Study of glyphosate impact on the genomic evolution of the target species

FERNÁNDEZ, L; DISTÉFANO, A.J; DE HARO, L; HOPP HE; TOSTO D. S.

Buenos Aires Argentina

Congreso; 11º Simposio Internacional sobre la Bioseguridad de los Organismos Genéticamente Modificados (ISBGMO); 2010

Sociedad Internacional para la Investigación en Bioseguridad (ISBR)

Commercial cultivation of herbicide resistant GM have a complex impact in agroecosystems involving selective pressure over target species. Glyphosate resistance on soybean, canola, cotton and corn is the most widespread character in GM crops and the selection pressure exerted on the weeds that are targets of herbicide action are unique in history. This harsh selective pressure has allowed the selection of resistant mutants. Johnsongrass (Sorghum halepense L.) is a widely distributed weed in many countries and represents a major problem to agriculture. In Argentina, first glyphosate tolerant Johnsongrasses were detected as few focuses in Tartagal (Salta) and in Estación Aráoz (Tucumán), in 2005. More recently, about 10 more locations were notified to SENASA (quarantine authority in Argentina). The monitoring of the impact of commercial GM by molecular analysis of the population genomic, population dynamics, and phylogeographic characterization of target species, are very usefull tools to assess one aspect of the enviromental risk and to cary out a correct weed control management The problem to address is a product of the dynamics of an artificial system in nature where evolutionary processes are operating. One of the important points is to determine if the resistance has monophyletic or polyphyletic origins. In the first case, by founder effect (genetic drift) it is expected that invading resistant plants are much more alike (in their genetic background) than their neighboring native susceptible plants. Assuming polyphyletic hypothesis resistant plants have genetic backgrounds similar to their neighbor in ecotypes or populations. Fifty geographically diverse samples were analyzed for 10 heterologous microsatellites representative of 20 loci of the tetraploid genome in an ABI automatic sequencer denoting an elevated polymorphism and suggesting a strong allogamic behavior. Most of the samples clustered according to their geographic origins, regardless of their resistance to glyphosate, thus supporting a polyphyletic origin hypothesis (i.e. independent mutant events were selected).  A phylogeographic study by analising different polymorphic chloroplast regions is alsocarring out. Best studied resistance mechanisms are those derived from directed mutagenesis of EPSPS gene indicating that aminoacid positions 101 and 106 are critical to tolerate glyphosate while maintaining a good PEP substrate affinity. EPSPS genes from different sources of Johnsongrass (resistant and susceptible plants) and cultivated sorghum control were sequenced except for the leader peptide (90% of the total coding region). Different deduced aminoacid polymorphisms were detected, but none of them were located in positions 101 or 106, strongly indicating that resistance mechanism is not based on known mutations of the herbicide target enzyme gene.