Intra-Species Variation in Green Ash Response to an Invasive Insect

Green ash (Fraxinus pennsylvanica) is a medium-sized, ecologically and economically valuable tree species native to the eastern and central United States. However, the widely distributed species in North America is under severe threat from the rapid invasion of emerald ash borer (Agrilus planipennis; EAB), an Asian wood-boring beetle. While the majority of trees in attacked forest stands are killed by EAB, in some cases a low percentage of trees survive the infestation. Our project aims to uncover the gene expression variation and genetic sequence variation contributing to the observed resistance or tolerance to EAB in the surviving green ash trees. Our expression profiling results suggested that jasmonate (JA)-mediated defenses primarily contribute to the herbivory response in the putatively-resistant genotypes treated with EAB larvae. At the population level, we conducted a genetic diversity study with 429 green ash accessions collected from 60 provenances across the species’ natural range. Our results revealed three distinct sub-groups of provenances – Northern, Southern and Admixed groups. We found that the admixture group grew better (significantly larger diameters) than the Northern and Southern groups, while the Southern group showed most rapid mortality to EAB.

To further understand the role of genetic variation, we will conduct an association study to identify loci significantly associated with the growth and EAB-susceptibility traits. Hopefully, the combined expression profiling analysis and association results will provide a reliable explanation for the observed cases of EAB resistance/tolerance in green ash. Such evidence will be a big step towards providing tools to assist disease-resistance tree breeding and forest management programs. This project was supported by a grant from NSF’s Plant Genome Research Program (IOS-1025974).