vH9 avirulence gene discovery in the Hessian fly

The deployment of resistant (R) genes in wheat (Triticum spp) is the most effective method of Hessian fly (Mayetiola destructor) control. Similar to plant pathogenic bacteria, fungi and nematodes, resistance to this insect is hypothesized to depend on an interaction between plant R gene products and the corresponding “Avirulence” (Avr) gene products in the pest. Avirulent Hessian flies, expressing an Avr gene, are unable to survive and stunt resistant wheat that carries the corresponding R gene. However, loss-of-function mutations in the Avr gene create a virulent genotype that can overcome the resistance. These mutations therefore limit the durability of R gene protection. In this study, we used a FPC-based physical map of the small Hessian fly genome to map mutations associated with Hessian fly virulence to the wheat R gene H9. Using three different mapping populations, the position of these mutations was resolved to a 180-kb genomic position near the telomere of the short arm of Hessian fly chromosome X1. This region was completely sequenced revealing candidate H9 Avr genes. Structural comparisons of this region between H9-avirulent and H9-virulent genotypes suggested that only three candidate Avr genes were present in this region. Transcriptional analysis and association mapping narrowed the number of candidate genes to two adjacent genes putatively encoding proteins with 62% amino acid similarity. An insertion upstream of both genes was diagnostic for virulence in four field populations.