High genetic diversity in the clonal aquatic weed, Alternanthera philoxeroides in the United States

Authors: WILLIAMS, DEAN - Texas Christian University; HARMS, NATHAN - Environmental Laboratory, Us Army Engineer Research And Development Center, Waterways Experiment St; KNIGHT, IAN - Louisiana State University Agcenter; Grewell, Brenda; Futrell, Caryn; Pratt, Paul

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2020
Publication Date: 11/4/2020
Citation: Williams, D.A., Harms, N.E., Knight, I.A., Grewell, B.J., Futrell, C.J., Pratt, P.D. 2020. High genetic diversity in the clonal aquatic weed, Alternanthera philoxeroides in the United States. Invasive Plant Science and Management. 13(4):217-225. https://doi.org/10.1017/inp.2020.32.
DOI: https://doi.org/10.1017/inp.2020.32

Interpretive Summary: The distribution of genetic diversity in invasive plant populations can have important management implications. Alligator weed (Alternanthera philoxeroides (Mart.) Griseb.) was introduced into the United States around 1900 and has since spread throughout much of the southern U.S. and California. A successful biological control program was initiated in the late 1960s which reduced alligator weed in the southern U.S., although control has varied geographically. To support improved biological control, we conducted genetic studies on alligator weed sampled throughout the invaded U.S. range. We found high genetic diversity of alligator weed across the US that is structured geographically. Two of the chloroplast haplotypes (genotypes) we found correspond to two previously described narrow stem (NSA) and broad stem (BSA) biotypes that are reported to respond differently to both biological control and herbicides. The other genotypes we found may also differ in their invasiveness, such as differences in growth rates, or respond differently to control measures. The control of these different haplotypes should be tested with the currently available herbicides and biological control agents. These studies may encourage foreign exploration for additional agents that are better adapted to specific haplotypes and climates. Managers may need to consider that different areas will need different management strategies. Some states have mainly a single invasive haplotype and so may respond more uniformly to a single control strategy while other states and river basins have a variety of haplotypes, and therefore may require a variety of control strategies depending on the population genetics and specific locality. The markers we developed in this study could be used to quickly determine which haplotypes are present in a given area. Future studies can use these genetic data to determine if genotypes differ in their invasiveness or respond differently to control measures. Some states for instance, have mainly a single haplotype and so may respond more uniformly to a single control strategy compared to other states which may require a variety of control strategies. These data will also provide the basis for identifying the source regions in South America, which may lead to the discovery of new biological control agents more closely matched to particular genotypes.

Technical Abstract: The distribution of genetic diversity in invasive plant populations can have important management implications. Alligator weed (Alternanthera philoxeroides (Mart.) Griseb.) was introduced into the United States around 1900 and has since spread throughout much of the southern U.S. and California. A successful biological control program was initiated in the late 1960s which reduced alligator weed in the southern U.S., although control has varied geographically. The degree to which variation among genotypes maybe responsible for variation in control efficacy has not been well studied due to a lack of genetic data. We sampled 373 plants from 90 sites across the U.S. and genotyped all samples at three chloroplast regions to help inform future management efforts. Consistent with clonal spread, there was high differentiation between sites, yet we found seven haplotypes and high haplotype diversity (mean h = 0.50) across states, suggesting this plant has been introduced multiple times. Two of the haplotypes correspond to previously described biotypes that differ in their susceptibility to herbicides and herbivory. The three common haplotypes varied by latitude and longitude while the other haplotypes were widespread or localized to one or a few sites. The major haplotypes we screened are all hexaploid (6n = 102) which may enhance biological control. Future studies can use these genetic data to determine if genotypes differ in their invasiveness or respond differently to control measures. Some states for instance, have mainly a single haplotype and so may respond more uniformly to a single control strategy compared to other states which may require a variety of control strategies. These data will also provide the basis for identifying the source regions in South America, which may lead to the discovery of new biological control agents more closely matched to particular genotypes.