Data for: Multi-generational fitness effects of natural immigration indicate strong heterosis and epistatic breakdown in a wild bird population

The fitness of immigrants and their descendants produced within recipient
populations fundamentally underpins the genetic and population dynamic
consequences of immigration. Immigrants can in principle induce
contrasting genetic effects on fitness across generations, reflecting
multi-faceted additive, dominance, and epistatic effects. Yet, full
multi-generational and sex-specific fitness effects of regular immigration
have not been quantified within naturally structured systems, precluding
inference on underlying genetic architectures and population outcomes. We
used four decades of song sparrow (Melospiza melodia) life-history and
pedigree data to quantify fitness of natural immigrants, natives, and
their F1, F2, and backcross descendants, and test for evidence of
non-additive genetic effects. Values of key fitness components (including
adult lifetime reproductive success and zygote survival) of F1 offspring
of immigrant-native matings substantially exceeded their parent mean,
indicating strong heterosis. Meanwhile, F2 offspring of F1-F1 matings had
notably low values, indicating surprisingly strong epistatic breakdown.
Further, magnitudes of effects varied among fitness components, and
differed between females and males descendants. These results demonstrate
that strong non-additive genetic effects on fitness can arise within
weakly structured and fragmented populations experiencing frequent natural
immigration. Such effects will substantially affect the net degree of
effective gene flow and resulting local genetic introgression and
adaptation.