Investigation of the recent agricultural invasion of Palmer amaranth and waterhemp in California’s Central Valley

---

The most agriculturally problematic species in the weedy plant genus Amaranthus is A. palmeri (Palmer amaranth), which has rapidly evolved herbicide resistance: populations of the species are now insensitive to herbicides that target six different biological pathways. Agricultural research has focused on understanding the genetic mutations underlying the resistance mechanisms in Palmer amaranth. However, there has been no genetic investigation of the species’ remarkable range expansion out of the desert Southwest and aggressive invasion into agricultural fields in the eastern U.S. and Central California, and no development of a population genetic framework for the invasion that could relate neutral genetic variation (reflecting demographic changes) to agriculturally-adaptive genetic variation, such as herbicide resistance.

With funding from the USDA, we have begun a broad collaborative project with Dr. Chance Riggins at the University of Illinois and Dr. Anil Shrestha at CSU, Fresno) focused on exploring the evolution of agricultural invasiveness and herbicide resistance evolution in the species as a whole, with the applied goal of developing data-driven management strategies designed to curb resistance evolution and spread.  We have collected seed and leaf samples from populations of Palmer amaranth across its native range in the southwestern U.S., and from California’s Central Valley, including one population confirmed resistant to glyphosate (RoundUp).  This summer, genotyping-by-sequencing analysis will be performed for these populations, as well as Palmer amaranth populations from the eastern invaded portion of its range (the Midwest and Southeast U.S.).  Subsequent bioinformatics analyses will allow calculation of genetic diversity and structure metrics for Palmer amaranth across its native range; this SNP data will subsequently be used for tests of selection, and model-based inferences about the invasion process in the west (Central California) vs. the eastern U.S.  Simultaneously, our lab is performing molecular and greenhouse tests for resistance to several different classes of herbicides, including ALS-inhibitors, PPO-inhibitors, and glyphosate, on populations in the native and invaded ranges.  Finally, our bioinformatics results will be used as a genetic foundation for comparative common garden studies to test hypotheses about local adaptation in the native and invaded ranges of Palmer amaranth.

Our proposal to perform similar genetic and common garden studies with waterhemp (Amaranthus tuberculatus), a problematic Midwestern agricultural weed that has just begun to appear in the Central Valley, was recently funded by the CSU Agricultural Research Institute.