Molecular confirmation of hybridization with invasive curly-leaf pondweed in the Sacramento -San Joaquin Delta, California

Weed managers recognize that hybridization can influence invasiveness in target weeds. As such, the identification of hybridization in target weeds has become of fundamental interest. Curly-leaf pondweed (Potamogeton crispus) is a heavily managed invasive aquatic weed in the United States. The genus is known for extensive interspecific hybridization, but the extent to which invasive P. crispus in the United States hybridizes is unknown. In October 2018, an aquatic vegetation survey in the California Sacramento-San Joaquin river delta identified plants that were suspected as P. crispus hybrids. These plants closely resembled P. crispus but differed in several ways, including having smaller, finer leaves and lacking the presence of true turions. We performed genetic analysis on these plants by comparing the internal transcribed spacer (ITS) DNA sequences from the putative hybrids to those identified as pure P. crispus and to Potamogeton accessions retrieved from GenBank. The putative hybrids had two divergent ITS sequences, one of which corresponded to sequences from P. crispus, and the other of which corresponded to sequences from P. pusillus, providing strong evidence of interspecific hybridization between these two species. Further, we identified genetic diversity even among pure P. crispus in North America. The extent of genetic diversity and the relevance to P. crispus ecology or management are currently unknown. Given the extent of management of P. crispus in North America, and the recognition that hybridization and genetic diversity can impact management outcomes, a geographic survey of genetic diversity and hybridization in P. crispus is warranted. Key words: GenBank, Pondweed hybrid, Potamogeton berchtoldi, Potamogeton pusillus, putative hybrid.