Spatial and temporal aspects of the genetic structure of Juniperus communis populations

Juniperus communis is a dioecious, wind pollinated shrub or small tree that produces 'berries' (female cones) containing a small number of seeds that are thought to be dispersed by birds. The expectation, therefore, would be that populations of Juniper are genetically diverse with little structuring between them. In Britain, the species has two main centres of distribution : a highland zone in the north and west, in which populations are still lar ge and sexually reproducing, and a southern zone on chalk downlands in whic h populations are small and fragmented and individuals suffer from a declin e in fertility. Thus, one would expect the large sexually viable population s in the north to exhibit high levels of within-population genetic variatio n, while the declining southern populations would be genetically depauperat e. The analysis of amplified fragment length polymorphisms (AFLPs) was used to test this hypothesis. Surprisingly, all populations studied showed high levels of genetic variation although there was clear structuring between p opulations. On the basis of the geographical structuring of the populations it was hypothesized that J. communis colonized Britain via three separate routes.