Is Hybridization Responsible for Invasive Growth of Non-indigenous Water-milfoils?

Abstract

Heterosis, or hybrid vigor, has recently been proposed as a factor promoting invasive growth of some non-indigenous aquatic plant species, particularly those capable of spreading rapidly within and among lakes through clonal reproduction. We tested this hypothesis for variable-leaf water milfoil (Myriophyllum heterophyllum), a non-indigenous aquatic plant that has become a major management and conservation concern in New England. Using nuclear ribosomal DNA, we looked for F1 hybrid populations of invasive M. heterophyllum in 25 New Hampshire (NH) lakes. In contrast to a previous study that found F1 hybrid lineages of invasive M. heterophyllum in Connecticut, we did not find hybrids in our study lakes. This result has two implications: (1) pure lineages of M. heterophyllum are also capable of invasive growth, and (2) the distribution of invasive M. heterophyllum lineages (hybrid vs. pure) may be spatially structured across New England. We stress the importance of more detailed distributional and ecological studies for understanding the invasive potential of this species.