Evidence for the parasexual cycle in a strain of Aspergillus flavus containing virus-like particles

Mycoviruses in Aspergilli: A Comprehensive Review

Fungi, similar to all species, are susceptible to viral infection. Aspergillus is arguably the most well studied fungal genus because of its medical, ecological and economical significance. Mycoviruses were initially detected in Aspergillus species almost 50 years ago and the field continues to be active today with ground-breaking discoveries. The aim of the present review is to cover the scientific progress in all aspects of mycovirology as exemplified by Aspergillus-focused research. Initially an overview of the population studies illustrating the presence of mycoviruses in numerous important Aspergillus species, such as A. niger, A. flavus, and A. fumigatus with be presented. Moreover the intricacies of mycovirus transmission, both inter- and intra-species, will be discussed together with the methodologies used to investigate viral dispersion in a laboratory setting. Subsequently, the genomic features of all molecularly characterized mycoviruses to date will be analyzed in depth. These include members of established viral families, such as Partitiviridae, Chrysoviridae and Totiviridae, but also more recent, novel discoveries that led to the proposal of new viral families, such as Polymycoviridae, Alternaviridae and, in the context of the present review, Exartaviridae. Finally, the major issue of phenotypic effects of mycoviral infection on the host is addressed, including aflatoxin production in A. flavus, together with growth and virulence in A. fumigatus. Although the molecular mechanisms behind these phenomena are yet to be elucidated, recent studies suggest that by implication, RNA silencing may be involved.

Incidence and stability of infection by double-stranded RNA genetic elements inAspergillussectionflaviand effects on aflatoxigenicity

Ninety-two isolates belonging to Aspergillus sect, flavi were analyzed for double-stranded (ds) RNA via standard cellulose chromatography. Double-stranded RNA infection was detected in fungal isolates that had been in culture for long periods (5 of 26 were infected) and in those recently isolated (5 of 66 were infected). The number of dsRNA genetic elements differed among infected isolates and no two isolates contained identical dsRNAs on the basis of electrophoretic migration in agarose gels. Addition of micronutrients to culture media affected both the amount of dsRNA produced and the number of dsRNA genetic elements detected. Attempts to cure six fungal isolates of dsRNA by serial single conidial transfer, chlorate selection for nitrogen-metabolism mutants, and cycloheximide treatment, met with variable results. The frequency at which serial single conidial transfer and nitrogen-metabolism mutant (nit) selection successfully cured six Aspergillus sect, flavi isolates varied from 11 to 100% and 0 to 100%, respectively. The cycloheximide treatment was effective at curing 40% of the dsRNA-infected isolates. Comparison of aflatoxin production prior to and after dsRNA curing indicated that infection by dsRNA did not influence aflatoxin production. However, aflatoxin production by two isolates (91-031B and 91-184G) was reduced by both single conidial transfer and induction of nit mutants. Keywords: aflatoxin, cycloheximide, mycovirus, nitrogen-metabolism mutant.

Genetics of Aspergillus flavus: complementation and mapping of aflatoxin mutants

Mutants ofAspergillus flavusimpaired in aflatoxin production were induced withN-methyl-N′-nitrosoguanidine and analysed by means of the parasexual cycle. The gene symbolaflwas assigned to this type of mutation. Diploid complementation tests revealed that most of 14aflmutants belonged to different complementation groups. One mutant (afl-1) failed to complement or only partially complemented all other mutants. Haploidization of one diploid revealed the independent segregation of twoaflmutants. Heterozygous diploids were synthesized between twoaflmutants and tester strains genetically labelled on eight linkage groups. Haploidization of the diploids led to the assignment of two aft mutants to linkage groups. Linkage ofafl-4 tow+andafl-1 toleuon linkage groups II and VII, respectively, was demonstrated.