Hybridization in late stages of speciation: strong but incomplete
genome-wide reproductive isolation and “large Z-effect” in a moving
hybrid zone
Abstract
In organisms reproducing sexually, speciation occurs when increasing
divergence results in pre- or post-zygotic reproductive isolation
between lineages. Studies focusing on reproductive isolation origin in
early stages of speciation are common. Many rely on indirect measures of
introgression providing limited information on the genomic architecture
of reproductive isolation maintenance in the long term. This study
focuses on direct measures of introgression between two species in a
late stage of speciation. We used ddRADseq genotyping in a natural
hybrid zone between Podarcis bocagei and P. carbonelli to
examine admixture extent, estimate effective selection, analyse hybrid
zone stability, and assess variation in selection against introgression
across the genome. Hybridization was confirmed the narrow and bimodal
hybrid zone demonstrating the existence of strong mechanisms of
reproductive isolation. Simulations suggested that simple premating
barriers were not enough to explain the observed distribution of
admixture classes, pointing out the role of post-mating isolation. A
geographic cline approach confirmed strong reproductive isolation and
high effective selection preventing extensive introgression outside of
the contact zone; and detected a signal of hybrid zone movement towards
P. bocagei distribution. Genomic cline analysis revealed
heterogeneous patterns of introgression among loci within the syntopy
zone, but most of the loci do not introgress more or less than the
genomic average maintaining a strong association with the genomic
background of origin. However, genomic clines can be driven by
confounding effects resulting in incongruences between both cline
approaches. Importantly, overall patterns of introgression seem to
result from strong intrinsic barriers across the genome, without clear
“islands of differentiation”. Last, an important role of the Z
chromosome in reproductive isolation is suggested.