Trait Mapping Utilizing a Newly Constructed Genome for Allohexaploid Invasive Eurasian Watermilfoil ( <scp> <i>Myriophyllum spicatum</i> </scp> ) Reveals a Non‐Target Site <scp>QTL</scp> Associated With Fluridone Resistance

Herbicides are a valuable tool in agricultural ecosystems to manage nuisance species. Due to the reliance on herbicides for weed control, herbicide resistance is a growing concern. Herbicides are also used extensively in aquatic and natural systems, but the genetics and evolutionary dynamics of resistance are not as frequently incorporated into management plans in these systems. In Eurasian watermilfoil, a widespread and heavily managed invasive aquatic weed in the United States, clonal lineages have been characterized as resistant to fluridone, a commonly used phytoene desaturase (PDS)-inhibitor herbicide. In order to locate genomic loci associated with herbicide resistance, we created an F2 mapping population segregating for fluridone resistance. Using this population, we examined the <i>pds</i> gene for amino acid alterations in resistant individuals and performed bulk segregant analysis between the highly resistant and susceptible F2 individuals. Additionally, we compared <i>pds</i> gene expression between resistant and susceptible strains in control and treated environments using RT-qPCR. We found no evidence of amino acid alterations to the <i>pds</i> gene in fluridone resistant individuals or increased <i>pds</i> expression in the resistant strain, either in the presence or absence of fluridone. Our QTL mapping identified a putative QTL on chromosome seven, while the gene encoding fluridone's target molecule, phytoene desaturase (PDS) is located on chromosomes 10-12. Our results indicate that fluridone resistance in the Eurasian watermilfoil strain isolated from Lake Lansing, MI, is due to at least one non-target site mechanism. Characterizing mechanisms of herbicide resistance within invasive plants enables effective and thoughtful herbicide usage, as well as the development of diagnostic biomarkers for resistance in unknown populations.