When populations hybridize during secondary contact, introgression can enable rapid adaptation to novel environmental conditions. This is particularly relevant for invasive, or range expanding species, as it can influence the dynamics of spread. Introductions derived from multiple source populations can increase genetic diversity and allow invasive hybrid swarms to spread across a range of conditions. Therefore, observing secondary contact between introduced allopatric lineages in nature, and performing experimental tests of hybrid fitness in contrasting environments, allows us to explore the process of adaptive introgression. Here we are interested in two divergent threespine stickleback lineages that were introduced to Switzerland ~150 years ago. The western European G. gymnurus was introduced to Lake Geneva, while the northern European G. aculeatus was introduced to Lake Constance. They are now hybridizing in the wild. Though both lineages were introduced at roughly the same time, they are vastly different with respect to their age of freshwater colonization, with the Geneva lineage having a longer history of adaptation. We tested for fitness differences between parental lineages and F1 hybrids using a semi-natural pond experiment, in ponds with contrasting eutrophication histories. We also compared morphological traits between wild-caught adults, as these lineages differ in niche use.
Results/Conclusions
In this study, we found that fish from the Geneva lineage, and F1 hybrids consistently outperformed fish from the Constance lineage in both growth and survival, regardless of the environmental context that they experienced. Across all ponds, F1 hybrids exhibited the highest survival. While this result may indicate heterosis, it could also have implications for the outcomes of secondary contact in natural settings. Previous work has shown that stream populations in Lake Constance have a higher proportion of hybrid individuals than lake populations, thus introgression from the Western lineage seems to enhance fitness in stream habitats. Adult populations differed significantly in a number of functionally relevant morphological characteristics (e.g. gape width, mouth protrusion, suction force), suggesting that these populations are indeed adapted to specialize on different prey types. Whether the divergence in trophic morphology is responsible for fitness differences in these habitats remains to be tested. This work suggests that hybridization between these divergent stickleback lineages may therefore allow the population to expand into unoccupied habitats.
