Genetic homogeneity in Juglans nigra (Juglandaceae) at nuclear microsatellites

Broad-scale studies of genetic structure and diversity are indicative of the recent evolutionary history of a species and are relevant to conservation efforts. We have estimated current levels of genetic diversity and population structure for black walnut (Juglans nigra L.), a highly valuable timber species, in the central hardwood region of the United States. Black walnut trees from 43 populations across this region were genotyped at 12 highly polymorphic microsatellite loci. Genetic diversity was high and populations only slightly deviated from Hardy-Weinberg proportions (F-ts = 0.017). Considering the scale of our sampling, the species was remarkably genetically homogenous: F-ST was quite low (0.017), and in a Bayesian analysis the optimal higher-order partition was into a single group comprised of all 43 populations. Although black walnut is predominantly a bottomland species, very little genetic variance was partitioned among broad-scale hydrologic regions (F-PT = 0.002). However, a weak, but statistically significant pattern of isolation by distance was detected. The results are consistent with a scenario in which black walnut recolonized its current range from a single glacial refugium, and where subsequent genetic effects associated with deforestation and habitat fragmentation have been mitigated by high levels of pollen flow. Nuclear microsatellites alone may be insufficient to identify hotspots for black walnut conservation.