Genetic population structure of the wind‐pollinated, dioecious shrub Juniperus communis in fragmented Dutch heathlands

Abstract

AbstractThe wind‐pollinated, dioecious shrub Juniperus communis L. is declining in Dutch heathlands, mainly because recruitment is scarce. Aside from ecological factors, inbreeding associated with reduced population size and isolation in the currently fragmented landscape might explain this decline. However, the breeding system of this species largely prevents inbreeding. We assessed variation in 18 allozyme loci in 12 Dutch juniper populations to investigate population structure and to test the hypothesis that small populations have less genetic variation and show more inbreeding than large populations. Variation was high for a species with a fragmented distribution, but similar to values observed in other juniper species. The proportion of polymorphic loci (P) ranged between 0.72 and 0.83, expected heterozygosity (He) ranged between 0.16 and 0.27 and the mean effective number of alleles per locus (A) ranged between 2.5 and 3.3. Population size was not correlated with genetic variation or with sex ratio. Deviations from Hardy–Weinberg expectations, mainly heterozygote deficiencies, were mostly observed in larger populations, which appear to have been broken up into smaller subunits. Differentiation among populations was small (FST = 0.026) and there was a significant departure from random mating (FIS = 0.174). Geographic and genetic structures were not related. Our data suggest that gene flow in this wind‐pollinated shrub was high during the establishment phase of the current populations. Gene exchange by pollen and seeds may be less extensive in the present‐day landscape. However, juniper decline is not likely to be caused by genetic factors. Alternative hypotheses include changed heathland disturbance regimes, pollen limitation, seed predation and age‐related fecundity reductions. Implications for the restoration of juniper populations are discussed.