Comparative Genomics of Wheat Powdery Mildew

Wheat powdery mildew Blumeria graminis forma specialis tritici is a devastating fungal pathogen with a poorly understood evolutionary history. We have established a reference genome sequence of this obligate biotrophic pathogen. A comparative genome analyses of wheat and barley powdery mildew genomic sequences resulted in the identification of at least 602 candidate effector genes. This number is still increasing based on more refined sequence analysis. Many candidate effector genes show evidence for positive selection, suggesting that they indeed act as effectors and are possibly involved in host specialization. Re-sequencing of four isolates from different geographical regions in Europe and Israel revealed that their genomes differ in the presence or absence of some candidate effector genes. Importantly, mildew genomes were found to be mosaics of ancient haplogroups which existed prior to wheat domestication. This pattern of diversity in modern isolates suggests that there was no dramatic loss of genetic diversity upon formation of the new host bread wheat 10,000 years ago. We conclude that B. graminis tritici’s ready adaptation to the new host species was based on a diverse haplotype pool which provides a large genetic potential for pathogen variation.