Bigger, badder males drive unbalanced genetic introgression in wall lizards

Female mate choice is one of the major barriers to hybridization. But even when hybridization does happen, females are often identified as the primary drivers of how phenotypes move through a hybrid zone. A new paper appearing in Ecology Letters by While et al. goes against this trend by showing that male dominance drives asymmetric introgression in common wall lizards (Podarcis muralis).
In a very cool and integrative paper, While et al. established phenotypes (morphology, coloration, bite force and testes mass) likely under sexual selection in the core ranges of two wall lizard lineages (one from western Europe and one from northern Italy). They then created experimental populations that mimic a hybrid zone to describe male-male interactions and any asymmetries in reproductive success, including the survival of F1 hybrids. To make sure that any differences in mating success by the males of each lineage weren’t due to some post-copulatory mechanism, they additionally conducted sperm competition trials.
The Italian lineage males were all-around more competitive (larger heads, stronger bites, greater testes mass) and these phenotypes lead to dominance over males from the Western European lineage in the experimental populations: females from the Western European lineage were much more likely to give birth to young fathered by the opposite lineage.
The authors then looked at genetic patterns of introgression at three locations (one natural hybrid zone and two introduced areas) to understand if the results of their experiments match the observed patterns of genetic introgression in the wild. As predicted, the natural area of hybridization between the two lineages in northwestern Italy shows a much greater westward cline shift for microsatellites compared to that by mitochondrial DNA.

Our results show that divergence in male competitive ability in allopatry causes asymmetric hybridisation and gene flow upon secondary contact in wall lizards. As a consequence, sexually selected introgression shapes phenotypic and genetic variation in both native and non-native populations.

Male-male conflict may be a driver of genetic introgression in systems only when female choice is relatively weak (like in lizards), but this paper provides 1) an interesting perspective on how asymmetries in introgression are perpetuated and 2) a reason to investigate this phenomenon in other vertebrates that have varying strengths of female mate-choice.
While, G. M., Michaelides, S., Heathcote, R. J., MacGregor, H. E., Zajac, N., Beninde, J., … & Uller, T. (2015). Sexual selection drives asymmetric introgression in wall lizards. Ecology Letters. DOI: 10.1111/ele.12531

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