Assessment of Genetic Diversity and Relatedness in an Emerald Ash Borer-Resistant Green Ash Population

Populations of North American ash (Fraxinus sp.) are declining due to the introduction of emerald ash borer (EAB; Agrilus planipennis), a wood-boring beetle native to East Asia. At Oak Openings Preserve Metropark in northwest Ohio, a population of green ash (F. pennsylvanica) exists in which 99% of the trees have succumbed to emerald ash borer, but a small number of trees have remained healthy. These “lingering” ash may be closely related, thus sharing common alleles that may be contributing to EAB resistance. However, relatedness may also indicate an overall lack of genetic diversity in the lingering ash relative to the former intact population. To compare genetic diversity, 39 individuals were sampled from the lingering population as well as from 19 individuals that declined significantly or suffered above-ground mortality due to EAB infestation, but developed stump sprouts. This essentially creates a temporal sampling of the population, with the susceptible and lingering samples allowing estimation of the gene pool pre- and post-EAB arrival, respectively. Genotyping of trees at 12 EST-SSR loci revealed similar levels of allelic diversity and heterozygosity between the two groups. In addition, relatedness was not significantly higher among post-EAB trees (p>0.05), but a spatial autocorrelation analysis revealed that genetically similar trees were clustering close together in geographic space. These results suggest that green ash are buffering population loss and maintaining genetic diversity, probably through life history characteristics (e.g. dioecy, wind-pollination). Knowledge of the lingering ash’s genotypes will aid any breeding efforts undertaken by the US Forest Service to preserve ash on the North American landscape.