Natural auxotrophic heterozygosity in Candida albicans

Candida albicans is imperfect and diploid. This unusual genetic state permits the occurrence of strains heterozygous for recessive deleterious alleles. These alleles can be brought to homozygosity and therefore expression by UV irradiation-induced mitotic crossing over. Heterozygosity for recessive auxotrophic alleles is found in 10 to 15% of the strains. A wide variety of auxotrophic alleles have been found in such natural heterozygotes but one type, termed Suf +/- was found at an unusually high frequency. Some 5 to 10% of strains are of the Suf +/- type. Suf- homozygous auxotrophs are defective in sulfite reductase. Such auxotrophs require a source of reduced sulfur such as methionine or cysteine, alternatively inorganic thiosulfate will serve as a supplement. Complementation analysis of a variety of Suf- auxotrophs established that 11 out of 12 strains were in a single complementation group.