Résumé
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.