Spread of glyphosate‐resistant sourgrass (Digitaria insularis): Independent selections or merely propagule dissemination?

Abstract

The first reports of glyphosate-resistant (GR) sourgrass (Digitaria insularis) came from Paraguay (2005) and from South and Southeast Brazil, in Paraná (2008), and São Paulo state (2009). The hypothesis of this research is that the evolution of these resistant populations might have occurred independently. To test this hypothesis, a dose-response experiment was conducted in order to confirm resistance and inter simple sequence repeat markers subsequently were evaluated in five sourgrass populations from Paraguay (PY-R), Paraná (PR-S and PR-R), and São Paulo (SP-S and SP-R). Resistant populations were confirmed in PY-R, PR-R and SP-R. The resistant populations presented lower polymorphism than susceptible populations, possibly related to the selection pressure exerted by the herbicide. Both similarity matrix and dendrogram provided evidences that the origin of resistance in the populations from Paraguay and Paraná could be the same, which also is explained by their geographical proximity. In contrast, the evolution of GR populations in São Paulo was suggested to occur independently from other locations because resistance evolved in genetically divergent populations. This study's data provide evidences that GR sourgrass populations from Paraguay and Paraná share a similar genetic background; therefore, it is possible that resistance from Paraguay spread to Paraná through gene flow. The population from São Paulo probably was selected independently because the high values of its genetic structure and low level of gene flow. Independent selections and seed dispersion of GR sourgrass have contributed to the rapid spread of resistance across South America.

Monitoring of Resistance of Sourgrass to Glyphosate Herbicide in Urban Areas of the State of São Paulo, Brazil

“…Sourgrass possibly present independent selection. Takano et al (2018) evaluated the resistance of D. insularis to glyphosate in Brazil (states of Paraná and São Paulo) and in Paraguay and found that the selection of resistant populations in São Paulo possibly occurred independently of other sites, since the resistance evolved in highly divergent populations based on ISSR sequences. According to these authors, the level of genetic divergence in D. insularis populations was considered high (GST = 0.63) due to the high level of polymorphism found in the ISSR sequences.…”

“…Therefore, control of sourgrass using glyphosate herbicide has becoming less efficient. There are several reports of resistance of D. insularis to glyphosate in Brazil, as reported by Correia et al (2010), Carvalho et al (2011Carvalho et al ( , 2012, Reinert et al (2013), Barroso et al (2015), López-Ovejero et al (2017, and Takano et al (2018).…”

“…However, it is not well known how its dispersion has been occurring throughout the country. The current hypothesis is that resistance transmission occurs through the spread of propagules by people and machine traffic (López-Ovejero et al, 2017) and/or by independent transmission, which can result in a rapid spread of resistance (Takano et al 2018).…”

Herbicide Resistance in Brazil: Status, Impacts, and Future Challenges

“…The species of the genus Conyza, which have a high invasive potential due to the large seed production, the rapid and high germination capacity, cause great damage to agriculture, and due to their poor interspecific differentiation, it can be an exchange of resistant alleles between species [42]. However, D. insularis has been, among glyphosate resistant species, one of the main problems to be faced; therefore, greater efforts have been made to characterize the factors involved in its resistance, dispersal and management [12,[43][44][45][46][47][48]. Molecular studies showed that the first glyphosate resistant D. insularis populations found in the country (Guairá-Paraná) came from Paraguay…”

Effects of 2,4-D Herbicide on Species of the Digitaria Genus

“…Sourgrass (Digitaria insularis (L.) Mez ex Ekman) is a perennial, highly-competitive grass that can reduce maize and soybean yields in more than 32 and 44%, respectively (Gazziero et al, 2012;Gemelli et al, 2013). It is nowadays one of the main weed species in Brazil, mainly due to its high dispersion capacity, control difficulties, and increasing cases of resistance to herbicides (Barroso et al, 2015;López-Ovejero et al, 2017;Takano et al, 2018).…”