Glyphosate-resistant Lolium (Poaceae): Population subdivision in the presence of selection on a new adaptive trait

Glyphosate is a commonly used broad spectrum herbicide.  It is also a potent selection agent; since 1996, scientists have identified nine different weed species that have evolved resistance to glyphosate.  One of the species, Lolium spp. (Poaceae), is a common agricultural and wildland weed in California.  In 1998, UC farm advisors identified a population of Lolium spp. near Chico California with resistance to the herbicide glyphosate.  Currently, both the novel adaptive trait of glyphosate resistance and the original susceptible phenotype are present within the Central Valley of California.  We are using this population to study the effects of selection by glyphosate on both neutral genetic population subdivision and the distribution of the phenotype.  SSR (simple sequence repeats) markers applied to at least 600 individuals indicate there is population structure within the neutral genetic data.  We are testing if this structure is related to varying glyphosate environments and resistant (R) and susceptible (S) phenotypes.  To determine if selection is affecting the distribution of the phenotype, we are testing if resistant and susceptible phenotypes are randomly distributed or instead are restricted to the environments in which they are most fit.