Genetics and the Origin of Two Flycatcher Species

Sammanfattning: In this thesis, different genetic tools are used to investigate pre- and postzygotic barriers to gene exchange and their role in speciation in the pied flycatcher (Ficedula hypoleuca) and the collared flycatcher (F. albicollis). This species complex consists of four genetically distinct clades that apparently diverged in allopatry (I). Sequencing of introns from autosomal and Z-linked genes from the two species reveals signs of selection on the Z-chromosome. Sexual selection acting on Z-linked genes might explain this pattern (II). By using large-scale genotyping of single nucleotide polymorphisms (SNPs), introgression is observed at autosomal- but not Z-linked loci, mostly from the pied- to the collared flycatcher. Male plumage characters and genes involved in hybrid fitness are largely mapped to the Z-chromosome (III). By studying mate choice of female hybrids I show that there is a link between female preferences and the Z chromosome (IV). The rate of introgression in island versus clinal hybrid zones is consistent with regional differences in hybrid fertility. Asymmetric gene flow from allopatry on the islands may oppose reinforcement, leading to introgression and a partial breakdown of postzygotic isolation. Adaptive introgression may explain the high rate of introgression observed at one of the genetic markers (V). For late breeding female collared flycatchers it appears to be adaptive to use pied flycatchers as social fathers but conspecific males as genetic fathers. Additionally, females in mixed species pairs may reduce hybridization costs by producing an excess of male hybrid offspring that are more fertile than females (VI).In conclusion, the Z-chromosome appears to play a major role in flycatcher speciation. Sexual selection and reinforcement are important mechanisms in the divergence of these birds. However, gene flow from allopatry, introgression of adaptive genes and adaptive hetrospecific pairing by late breeding collared flycatcher females may work in the opposite direction.