Geographical Distribution of Genetic and Phenotypic Diversity of Dirca palustris

We sought to document genetic and phenotypic diversity within eastern leatherwood (Dirca palustris), a North American shrub that is rare in horticulture despite its yellow flowers and autumnal leaves, arborescent form, disease resistance, and shade tolerance.  Strength of the genetic structure among 92 populations of D. palustris throughout its distribution was assessed to identify populations of special interest to horticulturists and conservationists.  The psbD-trnT^(GGU), trnL-trnF, and rpl32-trnL^(UAG) noncoding chloroplast regions of three plants from each population were sequenced, and haplotype diversity was determined from a concatenated sequence of 3236 bases.  High genetic diversity was found; there were 24 haplotypes, including private haplotypes in 14 populations.  Areas of diversity, which had either many haplotypes or unique haplotypes, included the Cumberland Plateau (Kentucky/Tennessee), the Ozark Mountains (Arkansas/Missouri), the southern Atlantic Seaboard (Carolinas), and the Gulf Coast (Mississippi/Florida).  Plants in northern latitudes appear derived from northern refugia (e.g., Cumberland Plateau or Ozarks), and two haplotypes farther south have not dispersed northward since the last glacial maximum ~18,000 years ago.  The Atlantic Seaboard and Northeast have a distinct collection of haplotypes, suggestive of dispersal limitation across the Appalachian Mountains.  Individuals in several populations in the Southeast were phenotypically distinct, with wider calyxes and bud scale pubescence that was white instead of the brown described for the species.  Plants in these populations were united by a chloroplast mutation, indicating they are derived from a common maternal ancestor.  The taxonomic status of these southeastern populations awaits clarification expected from the study of nuclear genotypes and morphology.  Our results should facilitate making horticultural selections of D. palustris and will inform decisions about the preservation of rare genotypes.