When the rate of gene flow is low relative to rates of genetic change due to chance, history, or selection, then population genetic structure may coincide with the spatial structure of a population. In this study, we sought to examine the concordance between genetic and geographical distances in local eastern leatherwood (Dirca palustris) populations found in deciduous forest understories of Central Minnesota. Leatherwood’s patchy distribution and its highly localized pollination and seed dispersal regimes make it a good candidate species to exhibit genetic differentiation on fine spatial scales. In this study, we addressed two predictions of the hypothesis that leatherwood will exhibit fine-scale genetic structure: (1) There will be a significant positive correlation between geographic and Nei's standardized genetic distances among local populations of eastern leatherwood separated by less than 10 Km. (2) There will be a significant negative correlation between geographic separation of individual leatherwood shrubs and the proportion of genetic markers they share. We sampled leaf material from plants in five local leatherwood populations, recording each plant's GPS coordinates. We assessed genetic variation and differentiation using inter-simple sequence repeat (ISSR) analysis, scoring the presence or absence in each plant of markers revealed by three different primers.
Our ISSR analysis revealed 79 unique markers among the 50 individuals we sampled for genetic analysis. However, the relationship between geographic and genetic distances did not provide clear support for either of our hypotheses. Mantel tests revealed no significant associations between geographic distances and either Nei's standardized genetic distances among the local populations or the proportion of markers shared by individuals. Clustering analyses based upon genetic differentiation did not produce patterns of relationship among either local populations or individuals that corresponded to expectations based upon geographic separation. The most striking result revealed by this study was the impressive amount of genetic variation that we observed among individuals within local populations that cover only 25 m2. Our results suggest that long-range pollen transport or seed dispersal may occur more frequently than we suspect, or that leatherwood populations inherently contain a great deal of genetic variation, at least that measured by ISSR analysis. If the latter is the case, then tools other than ISSR analysis may show more promise in the elucidation of fine-scale genetic differentiation among local populations in ecological landscapes.