Hybridization and invasiveness in Eurasian watermilfoil: Is prioritizing hybrids in management justified?

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Hannah K. Hoff and Ryan A. Thum, Plant Science Plant Pathology, Montana State University, Bozeman, MT

Background/Question/Methods
Hybridization can play an important role in the evolution of invasiveness. Eurasian watermilfoil (Myriophyllum spicatum) is a widespread aquatic invasive plant species that hybridizes with native northern watermilfoil (Myriophyllum sibiricum). Previous studies have found mixed evidence for whether hybrid watermilfoil and pure Eurasian watermilfoil differ in vegetative growth rate and response to commonly-used herbicides such as 2,4-D. While several studies have emphasized variation among hybrid genotypes in their vegetative growth rate and herbicide response, genetic variation within pure Eurasian watermilfoil has not been considered. Therefore, it is unclear how important genetic variation within Eurasian watermilfoil versus between pure Eurasian and hybrid watermilfoil is in influencing invasive traits and management outcomes. If hybrid genotypes are always more invasive than pure Eurasian genotypes, simply distinguishing taxa may be sufficient for identifying management priorities; however, if significant phenotypic overlap is observed between Eurasian and hybrid watermilfoil, distinguishing individual genotypes may be warranted. We performed replicated trials of a vegetative growth and 2,4-D assay to measure clonal variation in growth rate and herbicide response in Eurasian and hybrid watermilfoil, and to test whether there is significant overlap between taxa in the traits previously associated with their invasiveness.
Results/Conclusions
Our results suggest that hybrid watermilfoil exhibits higher average vegetative growth than Eurasian watermilfoil, whether or not it is treated with 2,4-D. This is consistent with the notion that hybrid watermilfoil may generally be more invasive than pure Eurasian watermilfoil, and therefore warrants prioritization in lake management. We did not observe interactions between taxon and treatment or between genotype and treatment. Despite differences in average growth, there was substantial phenotypic overlap between Eurasian and hybrid watermilfoil. For example, the fastest-growing genotype of pure Eurasian watermilfoil did not differ significantly in average growth from the fastest-growing hybrid genotype in control or in treatment. Thus, pure Eurasian genotypes may be comparatively invasive to some hybrid genotypes. Further, the slowest-growing genotype of pure Eurasian watermilfoil did not differ significantly in average growth from the slowest-growing hybrid genotype. This pattern suggests that not all hybrid watermilfoil genotypes are fast growers, and that there is substantial overlap between genotypes of the two taxa, despite differences in average growth. The potential for overlap between invasive watermilfoil taxa suggests that distinguishing and characterizing genotypes may be more informative for managing invasive watermilfoil than simply distinguishing between Eurasian and hybrid watermilfoil.