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

Abstract

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.

Genetic diversity and structure of Lolium perenne ssp. multiflorum in California vineyards and orchards indicate potential for spread of herbicide resistance via gene flow

“…Short-and long-distance gene flow may also be mediated by seed movement on agricultural machinery and vehicles, or over short distances by wind or animals. The higher levels of admixture detected in this study compared to studies of other herbicide-resistant weeds (Kuester et al, 2015;Okada et al, 2013Okada et al, , 2014 reducing the selection pressure of glyphosate in these systems. To limit spread of resistance through gene flow, cleaning weed seed from equipment and shoes moved between infested fields may help reduce long-distance seed transfer.…”

“…Populations of species with an outcrossing mating system would be expected to have higher genetic diversity than those with predominantly self-pollinating mating systems. Glyphosateresistant populations of the two closely related selfing species Conyza canadensis and Conyza bonariensis in California displayed a wide range of genetic diversities (H E = 0.0-0.45 and H E = 0.009-0.513, respectively) based on microsatellite markers (Okada et al, 2013(Okada et al, , 2014. in California populations was also similar or higher than that seen in studies of L. perenne cultivars using some of the same microsatellite markers (N A = 19.4, 13.3, and 9.9; Kubik et al, 2001;Wang et al, 2009;Brazauskas et al, 2011, respectively).…”

“…Herbicide resistance alleles may be present in populations prior to the onset of selection pressure by an herbicide (Delye, Deulvot, et al, 2013), and may spread by gene flow even before the trait is selectively advantageous. Evidence for the spread of herbicide resistance among populations by seed dispersal has been shown in several highly selfpollinating weed species, based on patterns of molecular marker and phenotypic variation (Okada et al, 2013(Okada et al, , 2014Osuna, Okada, Ahmad, Fischer, & Jasieniuk, 2011). Interestingly, however, neutral genetic and phenotypic variation in Ipomoea purpurea, a weed species with a mixed mating system, provided support for independent origins of resistance in multiple geographic locations (Kuester, Chang, & Baucom, 2015).…”

Herbicide-resistant weeds challenge some signature cropping systems

“…Surveys can help inform growers of emerging herbicide-resistant weed populations while they are still localized; surveys are also often used to encourage adoption of resistance mitigation measures to minimize economic and environmental impacts. Further, surveys combined with population genetic research can determine the evolutionary and geographic origins, and routes of spread, of resistance across an agricultural landscape (e.g., Okada et al 2013;Okada et al 2014).…”

Carfentrazone‐ethyl and glyphosate drift inhibits uredinial formation of Austropuccinia psidii on Eucalyptus grandis leaves

“…However, in areas with frequent applications of glyphosate, some weed species such as Conyza sp. [2][3][4] and Commelina sp. 5,6 have evolved resistance and tolerance to the herbicide molecule, mainly due to the selection pressure.…”