Genetics of avirulence genes in Blumeria graminis f.sp. hordei and physical mapping of AVRa22 and AVRa12

Powdery mildew fungi are parasites that cause disease on a wide range of important crops. Plant resistance (R) genes, which induce host defences against powdery mildews, encode proteins that recognise avirulence (AVR) molecules from the parasite in a gene-for-gene manner. To gain insight into how virulence evolves in Blumeria graminis f.sp. hordei, associations between segregating AVR genes were established. As a prerequisite to the isolation of AVR genes, two loci were selected for further analysis. AVRa22 is located in a tightly linked cluster comprising AVRa10 and AVRk1 as well as up to five other AVR genes. The ratio between physical and genetic distance in the cluster ranged between 0.7 and 35 kB/cM. The AVRa22 locus was delimited by the previously isolated gene AVRa10 and two cleaved amplified polymorphic sequence (CAPS) markers, 19H12R and 74E9L. By contrast, AVRa12 was not linked to other AVR genes in two crosses. Bulk segregant analysis of over 100,000 AFLP fragments yielded two markers, ETAMTG-285 and PAAMACT-473, mapping 10 and 2 cM from AVRa12, respectively, thus delimiting AVRa12 on one side. All markers obtained for AVRa12 mapped proximal to it, indicating that the gene is located at the end of a chromosome. Three more AVRa10 paralogues were identified at the locus interspersed among genes for metabolic enzymes and abundant repetitive elements, especially those homologous to the CgT1 class of retrotransposons. The flanking and close markers obtained will facilitate the isolation of AVRa22 and AVRa12 and provide useful tools for studies of the evolution of powdery mildew fungi in agriculture and nature.