Patterns of hybridization in a secondary contact zone between two passerine species, the common nightingale Luscinia megarhynchos and the thrush nightingale Luscinia luscinia

Understanding how reproductive isolation arises and accumulates between incipient species is an important goal of evolutionary biology. Patterns of interspecific hybridization in naturally occurring hybrid zones can provide an important insight into this process since they reflect the strength, symmetry and nature of reproductive barriers separating the species. Here we studied patterns of hybridization in two closely related passerine species, the common nightingale Luscinia megarhynchos and the thrush nightingale L. luscinia, that diverged similar to 1.8 Mya and co-occur in a secondary contact zone spanning across Europe. Genome-wide genotyping of more than three hundred individuals from the sympatric population and adjacent allopatric populations revealed that the vast majority of sympatric individuals were pure parental species. Only 6.5% of sympatric individuals were hybrids, from which 3.4% were F1 hybrids and 3.1% backcross hybrids from the first to the fifth backcross generation. Most F1 hybrids arose from the cross of a thrush nightingale female and a common nightingale male. F1 hybrids showed intermediate morphology and could be distinguished with high confidence from the parental species based on several diagnostic traits. However, backcrosses were morphologically difficult to distinguish from the parental species from which they inherited most of the genome. Our results suggest strong, yet incomplete, reproductive isolation between the two nightingale species both at a prezygotic and postzygotic level. Nightingales thus represent a useful model system for exploring the late stages of speciation with ongoing gene flow after secondary contact.