Range expansion underlies historical introgressive hybridization in the Iberian hare

Introgressive hybridization is an important and widespread evolutionary process, but the relative roles of neutral demography and natural selection in promoting massive introgression are difficult to assess and an important matter of debate. Hares from the Iberian Peninsula provide an appropriate system to study this question. In its northern range, the Iberian hare, Lepus granatensis, shows a northwards gradient of increasing mitochondrial DNA (mtDNA) introgression from the arctic/boreal L. timidus, which it presumably replaced after the last glacial maximum. Here, we asked whether a south-north expansion wave of L. granatensis into L. timidus territory could underlie mtDNA introgression, and whether nuclear genes interacting with mitochondria ("mitonuc" genes) were affected. We extended previous RNA-sequencing and produced a comprehensive annotated transcriptome assembly for L. granatensis. We then genotyped 100 discovered nuclear SNPs in 317 specimens spanning the species range. The distribution of allele frequencies across populations suggests a northwards range expansion, particularly in the region of mtDNA introgression. We found no correlation between variants at 39 mitonuc genes and mtDNA introgression frequency. Whether the nuclear and mitochondrial genomes coevolved will need a thorough investigation of the hundreds of mitonuc genes, but range expansion and species replacement likely promoted massive mtDNA introgression.