Exploring genetic basis of Sickleweed (Falcaria vulgaris) invasion in the United States

Background/Question/Methods

Sickleweed (Falcaria vulgaris Bernh.; Apiaceae), native to Europe and Asia, occurs disjunctly in the Midwest and the East Coast of the United States. While there is no report of recent occurrence of sickleweed in the East coast, it is an aggressive weed potentially turning to invasive in the Midwest. The main objective of this study was to use molecular markers to study genetic basis of sickleweed spread in the United States. We screened nuclear microsatellite markers developed for Daucus carota  and tested for their transferability to sickleweed samples. We also sequenced trnL intron, trnL-F intergenic spacer and trnQ-rps16 regions of sickleweed chloroplast DNA and nuclear ITS region of nuclear ribosomal DNA to asseess sequence variation. The nuclear microsatellite markers were characterized and used  for genotyping sicklweeed samples from the Midwest.

Results/Conclusions

Microsatellite markers identified three genetic races of sickleweed in the Midwest. Overall population genetic diversity was very high: two populations from Fort Pierre National Grassland (South Dakota) exhibited the highest genetic diversity while two populations- one from Brookings County (South Dakota)  and another from Onawa County (Iowa) exhibited the lowest genetic diversity. Total genetic differentiation among population was also higher than previously reported values for the majority of invasive plant species. The low genetic differentiation between geographically closer populations suggested human mediated dispersal. Chloroplast DNA sequence data revealed six chloroplast haplotypes nested into two main lineages suggesting at least two independent introductions of sickleweed in the Midwest. High genetic differentiation and high genetic diversity also supported the occurrence of multiple introductions. Presence of multiple alleles across multiple microsatellite loci suggested polyploidy, which is perhaps a post introduction evolutionary strategy of sickleweed. The combined effect of multiple introductions, polyploidy and asexual reproduction in sickleweed may be inducing its invasiveness. This study provides valuable insights into the evolution of introduced species and will have implications in management practices before this this species emerge as a serious invader.