Population structure and origins of invasive King Ranch Bluestem (Bothriochloa ischaemum var. songarica)

Background/Question/Methods

Bothriochloa ischaemum var. songarica has established invasive populations throughout the southern and central United States and is considered a threat to native biodiversity in Texas. This bunchgrass is originally thought to have been introduced from China; however, experimental breeding between lineages from throughout the native range and release of engineered cultivars for agricultural purposes since the 1950’s have obscured the introduction history of current invasive populations in Texas. Multiple sources of introduction that create novel genotypes are thought to contribute to invasion success in many introduced plants. We examined genetic variation at eight microsatellite loci in ten invasive Texas populations and in 32 accessions representing populations from throughout the native range from the USDA’s Germplasm Resource Information Network (GRIN). We described population structure across the native range, within the invasive range, and assigned invasive genotypes to putative source locations to determine the origin of invasive Texas populations and the likely number of sources contributing to their invasion success.

Results/Conclusions

Accessions of Bothriochloa ischaemum var. songarica collected from throughout the native range exhibited strong population structure (e.g., F_ST = 0.812, P ≤ 0.001) and a pattern of genetic grouping that reflects geographic distribution, enabling the assignment of invasive populations. No significant genetic structure or differentiation was found for eight of ten invasive Texas populations (e.g., F_ST = 0.052, P = 0.058). Cluster analyses, including: Principle Coordinates Analysis, Neighbor-Joining Trees, and a Bayesian clustering method implemented in the software program STRUCTURE, consistently indicate Texas genotypes are most similar to accessions in eastern Asia and contain some admixture with accessions from the Middle East. A comparison with known bred cultivars, including the “Plains” and “King Ranch” cultivars, indicate Texas invasive populations are not directly derived from one of these cultivars, but may be derived from a separate experimental cultivar or formed via hybridization between “King Ranch” and another cultivar prior to invasive establishment. Understanding the invasion history of B. ischaemum var. songarica contributes to knowledge of the genetic and biological processes that influence invasion success and the growing evidence that multiple introductions can facilitate successful invasions.