Secondary contact zones of closely-related Erebia butterflies overlap with narrow phenotypic and parasitic clines

Zones of secondary contact between closely related taxa are a common
legacy of the Quaternary ice ages. Despite their abundance, the factors
that keep species apart and prevent hybridisation are often unknown. Here
we study a very narrow contact zone between three closely related
butterfly species of the Erebia tyndarus species complex. Using genomic
data, we first determined if gene flow occurs and then assessed whether it
might be hampered by differences in chromosome number between some
species. We found interspecific gene flow between sibling species that
differ in karyotype by one chromosome. Conversely, only F1 hybrids
occurred between two species that have the same karyotype, forming a steep
genomic cline. In a second step, we fitted clines to phenotypic,
ecological and parasitic data to identify the factors associated with the
genetic cline. We found clines for phenotypic data and the prevalence of
the endosymbiont parasite Wolbachia to overlap with the genetic cline,
suggesting that they might be drivers for separating the two species.
Overall our results highlight that some gene flow is possible between
closely-related species despite different chromosome numbers, but that
other barriers restrict such gene flow.