Determinants of Mating Type in Phytophthora

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Judelson 9722947 The goal of this research is to understand the regulation and evolution of mating systems in oomycete fungi. Using the heterothallic species Phytophthora infestans as a model system, the basis of mating type (sexual compatibility) will be determined by cloning the mating type locus and determining its function. This is expected to lead to studies of other heterothallic and homothallic species of Phytophthora, of mating type loci in other oomyceteous genera, and of other genes involved in the mating response. During previous studies by this laboratory DNA markers flanking the mating type locus were isolated and used to develop a genetic model to explain the determination of mating type; those markers are now being used to isolate the mating type locus by chromosome walking using cosmid and bacterial artificial chromosome libraries. In addition, the prior studies indicated that the chromosomal region containing the mating type determinants of P. infestans displayed several unusual genetic and physical characteristics. These included a non-Mendelian pattern of inheritance equivalent to that observed in systems of balanced lethal loci, and instability of the region as evidenced by its propensity to rearrange; these phenomena will continue to be addressed in this project. The oomycetes are an important but under-studied group of diploid lower eukaryotes that include many economically and environmentally significant species. These include pathogens of plants and animals, biocontrol agents of other pathogens, and saprophytes. P. infestans is a natural first choice for these studies due to the well-developed molecular and classical genetic techniques available for the species. P. infestans is also a widespread and enormously important pathogen of several food crops. Consequently, the importance of developing expertise in the molecular manipulation of this pathogen can not be over-stressed. Furthermore, the analysis of mating type in P. infestans represents an opportunity to study an important aspect of its biology. Mating systems are significant since they regulate the flow of genes through populations; for plant pathogens such as P. infestans, such genes include those determining pathogenicity, host preference, race specificity, and fungicide resistance. The mating cycle is also significant since the sexual spores (oospores) are long-lived and serve as an important source of inoculum for outbreaks of plant disease. Much will be learned from these studies including information about the regulation of mating type in diploids and the evolution of an unusual chromosomal region. This research will also illuminate fundamental aspects of oomycete biology, including those concerning gene regulation, cellular communication, signal transduction, and development. It is likely that mating in oomycetes is regulated by mechanisms very different from those of ascomycetes and basidiomycetes, which lack taxonomic affinity to the oomycetes and in which mating is controlled by different genetic and biochemical mechanisms. In the long term, these studies may have significant practical applications as well. If the factors which control mating can be identified, it might be possible to control plant diseases by manipulating or exploiting portions of the mating cycle. Studies in P. infestans will naturally lead to an understanding and ultimately control of other oomycete plant pathogens as well.