Population genetic structure and plant fitness of natural and ex situ populations in Silene chlorantha (WILLD.) EHRH. and Silene otites (L.) WIBEL

Within the presented thesis „Population genetic structure and plant fitness of natural and ex situ populations in Silene chlorantha (WILLD.) EHRH. and Silene otites (L.) WIBEL“ the effects of temporal and spatial isolation on population genetic structure and plant fitness were analysed. The hermaphroditic Silene chlorantha (Caryophyllaceae) and the dioecious S. otites in north-eastern Germany served as model species. Effects of inbreeding and outbreeding on genetic diversity and plant performance were investigated in a pollination experiment. Additionally, population structures between botanic garden ex situ populations and corresponding in situ source populations were compared. Genetic population structure was analysed using the DNA fingerprinting technique AFLP. Fitness evaluations were conducted in the field and in common garden approaches and data were analysed by statistical modelling. In both Silene species, population genetic studies, which used a Bayesian approach, AMOVA and neighbour-joining analyses, revealed a strong genetic differentiation among populations. Additionally, in both species no correlation between geographic and genetic distance could be found. The results indicate that patterns of population differentiation are mainly due to reproductive isolation and genetic drift. In S. chlorantha, fitness evaluation revealed pollinator limitation and habitat quality to be more important for reproductive fitness than genetic diversity by itself. Genetic diversity was similar in the botanic garden ex situ and the in situ source population. A very low level of differentiation between both populations could be observed. In S. chlorantha, plant performance was similar in ex situ and in situ progeny. F1 progeny of cross-pollinations in S. chlorantha featured heterosis and inbreeding depression whereas no outbreeding depression could be observed. Genetic diversity could only be enhanced, if one partner originated from a genetically diverse population. A high level of genetic differentiation among populations of S. otites in north-eastern Germany was found. Neither a correlation between geographic and genetic distance nor between genetic diversity and population size could be detected, which indicates reduced gene flow among populations and random genetic drift. Plant performance was positively related to population size and genetic diversity. Higher total vegetation coverage resulted in reduced plant fitness. Additionally, in S. otites a sex ratio bias towards more male plants in larger populations could be observed. The results indicate that dioecy does not necessarily prevent from genetic erosion in case of habitat fragmentation. In S. otites, genetic variation in the ex situ populations was lower than the variation found in corresponding in situ populations. Strong differentiation between corresponding in situ and ex situ populations was observed. This could be the result of small population sizes and unconscious selection during cultivation. Therefore, adequate sampling prior to ex situ cultivation and large effective ex situ population sizes are important to preserve genetic diversity.