Using SNPs to examine population structure and identify units for conservation of endangered northeastern bulrush, Scirpus ancistrochaetus, across its range

Background/Question/Methods

Scirpus ancistrochaetus, Northeastern bulrush, is a federally-endangered wetland sedge, found in 8 states in the Northeast.  Information is needed on the genetic population structure of this species to assure that the full complement of genetic resources is considered during conservation activities.  Because S. ancistrochaetus is clonal, there is the possibility of very little within and between pond variation in genotypes.  With over 50% of populations in need of management, population genetics studies can be used to prioritize key areas for management and monitoring.  Additionally, information on population genetics can be used when assessing federal–listing status of this species.  We collected leaf samples from over 80 separate wetlands, which represent over half of all known populations.  We collected primarily on public land, but also collected on private land, particularly when better geographic coverage was needed.   Using DNA sequencing, we identified SNPs (single nucleotide polymorphisms) that showed variation among the S. ancistrochaetus populations and developed 10 genetic markers for this species.

Results/Conclusions

Our results illustrate that Scirpus ancistrochaetus is genetically structured across its range, suggesting distinct population units that are not well-connected by gene flow. Overall there was a lack of genetic diversity within regions. For example, populations within Pennsylvania are generally homogenous, with most populations sharing identical genotypes. Likewise, populations in Vermont and New Hampshire are genetically similar, but well-differentiated from Pennsylvanian populations. Overall, a greater amount of genetic diversity was found in the southern range of S. ancistrochaetus, suggesting that this region might be a reservoir of diversity for the species.  However, some populations in Vermont and New Hampshire contain strongly divergent alleles suggestive of ongoing hybridization with other Scirpus species. Despite the strong geographic structure observed, there is some evidence of long-distance gene flow. In particular, one population from western Pennsylvania and one population from New Hampshire are more similar to populations in West Virginia and Virginia. Avian dispersal, particularly by waterfowl, is possible given that the barbed seeds readily adhere to clothing.  The results of this study help identify sites for priority protection and management, and they reveal a case of introgression which might be a new conservation threat to this endangered species.