3 years ago

Evolutionary analyses of the avirulence effector AvrStb6 in global populations of Zymoseptoria tritici identify candidate amino acids involved in recognition

Bruce A. McDonald, Patrick C. Brunner
We analyzed the population genetic diversity for AvrStb6, the first avirulence gene cloned from the wheat pathogen Zymoseptoria tritici, using 142 Z. tritici strains sampled from four wheat fields growing on three continents. Though AvrStb6 was located in a recombination hotspot, it was found in every strain, with 71 polymorphic sites that produced 41 distinct DNA haplotypes encoding 30 AvrStb6 protein isoforms. An AvrStb6 homolog was found in the closest known relative, Z. pseudotritici, but not in three other closely related Zymoseptoria species, indicating that this gene emerged in Zymoseptoria quite recently. Two AvrStb6 homologs with nucleotide similarities greater than 70% were identified on chromosome 10 in all Z. tritici isolates, suggesting that AvrStb6 belongs to a multigene family of candidate effectors that has expanded recently through gene duplication. The AvrStb6 sequences exhibited strong evidence for non-neutral evolution, including a large number of non-synonymous mutations, with significant positive, diversifying selection operating on 9 of the 82 codons. It appears that balancing selection is operating across the entire gene in natural field populations. There also was evidence for co-evolving codons within the gene that may reflect compensatory mutations associated with evasion of recognition by Stb6. Intragenic recombination also appears to have affected the diversity of AvrStb6. This article is protected by copyright. All rights reserved.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/mpp.12662

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