Tracking the global spread of a common and pest ambrosia beetle using genome sequencing

Each year new exotic species are transported across the world through global commerce. Many of these exotics never become successfully established. However, a few become destructive pests threatening economic and environmental resources. Some of the most successful exotic species are the ambrosia beetles. The ability to mate with siblings (inbreed) and their transportable food source (symbiotic fungus) enabled them to colonize most of the world. Increasingly, these exotic ambrosia beetles are becoming pests of plant nurseries, stored lumber, and forests. The purpose of my research is to uncover the population expansion history of one of the most widespread and abundant pest ambrosia beetles: Xylosandrus crassiusculus. Specifically, I am determining the source of introductions, assessing if the apparent ubiquity of this beetle is actually cryptic species diversity, and testing different expansion scenarios. Using restriction site associated DNA sequencing (RADSeq), which combines high-throughput genomic marker discovery and genotyping, I have genotyped 196 beetles from locations across the species native and introduced range at hundreds of genomic loci. Analysis of the genotype data indicates that there are two distinct linages of this “super-tramp” species. One linage originated in China and has spread through Southeast Asia and Africa. The other linage originated in Japan and has spread to the continental U.S. and Central America. Patterns of genetic differentiation suggest that there has been more than one introduction event from Japan in the continental U.S. while both Japanese and Chinese linages have been introduced to Hawaii. Currently, I am using Bayesian information criterion (BIC) to test different models of expansion to better infer the mechanisms facilitating the spread of this species in the U.S.