Enemy release mitigates inbreeding depression in native and invasive Silene latifolia populations: experimental insight into the role of inbreeding x environment interactions in invasion success

Inbreeding and enemy infestation are common in plants and can synergistically reduce their performance. This inbreeding x environment (IxE) interaction may be of particular importance for the success of plant invasions if introduced populations experience a release from attack by natural enemies relative to their native conspecifics. Using native and invasive plant populations, we investigate whether inbreeding affects infestation damage, whether inbreeding depression in performance is mitigated by enemy release and whether genetic differentiation among native and invasive plants modifies these IxE interactions. We used the plant invader Silene latifolia and its natural enemies as a study system. We performed two generations of experimental out- and inbreeding within eight native (European) and eight invasive (North American) S. latifolia populations under controlled conditions using field-collected seeds. Subsequently, we exposed the offspring to an enemy exclusion and inclusion treatment in a common garden in the species9 native range to assess the interactive effects of population origin (range), breeding treatment and enemy treatment on infestation damage as well as plant performance. Inbreeding increased flower and leaf infestation damage in plants from both ranges, but had opposing effects on fruit damage in native versus invasive plants. Both inbreeding and enemy infestation had negative effects on plant performance, whereby inbreeding depression in fruit number was higher in enemy inclusions than exclusions in plants from both ranges. Moreover, the magnitude of inbreeding depression in fruit number was lower in invasive than native populations. Our results support that inbreeding increases enemy susceptibility of S. latifolia, which magnifies inbreeding depression in the presence of enemies. Enemy release in the invaded habitat may thus increase the persistence of inbred founder populations and thereby contribute to successful invasion. Moreover, our findings emphasize that genetic differentiation among native and invasive plants can shape the magnitude and even the direction of inbreeding effects.