Hybridization and the evolution of invasiveness in herbicide-managed lakes

Background/Question/Methods

Hybridization may stimulate the evolution of invasiveness in human impacted habitats if unique hybrid genotypes have higher fitness than parental genotypes. Human efforts to control invasive taxa frequently involve the intentional alteration of habitats. For example, systemic herbicides are often applied to control invasive plants. However, few studies have considered whether hybridization can result in decreased susceptibility to herbicide. Decreased susceptibility to herbicide is a relevant invasive trait in herbicide-managed landscapes because it directly affects the establishment and spread of genotypes. 

Here, we use laboratory experiments and study natural distribution patterns to test whether interspecific hybrids between introduced Eurasian watermilfoil (Myriophyllum spicatum) and native northern watermilfoil (Myriophyllum sibiricum) are more invasive than parental Eurasian watermilfoil. Specifically, we focus on two key invasive traits in this study system: vegetative growth rate and susceptibility to the commonly-used systemic herbicide 2,4-Dichlorophenoxyacetic acid (2,4-D).

Results/Conclusions

Hybrids occurred more frequently than parental species in natural lakes that were previously treated with 2,4-D, suggesting that hybrid genotypes have higher fitness than parental genotypes in herbicide-treated lakes. Indeed, in two separate laboratory experiments, hybrids had higher vegetative growth rates and exhibited considerably less susceptibility to 2,4-D. These two traits appear to be predictable outcomes of hybridization because they occurred in hybrid genotypes from distinct hybridization events. It is unclear whether these traits reflect hybrid vigor or increased response to selection compared to parental Eurasian watermilfoil, but ongoing studies address these hypotheses. 

Our results provide compelling empirical evidence that hybridization is associated with the evolution of increased invasiveness in watermilfoils.  Furthermore, our results identify hybridity as an important risk factor when determining and permitting control activities for this widely-managed invasive aquatic weed.