Isolation of auxotrophic mutants of the methylotrophic yeast Candida boidinii and determination of its ploidy.

Auxotrophic mutations in the methylotrophic yeast strain Candida boidinii 11Bh were induced by different mutagens and their combinations (nitrosoguanidine, UV light, HNO2+UV). Majority of the mutants obtained carried defects in histidine, arginine, proline and/or adenine biosynthetic pathways. His- mutants were distributed into four complementation groups using the protoplasts fusion technique. Ploidy determination of Candida boidinii 11Bh was performed by measuring its DNA content and by following its survival after chemical mutagens treatment. The DNA content of this strain was found to be similar to that of a Saccharomyces cerevisiae diploid strain. Also the kinetics of survival of Candida boidinii cells indicate that Candida boidinii 11Bh is a diploid.