Candida albicans is among the most ubiquitous of all human pathogens, causing infections of the skin and mucosa in otherwise healthy individuals and severe systemic disease in individuals with impaired immunity. Just as C. albicans is the cause of a wide variety of infections, it can display a great variety of morphologies and other phenotypes. Much of the variability seen with C. albicans is due to high levels of genetic instability. This variability can be detected through high frequencies of morphologic changes, mutation to drug resistance, chromosome aberrations, and recombinational switching systems. The experiments in this proposal are designed to determine the molecular basis of this instability. Experiments in the proposal will: 1) examine the movement of a gene family that undergoes recombinational switching, 2) determine mechanisms for spontaneous mutation and morphologic variation, 3) construct a physical map of a Candida chromosome and use it to assess the nature of variant karyotypes seen in nature, and 4) examine the extent of genetic variation during infection. Several new techniques will be developed for the Candida genetic system. These include Candida-Saccharomyces shuttle artificial chromosome vectors and procedures for identifying homozygous site-- directed mutants of the Candida chromosomes. A polymerase chain reaction (PCR) assay will be developed for the detection of C. albicans in tissue samples. Efforts will be made to integrate the genetic analyses done on laboratory strains with natural variants that have been extensively characterized. These efforts should reveal which mechanisms for genetic variation are significant in the infection cycle and others which are more important for the emergence of new strains with altered virulence characteristics.