RESUMEN

Pistachio is a popular snack food. Aflatoxin contamination of pistachio nuts is a serious problem for many producing countries. The development of biological control methods based on ecological parameters is an environmentally friendly approach. Thirty-eight Aspergillus flavus isolates collected from a pistachio orchard in California (CA) were analyzed for production of aflatoxin (AF), cyclopiazonic acid (CPA), vegetative compatibility groups (VCGs), and mating types. All aflatoxigenic isolates produced both AFB1 and CPA. The most toxigenic one was CA28 which produced 164 µg AFB1 per 5 ml PDA fungal culture and small sclerotia (S strain, sclertoium size less than 400 µm). The other aflatoxigenic strains produce AFB1 ranging from 1.2 µg to 80 µg per 5 ml fungal culture. Twenty-one percent of the CA isolates produced AFB1, 84% produced CPA and half formed sclerotia on at least one of three tested media. The 38 CA isolates formed 26 VCGs, 6 of which had two or more isolates and 20 contained single isolates. The S strain isolates belong to 4 different VCGs. Genomic profiling by a retrotransposon DNA probe revealed fingerprint patterns that were highly polymorphic. The predicted VCGs (Pred-VCGs) based on a similarity coefficient >80% matched the VCGs of multiple isolates determined by complementation. All isolates within a VCG had the same mating-type gene of either MAT1-1 or MAT1-2. Uncorrected and VCG-corrected MAT1-1 and MAT1-2 among the isolates were equally distributed.

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RESUMEN

The objective of this study was to evaluate the ability of the Aspergillus flavus pAF28 DNA probe to produce DNA fingerprints for distinguishing among genotypes of Petromyces alliaceus (Aspergillus section Flavi), a fungus considered responsible for the ochratoxin A contamination that is occasionally observed in California fig orchards. P. alliaceus (14 isolates), Petromyces albertensis (one isolate), and seven species of Aspergillus section Circumdati (14 isolates) were analyzed by DNA fingerprinting using a repetitive sequence DNA probe pAF28 derived from A. flavus. The presence of hybridization bands with the DNA probe and with the P. alliaceus or P. albertensis genomic DNA indicates a close relationship between A. flavus and P. alliaceus. Twelve distinct DNA fingerprint groups or genotypes were identified among the 15 isolates of Petromyces. Conspecificity of P. alliaceus and P. albertensis is suggested based on DNA fingerprints. Species belonging to Aspergillus section Circumdati hybridized only slightly at the 7.0-kb region with the repetitive DNA probe, unlike the highly polymorphic hybridization patterns obtained from P. alliaceus and A. flavus, suggesting very little homology of the probe to Aspergillus section Circum dati genomic DNA. The pAF28 DNA probe offers a tool for typing and monitoring specific P. alliaceus clonal populations and for estimating the genotypic diversity of P. alliaceus in orchards, vineyards, or crop fields.

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Petromyces alliaceus Malloch and Cain is the only known sexually reproducing fungus classified in Aspergillus section Flavi. The goal of this research was to identify culture media and sources of nitrogen that best support the formation of stromata with ascocarps. Three cultures of P. alliaceus isolated from crop field soils were grown on selected agar media in Petri dishes for 7 months at 30 degrees C in darkness. The largest numbers of stromata were recorded for cultures grown on Czapek's agar (CZA) and a mixed cereal agar (MCA), while the percentage of stromata containing ascocarps was greatest (P

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The ability of Aspergillus caelatus, a species in Aspergillus section Flavi, to produce synnemata and sclerotia was investigated. Forty-eight isolates of A. caelatus differed widely in their production of synnemata and sclerotia; 83% of the isolates produced varying numbers of synnemata and sclerotia, and 17% produced neither sclerotia nor synnemata. Most strains produced synnemata and mostly sessile and few stipitate sclerotia on the same Czapek agar (CZA) plate. Two strains of A. caelatus were selected for further study because of the contrasting morphology of their synnemata and sclerotia. Those strains are NRRL 25528, the type species and a representative of the synnema- and black sclerotium-forming isolates, and NRRL 26119, considered an atypical strain that produced numerous synnemata and few slightly melanized or tan sclerotia. The induction and maturation of sclerotia in A. caelatus were affected greatly by the type of media as well as the kind and concentration of the carbon and nitrogen sources. CZA induced synnema and sclerotium production in both strains, whereas other media did not. Production of abundant synnemata and sclerotia also occurred when the carbon source in CZA is replaced with dextrose, xylose, cellobiose, melibiose and trehalose. CZA amended with serine, threonine, KNO(3) and NaNO(3) induced the production of numerous sclerotia and synnemata. For both strains, the optimal levels of sucrose and NaNO(3) for maximum production of synnemata or sclerotia were 3 and 0.9%, respectively. The production of synnemata and stipitate/sessile sclerotia by several wild-type strains of A. caelatus further substantiates previous suggestions for an evolutionary link between Aspergillus section Flavi and synnematal species A. togoensis, which also produces stipitate sclerotia.

RESUMEN

A genomic clone of the aflatoxin-producing fungus Aspergillus flavus, designated pAF28, has been used as a probe for Southern blot fingerprinting of fungal strains. A large number of A. flavus strains isolated from corn fields and tree-nut orchards can be distinguished because the DNA fingerprint patterns are highly polymorphic. We have completed the sequencing of a 6355 bp insert in pAF28. The sequence features motifs and open reading frames characteristic of transposable elements of the gypsy class. We have named this new element AfRTL-1, for A. flavus retrotransposon-like DNA.

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