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.

The Morphology.txt file contains the Procrustes coordinates from 19 landmarks that were set across the right forewing of Erebia butterflies (for each landmark the respective x and y coordinates are indicated). For a visual depiction of the landmarks please see Figure S1 of the study. For each specimen its ID, species, collection date, latitude (°N), longitude (°E) and sex are indicated.

The Niche.txt contains the ecological data for each specimen that were used for the multivariate niche reconstruction.
For each specimen its ID, species, collection date, latitude (°N), longitude (°E) and sex are indicated. Additional variables are:
clou7          monthly mean cloudines July
clou8          monthly mean cloudines August
clou9          monthly mean cloudines September
mind7          monthly moisture index July
mind8          monthly moisture index August
mind9          monthly moisture index September
prec7          monthly mean precipitation July
prec8          monthly mean precipitation August
prec9          monthly mean precipitation September
tave7          monthly mean of average temperature July
tave8          monthly mean of average temperature August
tave9          monthly mean of average temperature September
ddeg        annual degreedays with 3°C threshold
slope_25       slope
tmax_sum25     maximum temperature in Summer
tmax_wint25    maximum temperature in Winter
tmin_fall25    minimal temperature in Fall
tmin_spr25     minimal temperature in Spring
swb            annual average site water balance
sfroyy         annual average # of frost days during growing season
pday           # of precipitation days per growing season