Résumé
Recently diverged taxa showing marked phenotypic and ecological diversity
are optimal systems to understand the genetic processes underlying
speciation. We used genome-wide markers to investigate the diversification
of the Reunion grey white eye (Zosterops borbonicus) on the small volcanic
island of Reunion (Mascarene archipelago), where this species complex
exhibits four geographic forms that are parapatrically distributed across
the island and differ strikingly in plumage colour. One form restricted to
the highlands is separated by a steep ecological gradient from three
distinct lowland forms which meet at narrow hybrid zones that are not
associated with environmental variables. Analyses of genomic variation
based on SNP data from genotyping-by-sequencing and pooled RADseq
approaches, reveal that signatures of selection associated with elevation
can be found at multiple regions across the genome, whereas most loci
associated with the lowland forms are located on the Z sex chromosome. We
identified TYRP1, a Z-linked colour gene, as a likely candidate locus
underlying colour variation among lowland forms. Tests of demographic
models revealed that highland and lowland forms diverged in the presence
of gene flow, and divergence has progressed as gene flow was restricted by
selection at loci across the genome. This system is promising to
investigate how natural selection and reproductive isolation shape the
genomic landscape of divergence at multiple stages of the speciation
process.