Spatial genetic structure of delphinium nuttallianum populations: inferences about gene flow

The spatial genetic structure of a plant population provides a potential record of past gene flow and mating. We used hierarchical F-statistics and spatial autocorrelation to characterize spatial genetic differentiation of allozymes in adult Delphinium nuttallianum plants within and among six natural populations separated from one another by up to 3 km. Previous direct estimates suggested that gene flow is highly localized, averaging << 10 m. Earlier studies of seed-set, pollen-tube growth and progeny fitness suggested that partial reproductive isolation exists between plants growing too close together (<3 m) and too far apart (>100 m). Thus we anticipated substantial genetic differentiation on scales of a few to hundreds of metres. However, we detected little differentiation among the six populations, among replicate study plots within populations, or among subsections of study plots, except at the smallest scale of cm to m. These results suggest that relatively rare long-distance pollen movement has gone undetected and that postpollination selection may further modify genetic structure during the life cycle. Lack of differentiation is not at odds with the observation of partial reproductive isolation, because some loci may respond to spatial variation in selection without this response being evident at marker loci.