Population structure and genetic diversity of relict populations of Alyssum montanum on limestone cliffs in the Northern Swiss Jura mountains

Exposed cliffs in the Northern Swiss Jura mountains harbour a highly diverse flora with numerous rare and relict plant species. The genetic structure of cliff populations is of particular interest because in a variety of plant species the populations are small, isolated and separated from their main distribution area in the Alps and the Mediterranean. We examined possible relationships between population size and size structure, sexual reproduction and genetic diversity of Alyssum montanum, a relict plant species occurring on limestone cliffs in the Northern Swiss Jura mountains. The population size of A. montanum ranged from 8 to 248 individuals on the 12 cliffs examined. Most populations contained a high proportion of small-sized (= young) plants indicating a successful establishment of seedlings. Fitness-related traits of A. montanum (percentage of reproductive individuals, number of fruits, fruit weight) varied widely between cliffs and were neither related to the size of the populations nor to the percentage of vegetation cover on the cliffs. RAPD-PCR analysis revealed that A. montanum populations exhibit a remarkably high genetic diversity. However, genetic diversity decreased with decreasing population size. Moreover, the positive relationship found between genetic variability and fitness-related traits indicates that population size is a key factor for the persistence of A. montanum on limestone cliffs.