Patterns of differentiation among endangered pondberry populations

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Pondberry, Lindera melissifolia, is an endangered and partially clonally reproducing shrub species found in isolated populations that inhabit seasonally wet depressions in forested areas of the lower Mississippi River alluvial valley and southeastern regions of the United States. With eleven microsatellite loci, we quantified population genetic differentiation and diversity among 450 genets in 10 locations distributed across pondberry’s range. We used estimates of F st and Jost’s D est to measure genetic differences between populations and between geographic regions. The largest pairwise regional difference was found between eastern and western regional population groups (F st = 0.23, D est = 0.67), with the northern-most population groups in each region exhibiting larger divergence from each other than the southern-most population groups. Genetic diversity was lowest in the Sand Pond Conservation Area (A e = 1.9, H e = 0.36), which was the northern-most pondberry population, and highest in the Francis Marion National Forest (A e = 4.1, H e = 0.69), although we identified only 17 genets in that admixed population. Following adjustments for estimated null allele frequencies, we identified heterozygote excess in four eastern populations and found no evidence for inbreeding in any population. The observed patterns of differentiation indicate a phylogeography that exhibits an Appalachian Mountain discontinuity coupled with northward migrations along the Southern Atlantic Coastal Plain and into the Mississippi Alluvial Plain. The genetic consequences of this proposed phylogeographical structure may affect selection of germplasm sources for population reestablishment programs across pondberry’s range.