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BIOlogie et GEstion des Risques en agriculture - Champignons Pathogènes des Plantes

New Phytologist ‘Tansley Insight’ on evolution of fungal avirulence effectors under R gene selection

New Phytologist ‘Tansley Insight’
T. Rouxel and MH Balesdent published a ‘Tansley Insight’ paper, critically reviewing our current knowledge on evolution of fungal avirulence effector genes under resistance gene selection. Comparison of evolution of seven different avirulence effectors from the same species, Leptosphaeria maculans, highlights some specific features : (i) evolution towards virulence is a highly dynamic process, with stages of near-extinction, followed in some cases by resurgence of alternate virulent alleles; (ii) the mode of evolution is independent of the mode of “recognition” by the cognate resistance gene; (iii) the vast number of effector genes in fungal genomes is not always indicative of functional redundancy and, in many cases, maintenance of the effector gene is the favoured option.

In agricultural systems, major (R) genes for resistance in plants exert strong selection pressure on cognate/corresponding avirulence effector genes of phytopathogens. However, a complex interplay often exists between trade-offs linked to effector function and the need to escape R gene recognition. Here, using the Leptosphaeria maculans–oilseed rape pathosystem we review evolution of effectors submitted to multiple resistance gene selection. Characteristics of this pathosystem include a crop in which resistance genes have been deployed intensively resulting in ‘boom and bust’ cycles; a fungal pathogen with a high adaptive potential in which seven avirulence genes are cloned and for which population surveys have been coupled with molecular analysis of events responsible for virulence. The mode of evolution of avirulence genes, all located in dispensable parts of the ‘two-speed’ genome, is a highly dynamic gene-specific process. In some instances, avirulence genes are readily deleted under selection. However, others, even when located in the most plastic genome regions, undergo only limited point mutations or their avirulence phenotype is ‘camouflaged’ by another avirulence gene. Thus, while hundreds of effector genes are present, some effectors are likely to have an important and non-redundant function, suggesting functional redundancy and dispensability of effectors might not be the rule.

Reference : Rouxel T, Balesdent MH (2017) Life, death and rebirth of avirulence effectors in a fungal pathogen of Brassica crops, Leptosphaeria maculans. New Phytol. DOI: 10.1111/nph.14411