bill morphology candidate genes are included. The second region, residing on the sex chromosome Z, contains 8 genes, all of which are associated with previously known bill morphology candidate genes. One of them is ISL1, a homeobox transcription factor included in the neural crest differentiation pathway, which affects bill morphology in Darwin’s finches through the expression of bone morphogenetic proteins. The effect of the combined chromosome 1/Z genotypes on bill phenotype indicates an epistatic interaction between the regions. Our results show that minute proportions of the genome contribute to extreme bill variation in the Inaccessible Island finch, which in turn is associated with strong differential food preference, habitat selection and assortative mating.