THE EFFECT OF BREEDING SYSTEMS AND POLLINATION VECTORS ON THE GENETIC-VARIATION OF SMALL PLANT-POPULATIONS WITHIN AN AGRICULTURAL LANDSCAPE

Within an agricultural landscape (approximately 8 km(2)) the genetic s tructures of the herbs Festuca ovina, Lychnis viscaria and Arabis thal iana were examined using isoenzymes. The species have different breedi ng systems and pollination vectors, and occur in small populations in remnant patches of semi-natural habitats throughout the landscape. The re was no or only a weak correlation between population size and genet ic diversity. The wind-pollinated, outcrossing species F. ovina had th e highest levels of genetic variation, intermediate levels were found in the largely outcrossing and insect pollinated L. viscaria, while lo west levels of variation were found in the inbreeding A. thaliana. Rel ative to their breeding systems and the small geographical scale of th e study all species exhibited remarkably high levels of total genetic variation. In the outbreeding species most of the genetic variation wa s found within populations (F. ovina: F-ST=0.059 and L. viscaria: F-ST =0.092). No geographic pattern was found between populations in the ou tbreeding species, indicating high levels of gene flow. The cereal fie lds separating the populations may function as conduits rather than ba rriers to pollen dispersed by wind, and pollinating insects (bumblebee s and butterflies) may have no problems moving between patches at this scale, thus alleviating the danger of genetic erosion associated with small population size. In contrast, the fields appear to be efficient barriers to gene flow by seed dispersal. In the inbreeding Arabis tha liana a geographic pattern was evident. Most of the variation in this species was found between populations (F-ST = 0.722) indicating low le vels of gene flow; single populations exhibited large variation in the number of genotypes present (1 to 15).