The toad's warts: Discordance creates bumpy expectations of mitochondrial-nuclear evolution between species.

Discordance between the mitochondrial and nuclear genomes is a prevalent phenomenon in nature, in which the underlying processes responsible are considered to be important in shaping genetic variation in natural populations. Among the evolutionary processes that best explain such genomic mismatches incomplete lineage sorting and introgression are commonly identified, however, many studies are unable to distinguish between these hypotheses, which has become a major challenge in the field. In this issue of Molecular Ecology, Firneno et al. (2020) present an elegant exploration of mitochondrial-nuclear discordance in Mesoamerican toads. Integrating genome-scale and spatial data to test between these hypotheses within an empirical model testing framework, they find strong support that incomplete lineage sorting explains the observed discordance. Their work, along with many previous articles in Molecular Ecology, highlights the commonality of mito-nuclear discordance among species despite the expectations of tightly concerted mitochondrial and nuclear genome evolution. It is increasingly clear that the nuclear genomes of many species are (at least for short periods of evolutionary time) functionally compatible with multiple, divergent mitochondrial haplotypes. As such, we suggest future research not only seeks to understand the processes causing spatial mito-nuclear discordance (e.g. incomplete lineage sorting, introgression), but also explores those that maintain discordance through time and space (e.g. relaxed selection on mito-nuclear interactions, heterozygosity, population demographics). We also discuss the vital role that taxonomy plays in interpreting patterns of mito-nuclear discordance when data-consistent yet differing taxonomies are used, such as treating allopatrically distributed taxa as multiple isolated populations versus multiple micro-endemic species.