Mitotic non-conformity in Aspergillus nidulans: the production of hypodiploid and hypohaploid nuclei

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.

The effects of temperature on genetic instability in 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 most deletions occur from the translocated duplicate segment. Furthermore, it has been found that the overall frequency of deletions from a duplication is dependent upon the temperature of growth. The overall frequency of deletions from a chromosome III duplication is greatly enhanced by low temperatures, while the overall frequency of deletions from a chromosome I duplication is markedly enhanced by high temperatures. A temperature of 39.5 degrees C appears to enhance to overall frequency of deletions from the I duplication to the greatest extent. With regard to the non-translocated duplicate I segment, an increase in temperature progressively enhances the frequency of those deletions to which it is subject to far more deletions during a particular period of growth than during any other period, and at 42 degrees C, a section of the III duplication is subject to far more deletions during a given period of growth than during any other period. Comparisons with other cases of genetic instability are made and common underlying connections are proposed.

Genetic control of chromosome instability in Aspergillus nidulans as a mean for gene amplification in eukaryotic microorganisms

A haploid strain of Aspergillus nidulans carrying I-II duplication homozygous for the leaky mutation adE20 shows impreved growth on minimal medium. The duplication, though more stable than disomics, still shows instability. Several methods were used for detecting genetic control of improved stability. (a) visual selection, using a duplicated strain which is very unstable due to UV sensitivity, (adE20, biAl/dp yA2; uvsB). One stable strain showed a deletion (or a lethal mutation?) DISTAL TO BIA on the segment at the original position (on chromosome I). This deletion reduces crossing over frequency between the two homologous segments. As the deletion of the non-translocated segment (yelow sectors) must be preceded by crossing over, the above reduces the frequency of yellow sectors. A deletion of the translocated segment (green sectors) results in non-viability due to the deletion, and such sectors do not appear. The net result is a stable duplication involving only 12 C.O. units carrying the gene in concern. (b) Suppressors of UV sensitivity (su-uvsB) were attempted using the above uvs duplicated strain. Phenotypic revertants were easily obtained, but all were back mutations at the uvsB locus. (c) Mutations for UV resistance higher than that of the wild type were not obtained, in spite of the strong selective pressure inserted. (d) Recombination deficient mutations (rec), six altogether, all uvs+, did not have any effect on stability.

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.

Mutations affecting mitotic recombination frequency in haploids and diploids of the filamentous fungus Aspergillus nidulans

A haploid strain of Asp. nidulans with a chromosome segment in duplicate (one in normal position on chromosome I, one translocated to chromosome II) shows mitotic recombination, mostly by conversion, in adE in a frequency slightly higher than in the equivalent diploid. A method has been devised, using this duplication, for the selection of rec and uvs mutations. Six rec mutations have been found which decrease recombination frequency in the haploid. One mutation selected as UV sensitive showed a hundred fold increase in recombination frequency in the haploid (pop mutation) and probably the same in diploids. The increased frequency is both in gene conversion and in crossing over, and the exchanges appear in clusters of two or more. pop is allelic to uvsB (Jansen, 1970) which had been found to affect mitotic but not meiotic recombination. It is suggested that mutations of this type interfere with the control mechanism which determines that high recombination is confirmed to the meiotic nuclei and avoided in somatic nuclei.

A variegated position effect in Aspergillus nidulans

In mutants at the ‘bristle’ locus ofAspergillus nidulansthe conidiophore remains as a stiff hypha rather than developing a vesicle, sterigmata and conidia. ThebrlA12 allele of this locus has a variegated phenotype, and genetic analysis has shown that this is associated with a translocation which has a breakpoint in the map interval adjacent to thebristlelocus.

The mutant phenotype is partially repaired on high-salt medium at low pH, and can also be repaired by suppressors, one of which has been mapped at a locus unlinked tobrlA12.

The mutant provides proof that variegation is due to instability of gene expression and not to mutability sincebrlA12 is genetically stable and can be propagated from either conidia or sterile conidiophores, the structures formed at the two extremes of variegation, and the resulting colonies in both cases are identical to the original strain.

It has been shown by mitotic recombination that the translocation associated with the variegated mutant is a ‘simple translocation’ in which the distal half of linkage group VIII is attached to the end of linkage group III. This terminal attachment site does not appear to be damaged in any genetically detectable way.

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

Strains ofAspergillus nidulanswith, a duplicate chromosome segment are mitotically unstable; in addition to phenotypically improved variants, arising following deletions in either duplicate segment, they give morphologically deteriorated types, some with, enhanced stability. In one isolate, deterioration and increased instability were determined by mutation in a duplicate segment; a more stable derivative no longer had this mutation but had one in another linkage group. Another variant, too unstable for analysis, gave derivatives whose single, new mutations were in different linkage groups. It is proposed that deterioration and increased instability result from tandem duplications on either duplicate segment; transposition of these to non-duplicated regions reduces instability. Another 17 variants had a single new mutation each; mutations, possibly clustered, occurred in all linkage groups. In these strains perhaps transposition preceded analysis. Deteriorated variants gave lineages of types with morphological changes caused by further, superimposed mutations. This continued instability is explained as interaction, in fidelity of replication, of non-homologous chromosome segments.

Instability inA. nidulansstems from chromosome imbalance. As imbalance is known or suspected in other cases of instability it may be possible to show common mechanisms for apparently diverse phenomena.