Perennial pepperweed is an invasive plant species in North America, native to temperate Eurasia and northern Africa. Effective biological control depends upon correct taxonomic identification. Therefore, we investigated morphological and genetic data (cpDNA sequences and amplified fragment length polymorphisms [AFLP]) in its native range, where the species is at times treated as multiple taxa (L. latifolium, L. affine and L. obtusum). We also analyzed genetic data to determine the number and distribution of haplotypes and genotypes in the invaded range. Using Bayesian analysis, we found three clusters of AFLP genotypes in the native range, but little correlation between these clusters and morphological characters used to distinguish taxa. Also, we found combinations of morphological character states within many native range plants that are incompatible with current species descriptions, offering no support for splitting L. latifolium sensu lato into three species. In North America 97% of the genetic variation was among populations and there were only eight AFLP genotypes in 288 plants, suggesting few introductions or a severe bottleneck, and little or no creation of new genotypes since introduction. We found plants in the native range that are genetically similar (88 to 99%) to six of the eight invasive AFLP genotypes, suggesting that Kazakhstan and China are origins for much of the North American invasion.
Nomenclature: Perennial pepperweed, Lepidium latifolium L. LEPLA, Lepidium affine Ledeb., Lepidium obtusum Basiner
Management Implications: Clear taxonomic identification of an invasive plant in its native and introduced ranges is a prerequisite for the success of any biological control project. Also, invasions can contain diverse assemblages of genotypes, which can affect herbivory. To verify the identity of perennial pepperweed populations from Eurasia that are being explored for biological control agent candidates we collected and analyzed morphological and genotypic data from populations in Eurasia and North America. In addition, we analyzed genotypic data to determine the diversity and distribution of perennial pepperweed genotypes in North America. Our results indicate that perennial pepperweed most probably consists of one taxon; not two or three as suggested in some recent floristic studies. We found that origins of common invasive genotypes are in Kazakhstan and China, suggesting that these regions should preferentially be searched in future foreign exploration for additional biological control agents. We found only eight invasive genotypes in North America, suggesting few introductions or a severe bottleneck, and little or no creation of new genotypes since introduction. The low diversity allowed us to provide seed representing all cpDNA sequence haplotypes for host-specificity studies, which should reduce the risk of any resistance to biological control agents being found after release.