An Aspergillus flavus mutant producing stipitate sclerotia and synnemata

Abstract

The ability to produce stipitate sclerotia by a mutant culture NRRL 29254 derived from Aspergillus flavus NRRL 3357 that produces sessile sclerotia is reported for the first time. The mutant produced stipitate sclerotia on Czapek agar (CZA) and synnemata formed on Murashige-Skoog agar (MSA) and on oatmeal agar. Temperature, light, and pH influenced stipitate and sessile sclerotium formation. Sclerotial yield was greatly affected by the type and concentration of carbon and nitrogen sources but not significantly by the C:N ratio. Stipitate sclerotia were abundant when the carbon source in CZA was replaced with dextrose, fructose, melibiose or xylose whereas MSA amended with fructose, mannitol or sorbitol produced numerous erect synnematous structures instead of stipitate sclerotia. Glycine-, asparagine-, or threonine-amended CZA supported abundant production of stipitate sclerotia comparable to those obtained when the nitrogen sources were NaNO₃ and KN0₃, while CZA amended with lysine, serine, proline or hydroxyproline produced mostly synnemata. The wild type and the mutant were very similar in morphology, cultural and physiological characteristics, and DNA fingerprints, but the wild type produced only sessile sclerotia and no synnemata. The production of stipitate sclerotia by the mutant substantiates previous suggestions for an evolutionary link between A. flavus and Stilbothamnium togoense, a tropical fungus that produces stipitate and sessile sclerotia. Weak bands were detected when the genomic DNA of S. togoense was fingerprinted using the DNA probe pAF28 derived from A. flavus, suggesting some degree of DNA homology between these two fungi.

Key Words:

• Carbohydrates
• C:N ratio
• DNA fingerprinting
• Nitrogen
• Stilbothamnium
• Trichocomaceae