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 structures of the herbs Festuca ovina, Lychnis viscaria and Arabis thaliana were examined using isoenzymes. The species have different breeding systems and pollination vectors, and occur in small populations in remnant patches of semi-natural habitats throughout the landscape. There was no or only a weak correlation between population size and genetic diversity. The wind-pollinated, outcrossing species F. ovina had the highest levels of genetic variation, intermediate levels were found in the largely outcrossing and insect pollinated L. viscaria, while lowest levels of variation were found in the inbreeding A. thaliana. Relative to their breeding systems and the small geographical scale of the study all species exhibited remarkably high levels of total genetic variation. In the outbreeding species most of the genetic variation was 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 outbreeding species, indicating high levels of gene flow. The cereal fields separating the populations may function as conduits rather than barriers to pollen dispersed by wind, and pollinating insects (bumblebees 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 thaliana a geographic pattern was evident. Most of the variation in this species was found between populations (F-ST = 0.722) indicating low levels of gene flow; single populations exhibited large variation in the number of genotypes present (1 to 15).