Genetic diversity of glyphosate resistant Echinochloa colona

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.