Submitted to: Journal of Aquatic Plant Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2016
Publication Date: 1/2/2017
Citation: Williams, D., Harms, N., Dodd, L., Grodowitz, M.J., Dick, G. 2017. Do the U.S. Dioecious and Monoecious biotypes of Hydrilla verticillata L.F. Royle hybridize?. Journal of Aquatic Plant Management. 55:35-39.

Interpretive Summary: This technical note describes both controlled and field studies to determine if the two U.S. biotypes of hydrilla can hybridize. Based on collection of hydrilla from field sites where both biotypes occur in close proximity and controlled experiments where both hybrids were grown together no evidence of hybridization were observed based on genetic analyses (field studies) and examination of seed production under controlled (i.e., greenhouse) conditions. Although the sample size was small, these studies suggest that hybridization between the U.S. biotypes has not occurred or, if it has, those hybrids may be at an ecological disadvantage and so are rare. It is important to document if hybridization is occurring since hybrids may develop a competitive advantage in the environment and lead to reduced efficacy of chemical and biological control technologies.

Technical Abstract: This technical note reports the results of two studies to address the question of whether the two hydrilla biotypes present in the U.S. can hybridize. These include whether hybridization can occur under controlled conditions and in field populations where the two hybrids coexist in close proximity to one another. Biotypes were determined using modified genetic markers. PCA analysis clearly separated the field samples into two distinct clusters which corresponded to the two biotypes identified genetically. There were no intermediate samples between the two clusters suggesting there were no hybrid individuals. Although the sample size was small, the data suggest that hybridization between the U.S. biotypes has not occurred or, if it has, those hybrids may be at an ecological disadvantage in these areas and so are rare. Monoecious hydrilla began producing pistillate flowers in August, 12 weeks after planting. Staminate floating flowers were observed within several days of the female flowers. Dioecious plants began producing pistillate flowers approximately 3 weeks later. Flowering of both biotypes continued through November 2014, and pistillate ovaries remained intact through February 2015. Ovary dissections revealed only ovules in both biotypes, with no seed production found in either through February 2015. Pollination, fertilization, or embryo development did not occur in examined flowers of either biotype in this study. Because the number of sites in the U.S. where both biotypes coexist appears to be limited, continued monitoring for hybridization should take place. The generally low seed production of U.S. monoecious hydrilla dictates a much larger number of pairings under controlled conditions to better determine whether seed production can result from a U.S. monoecious-dioecious cross.