Do relaxed selection and habitat temperature facilitate biased mitogenomic introgression in a narrowly endemic fish?

Introgression might be exceptionally common during the evolution of narrowly endemic species. For instance, in the springs of the small and isolated Cuatro Ciénegas Valley, the mitogenome of the cichlid fish Herichthys cyanoguttatus could be rapidly introgressing into populations of the trophically polymorphic H. minckleyi. We used a combination of genetic and environmental data to examine the factors associated with this mitochondrial introgression. A reduced representation library of over 6220 single nucleotide polymorphisms (SNPs) from the nuclear genome showed that mitochondrial introgression into H. minckleyi is biased relative to the amount of nuclear introgression. SNP assignment probabilities also indicated that cichlids with more hybrid ancestry are not more commonly female providing no support for asymmetric backcrossing or hybrid‐induced sex‐ratio distortion in generating the bias in mitochondrial introgression. Smaller effective population size in H. minckleyi inferred from the SNPs coupled with sequences of all 13 mitochondrial proteins suggests that relaxed selection on the mitogenome could be facilitating the introgression of “H. cyanoguttatus” haplotypes. Additionally, we showed that springs with colder temperatures had greater amounts of mitochondrial introgression from H. cyanoguttatus. Relaxed selection in H. minckleyi coupled with temperature‐related molecular adaptation could be facilitating mitogenomic introgression into H. minckleyi.