Effects of outcrossing distance and selling on fitness components in the rare Gentiana pneumonanthe (Gentianaceae)

Variation in offspring fitness as a result of different distances of pollen donors has been described for several plant species. In some of these, inbreeding depression after selfing or mating between close neighbours and outbreeding depression following crossing over large distances may lead to an intermediate crossing distance with optimal fitness. We studied the relationship between outcrossing distance and offspring fitness in a large population of the rare herbaceous perennial Gentiana pneumonanthe. The relative success of selfing and cross pollination with pollen from increasing distances from maternal plants, including an ‘interpopulation’ cross, was measured as seed set and seed weight, and in a consecutive glasshouse experiment as the proportion of seeds germinating, seedling weight, adult weight and total relative fitness. Regression analyses showed significant positive relationships between outcrossing distance and the fitness components seed set, seedling weight and adult weight. Additional statistics showed, however, that the observed significant regressions were mainly the result of a relatively low fitness following selfing (inbreeding depression) and a comparatively high performance of the interpopulation crosses (heterosis). No significant effects of within-population crossing distance were found when selfing and interpopulation crosses were omitted from the analyses. We therefore conclude that there is no substantial evidence for an effect of outcrossing distance on fitness components in the studied population of Gentiana pneumonanthe. The results are discussed in the context of conservation biology of rare plant species. Artificial interpopulation crossing may be a way to counteract the losses of genetic variation and fitness in small, isolated populations. In our opinion, however, this is only a feasible management option for very rare and endangered species.