Journal of Research in Weed Science

Abstract Echinochloa colona (L) Link. is a problematic annual grass weed in summer fallows in northern Australian cropping regions where repeated use of glyphosate has resulted in the evolution of glyphosate resistance. Pot trials conducted on E. colona populations collected from northern Australia identified 34 glyphosate resistant populations out of 65 populations tested, with resistance levels varying between 2 and 11-fold. The technique of AFLP (amplified fragment length polymorphism) was used to investigate genetic diversity within and between two resistant and one susceptible population. Within these three populations, a total of 354 fragments were identified with 99.2% being polymorphic. The frequency of polymorphic fragments within the 30 individuals from each of the two resistant populations (81.0 and 83.9%) was similar to the susceptible population (80.8%), suggesting no apparent selection bottleneck associated with resistance evolution. The large genetic diversity present within populations suggests a significant level of outcrossing between individuals. A high level of genetic diversity among the individuals was identified across a single individual examined from each of 62 populations. These individuals clustered into four main groups with three isolated accessions. Individuals did not cluster geographically; additionally, individuals did not cluster by resistance or susceptibility to glyphosate either. The results of this study suggest glyphosate resistance evolved independently across a wide geographical region in northern Australia and the large genetic diversity within populations likely contributed to rapid resistance evolution.

Abstract Glyphosate resistant Echinochloa colona is widespread in fallow systems of northern Australia from intensive glyphosate use. The inheritance of glyphosate resistance and the potential for gene transfer through pollen from resistant to susceptible individuals were investigated. A glyphosate resistant population A533.1, containing a mutation in EPSPS, was used as the resistant parent and Echi S as the susceptible parent. Gene flow via pollen between adjacent susceptible and resistant individuals was examined by treating progeny from the susceptible individuals at 240 g ha^-1 glyphosate with 1.38% gene flow detected. Survivors were selfed and the progeny segregated 3:1 for survival to 240 g ha^-1 glyphosate, consistent with single dominant gene inheritance. Hand crosses between resistant and susceptible individuals produced a single F₁ seed which carried the mutation in EPSPS. The F₂ generation from the hand cross had a response to glyphosate, intermediate between the two parents and similar to that expected for a single largely dominant gene. Sequencing the EPSPS cDNA detected at least two EPSPS genes expressed in E. colona, only one of which carried the mutation. Glyphosate resistance in this population of E. colona is inherited as a single largely dominant allele.