No abstract

A collection of 17 atrazine-degrading bacteria isolated from soils was studied to determine the composition of the atrazine-degrading genetic potential (i.e. trzN, trzD and atz) and the presence of IS1071. The characterization of seven new atrazine-degrading bacteria revealed for the first time the trzN-atzBC gene composition in Gram-negative bacteria such as Sinorhizobium sp. or Polaromonas sp. Three main atrazine-degrading gene combinations (i) trzN-atzBC, (ii) atzABC-trzD and (iii) atzABCDEF were observed. The atz and trz genes were often located on plasmids, suggesting that plasmid conjugation could play an important role in their dispersion. In addition, the observation of these genes (i) on the chromosome, (ii) on the same DNA fragment but on different plasmids and (iii) on DNA fragments also hybridizing with IS1071 suggests that transposition may also contribute to disperse the atrazine-degrading genes.

Selection at the group level is proposed to be an evolutionary process occurring in the context of multilevel selection in natura. In artificial selection experiments, selecting at the community level can allow to find multispecies assemblages that are more efficient than a single species at solving a given problem. In such procedures, the main difficulty is to find a balance between variation and heritability, which are both essential for selection to act. The aim of our study was to determine if the way of creating offspring units of selection from parental units, called "reproduction method," could influence artificial selection efficiency through a differential in the variation/heritability balance. Selecting microbial communities depending on their biomass production and propagating them either one by one or in a mix of three communities, we showed that the effect of the reproduction method was not maintained over time with a loss of the effect of artificial selection on community phenotype at certain cycles and a very low heritability. However, mixing parental communities was more efficient at increasing biomass production than using a single parental community (+5% of biomass). We discussed the role of differences in community richness and structure in explaining these results.