A system generating spontaneous intrachromosomal changes at mitosis in Aspergillus nidulans

Phenotypic instability in fungi

Fungi are prone to phenotypic instability, that is, the vegetative phase of these organisms, be they yeasts or molds, undergoes frequent switching between two or more behaviors, often with different morphologies, but also sometime having different physiologies without any obvious morphological outcome. In the context of industrial utilization of fungi, this can have a negative impact on the maintenance of strains and/or on their productivity. Instabilities have been shown to result from various mechanisms, either genetic or epigenetic. This chapter will review different types of instabilities and discuss some lesser-known ones, mostly in filamentous fungi, while it will direct readers to additional literature in the case of well-known phenomena such as the amyloid prions or fungal senescence. It will present in depth the "white/opaque" switch of Candida albicans and the "crippled growth" degeneration of the model fungus Podospora anserina. These are two of the most thoroughly studied epigenetic phenotypic switches. I will also discuss the "sectors" presented by many filamentous ascomycetes, for which a prion-based model exists but is not demonstrated. Finally, I will also describe intriguing examples of phenotypic instability for which an explanation has yet to be provided.

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.

Selection for growth-rate during asexual and sexual propagation in Phytophthora cactorum

Selection for high and low growth-rate was carried out during eight generations of asexual propagation by zoospores and seven generations of sexual reproduction by oospores. The fungus has previously been shown to be diploid during its vegetative phase. In the zoospore lines there was no significant variation and no response to selection, except for the occasional appearance of fast-growing sectors. A high line was established from such a sector; in its sexual progeny the inheritance of growth-rate was non-Mendelian. Propagation through self-fertilized oospores released very considerable genetic variation, and both high and low lines responded to selection. At first the variation within families, and the response to selection, increased with succeeding generations, despite the intense inbreeding. In later generations the high line became less variable, and the progeny oospore cultures resembled the fast-growing sectors. It is concluded that growth-rate is controlled by a polygenic system and by cytoplasmic determinants, a mutant form of which is responsible for the fast-sectoring phenotype.

Genetic analysis of an unequal chromosomal translocation in Aspergillus nidulans

Translocation T(III–VIII) in Aspergillus nidulans has been analysed by the detection of meiotic linkage between markers previously located separately on linkage groups III and VIII. The breakage points have been mapped by the detection of linkage between the crinkled type and genetic markers in the region of the break. A segment from linkage group III, approximately 43 units long and including the markers moC96, sC12, sA1 and cnxH3, has been translocated into linkage group VIII. The breakage point is between su6proA and moC96 and the attachment point is close to cha in linkage group VIII. It seems probable that the segment has been inserted into linkage group VIII.

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.

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.

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.

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.