Premise of the study: Inbreeding depression is a major evolutionary force and an important topic in conservation genetics because habitat fragmentation leads to increased inbreeding in the populations of many species. Crosses between populations may restore heterozygosity, resulting in increased performance (heterosis), but may also lead to the disruption of coadapted gene complexes and to decreased performance (outbreeding depression).
Methods: We investigated the effects of selfing and of within and between population crosses on reproduction and the performance of two generations of offspring of the declining grassland plant Saxifraga granulata (Saxifragaceae). We also subjected the first generation of offspring to a fertilization and two stress treatments (competition and defoliation) to investigate whether the effects of inbreeding and interpopulation gene flow depend on environmental conditions.
Key results: Inbreeding depression affected all traits in the F(1) generation (δ = 0.07-0.55), but was stronger for traits expressed late during development and varied among families. The adaptive plasticity of offspring from selfing and from interpopulation crosses in response to nutrient addition was reduced. Outbreeding depression was also observed in response to stress. Multiplicative fitness of the F(2) generation after serial inbreeding was extremely low (δ > 0.99), but there was heterosis after crossing inbred lines. Outbreeding depression was not observed in the F(2).
Conclusions: Continuous inbreeding may drastically reduce the fitness of plants, but effects may be environment-dependent. When assessing the genetic effects of fragmentation and interpopulation crosses, the possible effects on the mean performance of offspring and on its adaptive plasticity should be considered.