Weak but repeated patterns of co-introgression of nuclear OXPHOS genes and mitochondrial DNA in Iberian wall lizards

In this study, we took advantage of the previously reported instances of mitochondrial DNA capture in the Podarcis Iberian group, a speciose group of Iberian wall lizards, to test the hypothesis that nuclear genes from the OXPHOS (Oxidative phosphorylation) chain can co-introgress with the mitochondria as an evolutionary response to mitigate the costs of mitonuclear incompatibilities. Using dense population sampling and transcriptome data, we generated capture-sequence datasets for nuclear OXPHOS chain genes (nucOXPHOS), random nuclear loci (nucControl) and the complete mitochondrial genome. Phylogenetic analyses of nuclear and mitochondrial genes confirmed two previously identified events of mitochondrial introgression in the Podarcis Iberian group and revealed two new cases. Three of these cases have led to complete local mitochondrial DNA replacements, where the introgressed mitotypes have replaced the native ones in several populations, and involve a currently unknown and presumably extinct donor species, so-called "ghost lineage". Detecting introgression from ghost lineages, whose genomes are not accessible, remains challenging. To overcome this issue, we designed or adapted several tests aimed at detecting differential signals of introgression between our nucOXPHOS and nucControl gene sets. One of these tests, based on the effects of introgression on branch lengths in phylogenetic trees, uncovered a weak but consistently significant signal of partial co-introgression of nucOXPHOS genes compared to the genomic background (represented by the nucControl gene set) in three out of four cases of mtDNA introgression.