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Geographical variation and positive diversifying selection in the host-specific toxin SnToxA
Eva H Stukenbrock1, Bruce A McDonald
1Plant Pathology, Institute of Integrative Biology, ETH Zurich LFW, Universitätstrasse 2, CH-8092 Zurich, Switzerland.
Molecular Plant Pathology
|May 29, 2010
Summary
The wheat pathogen Phaeosphaeria nodorum shows high variation in its ToxA gene, including deletions, unlike Pyrenophora tritici-repentis. This genetic diversity suggests host-driven evolution.
Area of Science:
- Plant pathology
- Molecular evolution
- Genetics of host-pathogen interactions
Background:
- The wheat pathogen Phaeosphaeria nodorum produces the host-specific toxin ToxA, which interacts with the Tsn1 gene in wheat to cause necrosis.
- The ToxA gene is believed to have been transferred horizontally from Ph. nodorum to Pyrenophora tritici-repentis.
- Significant variation, including gene deletions, exists in the ToxA gene of Ph. nodorum (SnToxA) compared to Py. tritici-repentis (PtrToxA).
Purpose of the Study:
- To investigate the patterns of SnToxA deletions and sequence variation in Ph. nodorum populations across different geographical regions.
- To understand the evolutionary forces shaping the ToxA locus in Ph. nodorum.
- To explore the relationship between SnToxA allele distribution and Tsn1 allele distribution in wheat populations.
Main Methods:
- Polymerase Chain Reaction (PCR) screening of 788 Ph. nodorum isolates for SnToxA deletions.
- DNA sequencing of the SnToxA gene in 123 Ph. nodorum isolates to identify haplotypes.
- Application of selection models to analyze the evolutionary mode at the ToxA locus.
Main Results:
- SnToxA deletion frequencies varied significantly among geographical populations, from 0% in Australia to 98% in China.
- Sequence analysis revealed 13 distinct SnToxA haplotypes, with significant variations in distribution and diversity among populations.
- The majority of SnToxA mutations were non-synonymous, leading to protein-level changes.
- Evidence for positive diversifying selection at the ToxA locus was found.
Conclusions:
- The high variation in SnToxA, including deletions and non-synonymous mutations, indicates rapid evolution driven by host selection.
- The distribution patterns of SnToxA alleles and deletions likely correlate with the distribution of Tsn1 alleles in host wheat populations.
- Understanding ToxA evolution is crucial for managing wheat diseases caused by these pathogens.
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