Recombination between the extranuclear genes conferring oligomycin resistance and cold sensitivity in Aspergillus nidulans

Interaction between mitochondria derived from incompatible black Aspergillus isolates

As black Aspergillus isolates are highly heterokaryon-incompatible mitochondrial transmissions were performed by protoplast fusion. Donor strains with oligomycin-resistant mitochondria and sensitive recipient partners of various A. japonicus isolates were applied and the progeny were selected for oligomycin resistance and for recipient nuclear phenotype. These strains basically inherited the mitochondrial DNA of the donor strain, which might remain unchanged (substituted progeny) or might be modified by specific sequences of the recipient mtDNAs (recombinant progeny). Different mobile elements characteristic of the recipient parents were exclusively responsible for the development of the feature of recombinant mtDNAs. Substituted progeny were either stable wild-type-like strains as a result of compatible co-operation between donor mitochondria and recipient nuclei, or aconidial strains with a reduced fitness, exhibiting a certain instability. The latter type was probably due to the less compatible communication between nuclear and extrachromosomal genetic systems originating from different parents. These progeny were able to undergo some developmental (segregation) processes during subsequent cultivation, resulting in a stable, wild-type phenotype which possessed a new type of mtDNA resembling that of the acceptor parents.

Uniparental inheritance of mitochondrial and chloroplast genes: mechanisms and evolution

In nearly all eukaryotes, at least some individuals inherit mitochondrial and chloroplast genes from only one parent. There is no single mechanism of uniparental inheritance: organelle gene inheritance is blocked by a variety of mechanisms and at different stages of reproduction in different species. Frequent changes in the pattern of organelle gene inheritance during evolution suggest that it is subject to varying selective pressures. Organelle genes often fail to recombine even when inherited biparentally; consequently, their inheritance is asexual. Sexual reproduction is apparently less important for genes in organelles than for nuclear genes, probably because there are fewer of them. As a result organelle sex can be lost because of selection for special reproductive features such as oogamy or because uniparental inheritance reduces the spread of cytoplasmic parasites and selfish organelle DNA.

Nuclear-extranuclear interactions affecting oligomycin resistance in Aspergillus nidulans

The extranuclear mitochondrial oligomycin-resistant mutation of Aspergillus nidulans, (oliA1), was transferred asexually into four nuclear oligomycin-resistant strains of different phenotypes. In all four cases, the possession of the nuclear plus extranuclear mutation led to an increase in the in vivo level of oligomycin resistance. In two cases, the altered cytochrome spectrum and impaired growth ability determined by (oliA1) were suppressed by the nuclear mutations. In the third case, the in vitro oligomycin resistance of the double mutant ATPase was dramatically increased above that of either of the component single mutant strains, indicating a synergystic interaction between the nuclear and extranuclear gene products. In the fourth case, the double mutant became cold-sensitive. A new extranuclear mitochondrial oligomycin-resistant mutation (oliB332) is described. This mutant is phenotypically similar to, though not identical with, (oliA1) but is separable by recombination. A range of nuclear oligomycin-resistant mutants have been mapped. Despite presenting five distinctly different phenotypes, they all map at the same locus.

Mitochondrial genes in Podospora anserina: recombination and linkage

A fifth cytoplasmic mutation (capr 1) obtained in Podospora anserina is described. In addition to chloramphenicol resistance it confers a strong deficiency in cytochrome aa3 and impairs the germination of ascospores. Genetic analysis shows: 1) strict maternal inheritance of (capr 1) allele; 2) selection against the (capr 1) allele as well in sexual crosses as during vegetative growth; 3) complete reversion of this selection by even low concentration of CAP. On the basis of their cytoplasmic inheritance and altered cytochrome spectra the five cytoplasmic mutations are assumed to be mitochondrial. Analysis of crosses between them allows to class them in 3 loci, 2 of which being closely linked.

Maternal inheritance of extranuclear mitochondrial markers in Aspergillus nidulans

Maternal inheritance of extranuclear mitochondrial genes has been demonstrated inAspergillus nidulansusing the ‘blue’ ascospore colour mutants in combination with heterokaryon incompatible strains. It appears that heterokaryosis is not a prerequisite of sexual outcrossing, and that recombination of extranuclear mitochondrial markers does not occur in the sexual stage of the cell cycle.

Maintenance of genetic homogeneity in systems with multiple genomes

Genes or sequences of DNA present in multiple copies per cell include entire genomes of mitochondria and chloroplasts, nuclear ribosomal RNA genes, and highly repetitive sequences in heterochromatin. All copies are nearly identical, in spite of mutational pressure and weak selection. A zygote containing mitochondrial or chloroplast genophores of two different genotypes quickly produces progeny pure for one genotype or another (vegetative segregation). Evidence from yeast andChlamy-domonassuggests that organelle genophores undergo repeated rounds of random mating and recombination. When two molecules carrying different alleles at a locus recombine, gene conversion can result in the cell becoming pure for one allele. Stochastic matching and conversion (SMAC) has been studied by computer simulations which suggest that it will tend to eliminate new mutations in yeast mitochondrial DNA and speed up vegetative segregation. We have verified previous suggestions that gene conversion, occurring during unequal mitotic sister-strand crossing-over, provides an efficient mechanism for maintaining the homogeneity of repeated sequences in eukaryotic chromosomes.

Cytochrome abnormalities and cyanide-resistant respiration in extranuclear mutants of Aspergillus nidulans

The cytochrome spectra of two extranuclear mutants of Aspergillus nidulans and the double-mutant recombinant formed from them have been examined both at room temperature and at the temperature of liquid N2 and compared with those of the wild-type strain. The oligomycin-resistant, slow growing mutant contained an increased amount of cytochrome c without any loss of cytochromes b and a,a3. The cold-sensitive mutant, apparently normal when grown at 37 C, showed an increased amount of cytochrome c and a partial loss of cytochromes b and a,a3 when grown at 20 C. A combination of these effects was observed in the double-mutant recombinant. Cyanide-resistant respiration was present in both mutant strains and in the recombinant at much higher levels than in the wild-type strain. In the oligomycin-resistant mutant, this was usually present together with cyanide-sensitive respiration, whereas in the cold-sensitive mutant and recombinant grown at 20 C cyanide-resistant approached 100%. Inhibitor and growth yield studies indicated that the cyanide-resistant pathway was not used by the cold-sensitive mutant during growth at 20 C.

Transmission and recombination of extranuclear genes during sexual crosses in Aspergillus nidulans

Three extranuclear mitochondrial mutations in Aspergillus nidulans, (oliA1), (camA1) and (cs67), were used as markers in sexual crosses to provide information on the frequencies of transmission and recombination of the mitochondrial genome. Any individual perithecium contained ascospores of only one extranuclear genotype. Using mono-, bi- and trifactorial crosses it was found that all three markers could be recovered from the progeny, although the transmission frequencies were different for each marker. This bias was present irrespective of the nuclear background or the presence of selective agents in the medium on which the cross was established. These findings enable a series of "transmission strength" to be established, as shown below:-- (camA1) greater than (cs67, camA1) greater than (+) = (cs67) greater than (oliA1, cs67) greater than (oliA1) greater than (oliA1, camA1). However, the numbers of recombinants isolated were so variable as to make this form of analysis unsuitable for mapping the mitochondrial genome.

Three-marker extranuclear mitochondrial crosses in Aspergillus nidulans

Two- and three-point extranuclear crosses have been carried out via heterokaryons involving the three extranuclear mitochondrial markers of Aspergillus nidulans: (oliA1), (cs67) and (camA112). All three markers appear to be located on a single functional mitochondrial genome. Recombination between all three pairs of extranuclear markers appears to be equally frequent, suggesting a lack of genetic linkage. An important feature of these results is the variable and often marked non-equality of frequency of reciprocal classes of recombinants.

Cytoplasmic and nuclear mutations to chloramphenicol resistance in Aspergillus nidulans

Two chloramphenicol resistance mutations out of 123 tested in Aspergillus nidulans are inherited extranuclearly as judged by transmissibility in heterokaryons, lack of segregation at meiosis, and independent segregation from all of the eight nuclear linkage groups. They do not recombine with each other. However, experiments in collaboration with G. Turner and R.T. Rowlands show that they do recombine with cytoplasmic mutations to oligomycin resistance (Rowlands and Turner, 1973) and cold-sensitivity (Waldron and Roberts, 1973). These cytoplasmic chloramphenicol resistance mutations are stable and do not affect growth or morphology on antibiotic-free media. Nuclear mutations to chloramphenicol resistance map at a minimum of three loci. At one of these loci, most, but not all, mutations lead pleiotropically to cycloheximide hypersensitivity, and most of these, but not all, also confer pleiotropic hypersensitivity to salicylhydroxamic acid.