Evolution and spread of glyphosate resistance in Conyza bonariensis in California and a comparison with closely related Conyza canadensis

Glyphosate-resistant weeds are an increasing problem in perennial cropping systems in the Central Valley of California, USA. To elucidate the evolutionary origins and spatial spread of resistance, we investigated the geographical distribution of glyphosate resistance and the population genetic diversity and structure of Conyza bonariensis and compared the results with previously studied C.canadensis. Thirty-five populations from orchards and vineyards across the Central Valley were sampled. Population genetic structure was assessed using microsatellite markers. Population-level resistance was assessed in glasshouse screening of plants grown from field-collected seed. Bayesian clustering and analyses of multilocus genotypes indicated multiple origins of resistance, as observed in C.canadensis. Pairwise F-ST analysis detected spatial spread of resistance in the south of the Central Valley, also similar to C.canadensis. The results strongly indicate that the southern valley was an environment markedly more suitable than the northern valley for resistance spread and that spread in Conyza species was driven by increased uniformity of strong selection in the southern valley, due to recent regulation on herbicides other than glyphosate. Accordingly, resistant C.canadensis individuals occurred at high frequencies only in the southern valley, but interestingly resistant C.bonariensis occurred at high frequencies throughout the valley. Expression of resistance showed varying degrees of plasticity in C.bonariensis. The lower selfing rate and substantially greater genotypic diversity in C.bonariensis, relative to C.canadensis, indicate greater evolutionary potential over shorter time periods. Interspecific hybridisation was detected, but its role in resistance evolution remains unclear.