Cryptic Speciation, Genetic Diversity and Herbicide Resistance in the Invasive Aquatic Plant Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae)

Hydrilla verticillata (L. f) Royle (Hydrocharitaceae; commonly hydrilla ) is a submersed aquatic plant with a cosmopolitan distribution. It is a nonindigenous, highly invasive weed that causes serious ecological and economic harm in the United States, and consequently is of great management concern. Repeated use of the herbicide fluridone has led to the evolution of resistant strains in Florida hydrilla populations. I developed a standard method to screen the nuclear phytoene desaturase (pds) gene for three previously identified mutations that confer resistance. I screened accessions from the US and other countries and detected hydrilla with pds mutations from five localities in Florida and one in Georgia. All pds mutations were found only in US dioecious hydrilla, and all were located on the same homologous allele. Using this new method, lake managers may have hydrilla tested for resistance-conferring mutations prior to choosing a control treatment. ^ Hydrilla is a monotypic genus with H. verticillata comprising the sole species. I used phylogenetic and morphological analyses to identify and delineate three cryptic species within the genus Hydrilla. These lineages, based on the geographic areas found, are 1) an Indian/Nepal/US dioecious species; 2) a Japan/Korean/European species; and 3) an Indonesian/Malaysian species that is found also as an escape in Australia. All data indicated that US dioecious plants originated from India, a result consistent with previously published phenetic analyses. The diploid parents of triploid US monoecious hydrilla were not identified definitively, but this taxon is most likely a hybrid of the Indian and Indonesian lineages. ^ Sufficient genetic variation within invading populations may be necessary for successful colonization of new geographical regions. Hydrilla verticillata, which reproduces largely by asexual means, is an invasive aquatic weed in the United States, and a non-indigenous colonizer in China and Europe. I used data from six microsatellite loci to compare genetic diversity and structure within and among non-indigenous (US, China, Europe), and indigenous (Australia, India, Indonesia/Malaysia, Korea) hydrilla populations. The population genetic structure of hydrilla reflects the asexual reproductive history of the genus, the invasion history in different regions, and genetic divergence among hydrilla lineages that are in fact distinct species. Polyploidy and hybrid vigor are hypothesized to contribute to the success of introduced US and Chinese populations.