Understanding a cereal killer: The evolution of the avirulence gene Avr-Pita in the rice blast fungus
Abstract
Pyricularia oryzae (rice blast fungus) is one of the most prevalent and devastating diseases effecting rice agriculture today. Despite its impact on the economy of the globe, not much is known about the evolution of the avirulence genes within the fungus that triggers host defenses. Without understanding how these Avr genes are evolving in the field, breeders will never be able to engineer a rice cultivar that will have durable resistance because of rapid shifts to virulence on resistant rice cultivars. This dissertation focused on assessing the allelic diversity of the avirulence gene Avr-Pita, to provide a new insight into the evolution of avirulence genes in the rice blast fungus. First, an assessment of the frequency of mutational mechanisms was determined for the blast population in Colombia, South America. The results indicate that point mutation was the primary mode of Avr-Pita mutation in field isolates; this was contrary to previously published results which stated that Avr-Pita deletion was the primary mode of evolution. This data ultimately showed that field isolates favor Avr-Pita modification over deletion for shifts to virulence. A global data set was then analyzed to determine whether the results in Colombia were a good indicator of the type and frequency of mutational mechanisms on a global scale, as well as to determine migration and distribution patterns. 11 global rice growing regions (141 sequences) were included in the analysis and it was revealed that the distribution of diversity was heterogeneous, rather than country specific. New World (the Americas) vs. Old World (China, Thailand, India, and Indonesia) did show some pattern of distribution with the Old World isolates being distributed throughout the tree and the New World isolates being more distant from the base of the tree. The tree as a whole indicates a high level of spore dispersal (human trans-continental transportation) coupled with a highly conserved gene. This research has an impact on the rice agricultural community due to its effect on gene pyramiding schemes and other disease management strategies. It ultimately clarifies that avirulence genes remain a target for resistance breeding even in this rapidly evolving fungus.
Degree
Ph.D.
Advisors
Levy, Purdue University.
Subject Area
Genetics|Evolution and Development|Plant Pathology
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