Mitotic non-conformity in Aspergillus: successive and transposable genetic changes

Cytological characterization of an Aspergillus Nidulans mutant from a strain with chromosomic duplication

A development mutant, named V103, was obtained spontaneously from the A strain of A. nidulans. The A strain contains a duplicated segment of chromosome I that has undergone translocation to chromosome II (I II). It is mitotically unstable and generates phenotypically deteriorated types, some with enhanced stability. The deteriorated variants of A. nidulans show abnormal development, exhibiting slower colony growth, variations in colony pigmentation and changes in conidiophore structure. The alterations observed in the conidiophore include fewer metulae and phialides, further elongation and ramification of these structures, delayed nuclear migration and the presence of secondary conidiophores.

Horizontal transfer and hypovirulence associated with double-stranded RNA in Beauveria bassiana

Beauveria bassiana strains from different hosts and geographic origins were assayed for the presence of double-stranded RNA (dsRNA). Two of them (15.4%) showed extra bands, with approximately 4.0-3.5 kb and 2-0.7 kb, respectively, after electrophoretic separation of undigested nucleic acids. Virus-like particles were approximately 28-30 nm diam. The dsRNA was maintained after conidiogenesis (vertical transmission) and was transmitted horizontally by hyphal anastomosis. Strains purged of dsRNA obtained after cycloheximide treatment showed increased conidial production when compared with strains carrying dsRNA particles. Bioassays demonstrated hypovirulence associated with dsRNA. The mean mortality against the insect Euschistus heros was reduced in strains containing dsRNA when compared with the isogenic dsRNA-free ones.

RAPD analyses of recombination processes in the entomopathogenic fungus Beauveria bassiana

To understand the nature of recombination processes in Beauveria bassiana, double-auxotrophic complementary mutant strains were used to produce six heterokaryons by three different methods. Conidia from these heterokaryons were plated on selective media and stable haploid (but not diploid) recombinants were isolated. Single colony recombinants were recovered with both parental and non-parental random amplified polymorphic DNA (RAPD) profiles. These results suggest that a range of different recombination mechanisms may be occurring in B. bassiana.

Genetically modified crops: environmental and human health concerns

About 10,000 years ago subsistence farmers started to domesticate plants and it was only much later, after the discovery of the fundaments of genetics, those organisms were submitted to rational genetic improvement mainly by selecting of traits of interest. Breeders used appropriate gene combinations to produce new animal races, plant varieties and hybrids, as well as improved microorganisms such as yeasts. After the introduction of recombinant DNA techniques, the transfer of DNA between species belonging to different genera, families or kingdoms became possible. The release of transgenic plants has aroused debates about several aspects of the environmental and human risks that could result from the introduction of genetically modified crops. Less effort has been dedicated to evaluate the impact of transgenic plants on their associated microorganisms, some of which (e.g. nitrogen-fixing bacteria, mycorrhizal fungi and endophytic microbiota) are extremely important for the survival of the plant. Investigations have been made regarding the horizontal transfer of genetic material between transgenic plants and microorganisms and on the disturbance of useful symbiotic associations between plants and endophytic, epiphytic and rhizosphere communities. In most cases the results do no show any adverse effect of transgenic plants on autochthonous plant-associated microorganisms. Results from our laboratory show small changes caused by genetically modified endophytic bacteria on the indigenous endophytic population of the sweet orange Citrus sinensis. In tests using appropriated fungal strains preliminary results using extracts from transgenic plants indicate that these plants do not affect haploidization, mitotic crossing-over, mutation rate or chromosomal alterations.

Electrophoretic characterization of Aspergillus nidulans strains with chromosomal duplications

Pulsed-field gel electrophoresis was used to characterize strains of Aspergillus nidulans with a chromosomal duplication Dp(I-II). Morphologically deteriorated and improved variants of these strains were also analyzed. The electrophoretic karyotype demonstrated that in two duplicated strains (A and B) the 4.2 Mb band, which corresponds to chromosome II, was absent and a new band was observed. Hybridization studies using the uapA (chromosome I) and wA (chromosome II) genes demonstrated that the new band corresponded to chromosome II plus the duplicated segment of chromosome I. The size of the chromosomal duplication was approximately 1.0 Mb. Analysis of the chromosomal bands of a morphologically improved strain showed that the duplicated segment of chromosome I was completely lost. The morphologically deteriorated variants V9 and V17 had the same karyotype as the duplicated strains. However, the deteriorated variant V5 lost part of chromosome I and had a rearrangement involving chromosome V. This rearrangement may have resulted from the mutagenic treatment used to obtain the genetic markers. Pulsed-field gel electrophoresis was found to be an excellent tool for locating chromosomal rearrangements.

A simple method for the detection of recombinogenic substances in filamentous fungi

Several tests are available for detecting mutagenic and recombinogenic products in fungi. Since fungi are eukaryotic, these tests permit the detection of substances that produce chromosomal aberrations, or that are recombinogenic. We have developed a new method for testing substances that affect fungal stability using a duplication strain of Aspergillus nidulans and circular statistics for data analysis. The fungus was inoculated on the center of Petri dishes and the substance to be tested was applied to a defined area of the dish. Position of resulting sectors was measured by an angle and the data were analyzed by the Rayleigh test. Extensive testing with different compounds showed this method to be rapid and efficient for screening recombinogenic substances.

Characterization of an Aspergillus nidulans mutant with abnormal distribution of nuclei in hyphae, metulae, phialides and conidia

The V10 deteriorated variant of Aspergillus nidulans has hyphae, metulae, phialides and conidia with abnormal nuclear distributions. The alterations observed were: increase in the number of nuclei in hyphae, metulae and phialides, presence of anucleate, uninucleate and multinucleate conidia, abnormal vegetative growth and defection conidiation. When 0.5 M NaCl was added to the medium, an increase in the number of conidia was observed but their morphology and number of nuclei were not modified. The gene responsible for these alterations was named anuA1. The anuA1 gene is located on linkage group VII and is possibly involved in nuclear migration to hyphae, metulae, phialides and conidia.

Inheritance of Strain Instability (Sectoring) in the Commercial Button Mushroom, Agaricus bisporus

The button mushroom, Agaricus bisporus , is a commercially important cultivated filamentous fungus. During the last decade, the button mushroom industry has depended mainly on two strains (or derivatives of these two strains). Using one of these highly successful strains (strain U1) we examined the phenomenon of strain instability, specifically, the production of irreversible sectors. Three “stromatal” and three “fluffy” sectors were compared with a healthy type U1 strain and with a wild-collected isolate. Compost colonization and fruit body morphology were examined. The main objective of this study, however, was to examine the meiotic stability of the sectored phenotype. Single basidiospores were isolated and subjected to a grain bioassay in which the ability to produce sectors was measured. Our results were as follows: (i) basidiospore cultures obtained from a wild-collected isolate showed no tendency to produce sectors; (ii) approximately 5% of the basidiospore cultures obtained from healthy type U1 strains produced irreversible sectors in the grain bioassay; (iii) the five primary sectors examined produced basidiospore cultures, half of which produced normal-looking growth in the grain bioassay and half of which produced some degree of sectoring; and (iv) the one sectored isolate that represented the F2 generation gave ratios similar to the 1:1 ratio observed for the F1 cultures.

Effect of the bncA gene on the instability of Aspergillus nidulans

The presence of a gene designatedbncAwhich produces binucleate and trinucleate conidia inA. nidulansalters the instability of disomics, diploids, and strains with chromosome duplication. In disomics, the genebncAincreases instability. In duplicate and diploid strains, thebncAgene reduces instability by acting as a partial stabilizer. In the strain with chromosome duplication, thebncAgene produces increased percentages of bi- and trinucleate conidia, a fact that may be interpreted to be due to the larger conidial volume of this strain or to the combined effect ofbncAand of the strain, which normally already exhibits a small amount of binucleate conidia.

Two-way selection of mutants and revertants to chloroneb resistance in Aspergillus nidulans

5 mutants of Aspergillus nidulans, selected for resistance to chloroneb, were also partially dependent on it. The resistance of these mutants to chloroneb was about 20-150 times higher than that of the original strain. The resistance marker was due to a mutation in a single gene, located in linkage group III, and behaved as a recessive character. This genetic marker was distal in relation to galAl with a recombination frequency of about 30-35%. The different levels of resistance were attributed to mutations at different sites in the same locus. Both stable and unstable sectors were obtained from resistant strains inoculated on chloroneb-free medium. The emergence of stable sectors was due to back mutation, suppressor mutation or another mutation, which allows growth to the full extent in the absence of the drug. The unstable sectors showed better growth when compared with the resistant strain, kept their resistance and produced both resistant and non-resistant secondary sectors. This procedure of 2-way selection of mutants and revertants to chloroneb resistance could be useful for studying forward and back mutation in A. nidulans.

Reversion in variants from a duplication strain of Aspergillus nidulans

Strains of Aspergillus nidulans with a chromosome segment in duplicate, one in normal position and one translocated to another chromosome, are unstable at mitosis. In addition to variants which result from deletions in either of the duplicate segments, which usually have improved morphology, they produce variants with deteriorated morphology. Three deteriorated variants reverted frequently to parental type morphology, both spontaneously and after ultra-violet treatment. Of six reversions analysed genetically, five were due to suppressors and one was probably due to back mutation. The suppressors segregated as single genes and were not linked to the mutation which they suppress. The instability of these so-called "deteriorate"variants is discussed in relation to mitotic instability phenomena in A. nidulans.

Resistance and mitotic instability to chloroneb and 1,4-oxathiin in Aspergillus nidulans

Mutants resistant to two fungicides, chloroneb (1,4-dichloro-2,5-dimethoxybenzene) and vitavax (2,3-dihydro-5-carboxanilido-6-methyl-1,4-oxathiin) were spontaneously obtained from a strain of Aspergillus nidulans with frequencies of 12.5 and 1.1 respectively, in 10(8) conidia. One chloroneb-resistant mutant (Chl 1) segregated as a single gene and was mapped in linkage group IV. It also caused a partial dependence of the strain on the fungicide and was semi-dominant. The mutant resistant to vitavax (Vit 1) also segregated as a single gene and was dominant. Both fungicides altered the instability of diploid and duplication strains. Chloroneb mainly increased haploidization, and vitavax reduced the mitotic recombination in diploids. Chloroneb increased the instability of duplication strains, and vitavax reduced such instability. The possible mode of action of such fungicides affecting stability is discussed.

Effects of ethidium bromide in diploid and duplication strains of Aspergillus nidulans

Unstable duplication and diploid strains of Aspergillus nidulans were treated with ethidium bromide, and it was shown that this drug reduces the number of sectors produced by such strains. The mechanisms which could be responsible for the partial stabilization of the strains are discussed and it is suggested that a similar mechanism is responsible for the production of sectors in both strains. It is also suggested that ethidium bromide could be useful for the reduction of instability of industrial strains.

The genetic instability and mutagenic interaction of chromosomal duplications present together in haploid strains of Aspergillus nidulans

Previous work has shown that strains of Aspergillus nidulans with a chromosome segment in duplicate (one in normal position, one translocated to another chromosome) are unstable. Deletions occur from either duplicate segment. The present work has shown that when a chromosome I duplication and a chromosome III duplication are together in a haploid, deletions from the intact III duplication generally precede deletions from particular sections of the I duplication. Furthermore, the III duplication can enhance to some (but not major) extent the frequency of deletions from the I duplication. After the III duplication becomes reduced in size as a result of the loss of chromosomal material from the translocated duplicate III segment, such a reduced III duplication can greatly enhance the frequency of deletions from the I duplication. In other words, a III duplication of reduced size can promote far more deletions from the I duplication than the intact III duplication. The major increase in the deletional instability of the I duplication as promoted by the reduced III duplication is confined to the translocated duplicate I segment. The reduced III duplication can induce deletions from a section of the translocated duplicate I segment in accord with a temporal programme, and it appears that a particular region of the I duplication is far more under the mutagenic influence of the reduced III duplication than another region. Moreover, there is indication that there is a differential effect of two generally different genetic backgrounds on the susceptibility of duplication-regions to deletion. Possible mechanisms involved in such chromosomal instability are proposed. A manner in which genetic instability may be related to development is also proposed.

Altered instability due to genetic changes in a duplication strain of Aspergillus nidulans

Strains ofAspergillus nidulanswith a duplicate segment are mitotically unstable; they produce phenotypically improved variants following deletions in either duplicate segment, and morphologically deteriorated types. The number of variants produced is characteristic of each duplication strain under the same conditions. After ultraviolet treatment two variants, one more stable and the other less stable than the original strain, were selected. Genetic analysis showed that the increased instability in the less stable variant was due to a translocation involving linkage groups V and VIII. The increased stability of the more stable variant was due to a recessive factor (stf–1) located in linkage group VIII. In the homozygous condition this factor also reduces the number of sectors in a diploid strain. The possible genetic mechanisms explaining the instability alterations are discussed.