Departures from Mendelian inheritance of EPSPS gene copies in a herbicide-resistant weed

Here, we report on inheritance and intra-individual variation in gene copy number associated with the rapid evolution of herbicide resistance in the dioecious, agricultural weed Amaranthus palmeri. Copy number variation for the glyphosate resistance gene EPSPS was quantified using digital droplet polymerase chain reaction within individuals and for parents and offspring of controlled crosses, with copy numbers ranging from 1 to approximately 160. The nonlinear pattern of gene copy number inheritance from parent to offspring exhibits shifting directionality. Below the threshold of approximately 33 copies, the slope of the parent-offspring relationship is high (1.60) because parents with fewer gene copies yield similar or increased mean offspring copy numbers, relative to parents. By contrast, above the threshold, the slope is low (0.21), and offspring of high copy number parents carry increasingly fewer EPSPS copies than parents. Intra-individual variance in copy number increases with mean individual copy number, probably contributing to variation in offspring copy number, but not sufficient to explain extreme transgressive copy number variants nor the loss of high copy number. In short, the copy number is less stable and transmitted to offspring at a lower rate in parents with more copies. This suggests a constraint on the evolution of herbicide resistance through increased EPSPS copy number.