Gout, L; Fudal, I; Kuhn, ML; Vincenot, L; Rouxel, T; Balesdent, Marie-Hélène*.
Genome structure impacts molecular evolution at the AvrLm1 and AvrLm6 avirulence loci of Leptosphaeria maculans. In: International Meeting on "Population and Evolutionary Biology of Fungal Symbionts", Ascona, Switzerland, 2007. AB-19. Click here to download the presentation on pdf format.

The Dothideomycete Leptosphaeria maculans is the most damaging disease of oilseed rape (Brassica napus) worldwide. Genetic studies demonstrated the occurrence of gene-for-gene interactions in the L. maculans / B. napus system. In France, disease control relies mainly on the use of disease-resistant cultivars. The Rlm genes effectively control the disease as long as the corresponding avirulent allele (AvrLm) dominates in the pathogen population. However, L. maculans has the ability to very rapidly adapt to the selection pressure exerted by a novel resistance gene as exemplified by the 3-year evolution towards virulence at the AvrLm1 locus in French field conditions. Similarly, the Rlm6 resistance gene has been overwhelmed within three years in France in experimental fields. The AvrLm1 and AvrLm6 avirulence (Avr) genes were recently cloned and shown to be solo genes within non-coding, heterochromatin-like regions of 269 kb and 133 kb, respectively, consisting of mosaics of repeats degenerated following RIP (Repeat Induced Point mutations).
Polymorphism of AvrLm1 and AvrLm6 was evaluated in natural populations of L. maculans collected mainly in France and Australia in order to elucidate the mechanisms responsible for the gain of virulence at these two loci. As frequently observed for other fungal Avr genes, the two main events evidenced were deletion of the gene or accumulation of point mutations leading to inactivation of the genes. However, the genomic environment of these genes greatly influences both the size of the deletion events and the patterns of point mutations along the coding sequences. Following full or partial sequencing of the AvrLm1 genomic region in one avirulent and two virulent isolates, the gain of virulence was found to be linked for both isolates with a 260 kb deletion of a chromosomal segment spanning AvrLm1, with identical or similar deletion breakpoints for the two virulent isolates. Among 343 field isolates analyzed, a similar large deletion around AvrLm1, leading to chromosome length polymorphism, was evidenced in > 90% of the virulent isolates. Deletion breakpoints were strongly conserved in most of the virulent isolates leading to the hypothesis that a unique event of large deletion leading to the avrLm1 virulence has diffused in populations or is strongly constrained by the genome environment. For both AvrLm1 and AvrLm6, point mutations along the genes in the rare virulent isolates for which the gene was still present corresponded to RIP mutations, as observed in the nearby repeats. Interestingly, the two main mechanisms for gain of virulence in L. maculans, i.e. RIP inactivation and large deletions probably due to ectopic recombination between repeated sequences, are driven by the sexual reproduction, an essential stage of the pathogenic cycle of this fungus.

*Institution: INRA, France

Email: mhb@versailles.inra.fr