Behavioural foundation of a massive mitochondrial introgression in the fire salamander, Salamandra salamandra

Patterns of mito-nuclear discordance across secondary contact zones have been reported in a wide range of organisms. They consist of a spatial mismatch between nuclear and mitochondrial genomes, in terms of geographic location and/or extension of the contact zone between distinct evolutionary lineages. Several theoretical and empirical studies have identified massive mitochondrial introgression as putative source of these mismatches. Yet, we still lack a clear understanding of the phenotypic underpinnings of such massive introgression. We hypothesised that mtDNA variation across a contact zone could be associated with variation in phenotypic traits affecting dispersal propensity. We analysed patterns of behavioural and genetic variation across a mtDNA secondary contact zone of the fire salamander Salamandra salamandra in central Italy, which is over 600 km displaced from its nuclear counterpart. We found distinct behavioural profiles associated with the two co-occurring mitotypes. Counterintuitively, the introgressed mitotype was associated to a ‘slow-thorough’ dispersal profile, characterized by shy, less active and less exploratory personality traits. This pattern was consistent across life-stages and contexts, i.e., aquatic larvae and terrestrial juveniles. These results provide for the first time robust evidence that personality traits associated with distinct mitotypes could promote differential mitochondrial introgression within alternative nuclear backgrounds.