Phylogeography of eastern leatherwood examined with chloroplast sequencing

Background/Question/Methods

Dirca palustris L. (eastern leatherwood) represents a fascinating case study in plant persistence and colonization in eastern North America since the last glacial maximum ~18,000 years ago.  It occurs sporadically from Nova Scotia, west to North Dakota, and south to Louisiana and Florida as a deciduous understory shrub of mesic hardwood forests.  In the Southeast, it is especially infrequent in small and scattered populations, and it is listed as endangered in Florida.  Seed dispersal likely is limited, as ripe fruits fall to the ground beneath parent plants, and agents for long-distance dispersal have not been reported.  We recently noted several populations in the Apalachicola River Basin (SE Alabama, SW Georgia, NW Florida) with plants that have white-pubescent bud scales instead of the brown that is a hallmark of the species.  Therefore, we hypothesized that these phenotypically unique populations represent Pleistocene relicts that belong to a distinct glacial refugium.  To test our hypothesis, we sequenced the psbD-trnT(GGU), trnL-trnF, and rpl32-trnL(UAG) noncoding chloroplast regions of three plants from each of 31 populations across eastern North America.  We sequenced both forward and reverse primers, used three sequencing replicates to ensure accuracy of base calls, and aligned and edited sequences using Geneious Pro 5.5.7. 

Results/Conclusions

We found a total of six haplotypes, including a) six populations in Arkansas, Alabama, South Carolina, Ohio, and Nova Scotia; b) six populations in Alabama, Louisiana, Missouri, Iowa, and upper Michigan; c) five population in the Apalachicola River Basin and North Carolina; d) one population in Mississippi; e) one population in South Carolina; and f) 12 populations in Missouri, Arkansas, Georgia, Tennessee, South Carolina, Virginia, Alabama, North Dakota, Minnesota, and New York.  Our results show that the scattered populations in the Apalachicola River Basin and one population in North Carolina belong to a distinct glacial refugium.  Although all of these populations are characterized by white pubescence on bud scales, some plants of various haplotypes in South Carolina and Alabama also had white pubescence, indicating this trait is not unique to plants of this refugial haplotype.  The broad geographic distributions (e.g. from Louisiana to upper Michigan; from Georgia to North Dakota; from Arkansas to Nova Scotia) of some haplotypes suggest that multiple refugia provided propagules that colonized northern latitudes.  Further analyses incorporating additional markers, more populations, and spatial analyses will help to clarify our observations and elucidate patterns of seed- and pollen-mediated dispersal and gene flow in D. palustris.