Inferring colonization patterns of the invasive shrub Amur Honeysuckle (Lonicera maackii (Rupr.) Herder) in southwestern Ohio from the genetic structure of their populations

Background/Question/Methods
A fundamental question about invasive plants is whether they proceed along expanding fronts or through long-distance dispersal events followed by local expansion. Lonicera maackii (Rupr.) Herder (Caprifoliaceae), Amur Honeysuckle, is a self-incompatible shrub native to Asia that was originally brought to the United States in 1898 and introduced at least 14 more times over the following 86 years. Naturalized populations now occur in nearly every eastern state. Darke county, located in southwestern Ohio, is currently experiencing a rapid colonization of L. maackii. This study outlines a first attempt to describe the genetic structure of L. maackii populations in this region. Specifically, the aim is to find interpopulation genetic differences to support three proposed invasion paths: one from Dayton, Ohio to the southeast, one from Cincinnati, Ohio to the south, and one from Richmond, Indiana to the southwest. Multiple woodlots from Darke, Preble, and Miami counties in Ohio and Wayne County, Indiana with well established populations of L. maackii were included in this study. Distances between populations ranged from 2.00 km to 57.07 km. Genetic analysis was conducted using five polymorphic microsatellite loci specifically designed for this study.

Results/Conclusions

A total of 58 different alleles were found across five loci, with an overall average of 11.6 alleles per locus. Average mean Na per population was 7.0. Ne was lower, averaging 4.1.  Average observed heterozygosity came to 0.7066, while average expected heterozygosity was higher at 0.7415. High H_E and Na found in these populations suggest high levels of genetic diversity and a low level of inbreeding. The mean fixation index (F_IS) supports this and was found to be 0.0183 when calculated across all loci. The analysis of molecular variance (AMOVA) showed that 92% of the observed variation arises from within population differences. A Mantel test conducted to determine the relationship between the genetic and geographic distances between populations showed a positive correlation (r=0.5547, p-value < 0.001). Our analyses also reveal that L. maackii populations in this area can be clustered into five distinct groups. Detailed analysis examining the genetic diversity of recently established populations shows variation in colonizing abilities among different “clusters”, as the sources of most of these new populations are primarily assigned to one cluster. Taken together, our results appear to support the hypothesis that L. maackii spreads through long-distance dispersal events followed by outward expansion.