1. Hybridization is common in natural plant populations. Trait expression and ecological performance of hybrids determine the consequences of hybridization such as the degree and direction of gene flow or the generation of phenotypic novelty.
2. We investigated responses to shade and drought stress in crosses within the naturally hybridizing campions Silene dioica and S. latifolia and reciprocal crosses between them. We collected data on fitness proxies and on leaf and root traits in a 2-year greenhouse experiment.
3. Responses to drought stress did not differ between cross types. Shade stress, in contrast, led to a reduced flowering incidence in S. dioica but not in S. latifolia. Rapid flowering under stress conditions in S. latifolia could be an adaptation to disturbance in its habitat, whereas a delay of reproduction might be adaptive in the more predictable environment of S. dioica.
4. Hybrids exhibited intermediate, parental-like and transgressive trait expression. Both hybrid cross types were similar to S. latifolia in terms of biomass production possibly because of dominance of S. latifolia alleles or heterosis. Hybrids further had a strongly reduced flowering incidence under shade stress as did S. dioica, suggesting dominance of S. dioica alleles for flower induction. Under shade stress, both hybrid cross types produced much larger leaves than either of the two species suggesting that epigenetic interactions are disturbed. Reciprocal hybrids did not differ in fitness; however, maternal effects were observed for root cross-sectional area and mass per male flower, possibly supporting asymmetric gene flow in natural populations.
5. Synthesis. Silene latifolia and S. dioica responded to stress with differences in life history rather than in growth. Our results further suggest that different modes of gene action are responsible for the specific combination of intermediate, parental-like and transgressive traits observed in first-generation hybrids that may limit their performance and thus gene flow between the species.