Disjunct eastern hemlock populations: Ancient relicts or recent long distance dispersal events?

Background/Question/Methods

Eastern hemlock (Tsuga canadensis) is a conifer native to eastern North America found both within a large contiguous range and in many small, isolated populations.  These disjunct populations form clearly delineated, near monospecific stands associated with unique natural features such as north facing cliffs and box canyons.  Disjunct populations such as these have long been of interest and two hypotheses for their origins have frequently been proposed: 1) They are the product of rare long distance dispersal events, or, 2) Remnants of what was once a portion of the contiguous distribution.  Analysis of microsatellite loci can provide molecular data to evaluate these hypotheses.  Genetic bottlenecks imposed by long distance dispersal events should result in low within-population diversity, high between-population diversity, and correlation by distance to reference populations within the contiguous distribution.  Conversely, the remnant hypothesis predicts within-population diversity to be primarily correlated with population size, low between-population diversity, and little correlation by distance to reference populations.  To evaluate these hypotheses, microsatellite loci were amplified from DNA samples of 480 trees located in 18 disjunct and 6 reference populations using 21 published primer pairs.  Fragment analysis of PCR products was conducted with an ABI 3730 genetic analyzer and GeneMapper software.

Results/Conclusions

Of the 21 published microsatellite primer pairs for eastern hemlock we found 15 that consistently amplified products in the expected size range, of which 14 were polymorphic.  Initial results show that the number of alleles per polymorphic locus ranges from two to seven with an average of 3.7.  Values of observed and expected heterozygosity across all populations range from 0.046 to 1.0 (average = 0.382) and 0.095 to 0.50 (average = 0.385), respectively.  Disjunct populations had a lower level of genetic diversity (H_o = 0.316, H_e = 0.358) compared to reference populations in the contiguous range (H_o = 0.633, H_e = 0.392).  Additional analyses using F statistics and genetic isolation by distance calculations will also be presented to further evaluate the proposed origin hypotheses.