Mechanisms of Fungal Pathogenesis: Analysis of a Meiotically Unstable, Dispensable Chromosome

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9507263 Covert Certain isolates of Nectria haematococca, a fungal pathogen of plants, contain a chromosome that is often rearranged, partially deleted or completely lost during meiosis. Changes in the structure of the chromosome can be visualized, and the sizes of the resulting chromosome derivatives measured on pulsed field gels. The pattern of chromosomal alterations suggests that this chromosome contains sites that are particularly susceptible to breakage and reunion. The first goal of this proposal is to determine if this chromosome contains hotspots for meiotic breakage and reunion, and if so, to map the position of these sites on the chromosome. The unstable chromosome in N. haematococca is of further interest because it contains a gene, Makl, believed to be essential for pathogenicity on chickpea. The product of Makl detoxifies an antimicrobial compound produced by chickpea in response to infection. Genetic analysis of Makl indicated that it is either linked to genes for, or is itself responsible for high virulence on chickpea. Makl has been cloned and sequenced. The second goal of this research proposal is to directly test the importance of Makl in pathogenicity by disrupting it and assaying the resulting mutants for their ability to cause disease symptoms on chickpea. If the gene disruption experiment indicates that Makl is not essential for high virulence on chickpea, it will imply that additional genes involved in chickpea pathogenicity are on the same chromosome as Makl. In this case, the third goal of this proposal, mapping novel genes for chickpea pathogenicity on the 1.6 Meg abase Makl chromosome, will be pursued. %%% This research proposal focuses on a fungal pathogen of plants, Nectria haematococca. Two aspects of N. haematococca's biology are particularly interesting: (1) it has an unusual chromosome that is unstable during meiosis as well as dispensable for saprophytic growth, and (2) according to genetic evidence, genes on this chromosome are responsible for high virulenc e of N. haematococca on chickpea (Cicer arietinum). Because the unstable chromosome is present in field populations, it appears to confer a significant adaptive advantage to the isolates that contain it. Its instability may be a mechanism for shedding it in habitats in which it is extraneous. This research into chromosomal instability and plant pathogenicity will contribute to scientific knowledge on a number of fronts. The role of unstable, dispensable chromosomes in fungal-plant interactions will be clarified, the genetic machinery underlying pathogenicity in a model fungus will be characterized further, and perhaps most significantly, scholarship on the general mechanisms of chromosome instability will be advanced. ***