Submitted to: Aquatic Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2009
Publication Date: 4/5/2009
Citation: Okada, M., Grewell, B.J., Jasieniuk, M. 2009. Clonal spread of invasive Ludwigia hexapetala and L. grandiflora in freshwater wetlands of California. Aquatic Botany. 91(3):123-256. Interpretive Summary: This research provides genetic evidence for asexual reproduction and clonal spread of Ludwigia hexapetala and L. grandiflora, two invasive emergent macrophytes from South America, in California watersheds. The lack of genotypic variation among these aquatic weeds suggests that biological control may be a viable management strategy, and management should target reductions in vegetative growth and dispersal. The audience will include irrigation district and water management professionals, professional botanists, restoration practitioners, and plant ecologists and wildlife habitat managers in academia, government agencies, consulting, and conservation organizations.
Technical Abstract: Determining the reproductive mode contributing to dispersal within and between populations of invasive species is essential to understand the ecological and evolutionary processes underlying invasions and to guide management strategies. Ludwigia hexapetala and L. grandiflora are emergent aquatic species that have recently and aggressively invaded freshwater wetlands of California. We assessed the relative role of sexual versus asexual reproduction in 27 populations of L. hexapetala from two watersheds, and five populations of L. grandiflora from another watershed of California using AFLP markers. We also analyzed an invasive population from the state of Washington that originated as an ornamental release in 1956, for comparison. Of the total 794 L. hexapetala ramets analyzed from California, 95% represented a single genet. Only this genet was detected in 20 populations whereas two genets were detected in five populations, three in one population, and nine in the remaining population analyzed. The single genet detected in the population from Washington was identical to the predominant genet identified in California. Of the total 150 L. grandiflora ramets analyzed, only one genet was detected in the five populations sampled from the San Diego River basin. Based on the lack of genotypic variation observed in the majority of populations studied of the two species, our results strongly suggest that invasive spread, both within and between populations, and between watersheds, is almost exclusively asexual and clonal. The results indicate that management of the spread of invasive L. hexapetala and L. grandiflora should focus on reducing vegetative growth and dispersal of vegetative fragments. The genotypic uniformity of invasive populations also suggests that biological control of invasive L.hexapetala and L.grandiflora could be successful.