Environmentally independent selection for hybrids between divergent freshwater stickleback lineages in semi-natural ponds

Hybridization following secondary contact of genetically divergent populations can influence the range expansion of invasive species, though specific outcomes depend on the environmental dependence of hybrid fitness. Here, using two genetically and ecologically divergent threespine stickleback lineages that differ in their history of freshwater colonization, we estimate fitness variation of parental lineages and hybrids in semi-natural freshwater ponds with contrasting histories of nutrient loading. In our experiment, we found that fish from the older freshwater lineage (Lake Geneva) and hybrids outperformed fish from the younger freshwater lineage (Lake Constance) in terms of both growth and survival, regardless of the environmental context of our ponds. Across all ponds, hybrids exhibited the highest survival. Although wild-caught adult populations differed in their functional and defence morphology, it is unclear which of these traits underlie the fitness differences observed among juveniles in our experiment. Overall, our work suggests that when hybrid fitness is insensitive to environmental conditions, as observed here, introgression may promote population expansion into unoccupied habitats and accelerate invasion success.

Automated summary

Hybridization between genetically divergent populations can affect the expansion of invasive species, and our study shows that when hybrid fitness is not influenced by environmental conditions, introgression can promote population expansion and accelerate invasion success. Our experiment with threespine stickleback lineages in freshwater ponds demonstrated that fish from the older lineage and hybrids had better growth and survival than fish from the younger lineage, regardless of the nutrient loading history of the ponds. Hybrids exhibited the highest survival across all ponds.