DOMESTICATED PLANT - DOMESTICATED PATHOGEN?

Corn and corn smut

How has the evolutionary history of a crop plant affected the evolution of that plant's most common pathogen? In this BioComplexity project, we are interested in the impact of biogeographic history and spatial dynamics on the evolutionary interaction of corn (Zea mays) and corn smut (Ustilago maydis). Specific research questions are:

• What is the genetic basis of resistance to smut in corn?
• How does the biogeographic history and spatial population structure of the host affect the population dynamics and evolution of the pathogen?

Background

Knowing the evolutionary history of the host plant allows strong inferences to be made regarding the evolutionary history of the pathogen. The domestication of corn from teosinte as a food crop species occurred approximately 8,000 years ago in central Mexico. Maize was brought into South America ca. 4,000 years ago and into North America 800 - 1,000 years ago. The vast acreages that corn now occupies in the North American landscape and other parts of the world is very recent and represents one of the largest range expansions in the history of the continent.

Along with corn came corn smut. Intriguing references from early Native American history suggests that as these people brought corn northward, they also brought corn smut. The pathogen was not necessarily regarded as a pest but as a delicacy. Galls of smut on corn ears are sold in many Central American markets today. However, smut is an unwanted pathogen in modern agricultural practice and selective breeding programs of the 1940s and 1950s were apparently successful in preventing damaging epidemics. We ask whether corn smut has become "domesticated", because smut populations have not evolved strong virulence in response to resistance breeding, as occurs in many other crop pathogens.

Current Studies and Results

Brief descriptions of studies conducted under the BioComplexity project are listed below with the persons conducting the research noted.

• Genetic basis of resistance and susceptibility of corn to corn smut. Using recombinant inbred lines of corn and a QTL approach, loci of strong and of moderate effect on corn-smut interactions have been located (Andy Baumgarten, in collaboration with Dr. Ron Phillips, Dept. Agronomy and Plant Genetics).
• Population structure and dynamics of U. maydis. Results demonstrate a patchy distribution of smut genotypes across N. American populations that suggests relatively low levels of gene flow despite the widespread occurrence of the host (James Garton)
• Modeling the dynamics of U. maydis populations. Models to explore alternative hypotheses for the observed spatial structure of N. American populations. Early founder events or high levels of gene flow from C. American populations? (with Frank Shaw).
• Evolutionary relationship of U. maydis to members of the Ustilaginales. Does the grass host exert selective pressure on pathogen population and lead to population and species divergence? Sampling of Ustilaginales will be conducted in Central America on corn, teosinte and related grass hosts (Andy Munkacsi).
• Differentiation of smut populations in North and South America. Measurement of inbreeding within local populations and migration between populations is to determine underlying causes of dramatic differences we observe between N. and S. American smut populations (in collaboration with Charlie Barnes and James Groth, Dept. Plant Pathology).

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Last modified on Tue, Jun 29, 2004