Lepidium latifolium L. is a polyploid, perennial herbaceous plant with semi-woody stems, and creeping rhizomes, that is native or naturalized throughout much of Eurasia. This species was accidentally introduced into the USA, and has become a noxious weed in coastal New England and wetlands, meadows, pastures, and riparian areas of many western States, especially California and Nevada. The work reported here represents our initial attempt to determine the genetic diversity of L. latifolium in Eurasia. We assessed genetic diversity in six populations (380 individuals) of this species from the Mediterranean coast of southern France by staining for 14 enzymes that were coded for by 29 loci. Genetic diversity in L. latifolium is extremely low: we detected only two multilocus genotypes across all six populations. Five populations (280 individuals) were fixed for a single multilocus genotype, whereas all 100 individuals in the sixth (and easternmost) population were fixed for the other genotype. Enzyme banding patterns are consistent with allopolyploid gene expression, and chromosome counts reveal that all populations possess tetraploid chromosome numbers (2n = 24). The low levels of genetic diversity we report for L. latifolium may result from two factors: 1) these populations may have experienced a genetic bottleneck associated with their relatively recent naturalization in this region of southern France, and 2) these populations appear to reproduce almost exclusively through clonal means by rhizome expansion and fragmentation. Our results clearly indicate that further sampling and analysis of L. latifolium across larger portions of Eurasia is required before a better picture of genetic diversity in this species in its native range can be obtained.

Key words: Brassicaceae, clonal reproduction, genetic diversity, Lepidium latifolium, multilocus genotypes, naturalized populations