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Genetics Section

Meyer, Susan E. [1], Nelson, David L. [1], Clement, Suzette [1], Ramakrishnan, Alisa [2].

Ecological Genetics of the Bromus tectorum-Ustilago bullata Pathosystem: A Role for Frequency Dependent Selection?.

As part of studies to evaluate the use of Ustilago bullata, the pathogen that causes head smut of grasses, as a biocontrol agent for the inbreeding annual grass weed Bromus tectorum, we tested the hypothesis that epidemics rarely eradicate B. tectorum because of the presence of resistance polymorphism within host populations. We screened four host populations for resistance polymorphism with reference to virulence races present in co-occurring pathogen populations. Preliminary studies suggested that host lines are resistant to races from pathogen populations that are not co-occurring, but more detailed work revealed the presence of host lines in each population that had a resistance gene corresponding to an avirulence gene in a race from the co-occurring pathogen population. In these populations, the disease is present at endemic levels, most of the host lines are susceptible to all tested pathogen isolates, and host lines with resistance genes are rare, making it unlikely that resistance polymorphism plays much role in mediating disease levels. There is another line of evidence that resistance polymorphism is affecting population genetics of these populations, however. Using SSR's to define host genotypes, we found that the relative frequency of common genotypes differed between smutted and unsmutted subsets of two populations, and that these differences persisted across years. At a fifth site, where head smut disease had occurred at epidemic levels, over half the isolates tested were avirulent on several host lines, a much higher fraction than found at sites where the disease is endemic. Frequency-dependent selection may operate even where the disease is endemic, but it can be detected directly only when weather permits high rates of infection for susceptible individuals, so that non-smutted individuals are likely to represent lines that are resistant to the currently abundant pathogen race.

1 - USDA Forest Service, Rocky Mountain Research Station, Shrub Sciences Laboratory, 735 N 500 E, Provo, Utah, 84606, USA
2 - Brigham Young University, Department of Plant and Animal Sciences, Provo, Utah, 84606, USA

resistance polymorphism
biological control
downy brome
head smut disease

Presentation Type: Paper
Session: 41-7
Location: Peruvian (Cliff Lodge)
Date: Wednesday, August 4th, 2004
Time: 9:30 AM
Abstract ID:728

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