Population size, genetic variation, and reproductive success in a rapidly declining, self-incompatible perennial (Arnica montana) in The Netherlands

Arnica montana is a rare and rapidly declining, self-incompatible plant spe cies. In 26 populations in The Netherlands we investigated the relationship between population size and genetic variation using allozyme markers. Gene tic variation was low in A. montana (H-e = 0.088). There were positive corr elations between population size and the proportion of polymorphic loci, th e number of effective alleles, and expected heterozygosity, but not with ob served heterozygosity. There was a significantly positive correlation betwe en population size and the inbreeding coefficient. Generally, small populat ions showed heterozygote excess, which decreased with increasing population size. Possibly, the heterozygous individuals in small populations are surv ivors from the formerly larger populations with relatively high fitness. Th e F statistics showed a moderately high level of differentiation among popu lations (F-ST = 0.140 +/- 0.02), implying a low level of gene flow. For thr ee out of four allozyme loci, we found significant inbreeding (F-IS = 0.104 +/- 0.03). Only 14 of 26 populations were in Hardy-Weinberg equilibrium at all four polymorphic loci. In a subset of 14 populations of various size, rue investigated natural seed production and offspring fitness. Population size was positively coil-elated with seed set, seedling size, number of flo wering stems and flowerheads, adult survival and total relative fitness, bu t not with the number of florets per flowerhead, germination rate, or the p roportion of germination. Offspring performance in the greenhouse was not a ssociated with genetic diversity measured on their mothers in the field We conclude that the fitness of small population is significantly reduced, but that there is as yet no evidence that this was caused by inbreeding Possib ly, the self-incompatibility system of A montana has been effective in redu cing selfing rates and inbreeding depression.