A start has been made on establishing a collection of Aspergillus niger colour and auxotrophic mutants with an isogenic background for use as a source of genetic markers. All strains have short conidiophores (csp A1), which makes them easy to handle on test plates. Genetic markers were combined stepwise by somatic recombination. Somatic diploids were obtained at frequencies of 10(-6) -10(-5) with conidiospores collected from a heterokaryon. The haploidization of heterozygous diploids was induced by benomyl. For unlinked markers, the frequency of recombinants varied from 35%-65%. Low frequencies of recombinants were found between markers on a same chromosome, but this was sometimes disturbed by mitotic crossing-over during an early stage of the diploid. Master strains were constructed having markers for six linkage groups.

~~ ~~ ~-~ ~~ ~~ ~A mutant of Aspergillus niger unable to grow on D-XylOSe and L-arabinose has been isolated. Genetic analysis revealed that the mutation is located on linkage group IV. Enzymic analysis revealed a deficiency in D-XylUlOSe kinase activity. After transfer of growing mycelium to a medium containing either D-xylose or L-arabinose, the mutant accumulates large amounts of arabitol and xylitol, as shown by I3C NMR spectroscopy. These data and an analysis of enzyme activities induced by D-XylOSe and L-arabinose in the wild-type strain led to the following catabolic pathway for D-xylose : D-xylosexylitol -D-xylulose -D-xylulose 5-phosphate; and for L-arabinose :The reduction steps of the sugars to the corresponding polyols are all NADPH dependent. The oxidation steps of the polyols to the sugars are all NAD+ dependent. Fractionation of cell-free extracts gave information about the specificity of the enzymes and showed that all the reactions are catalysed by different enzymes.

In the filamentous fungus Podospora anserina, two phenomena are associated with polymorphism at the het-s locus, vegetative incompatibility and ascospore abortion. Two het-s alleles occur naturally, het-s and het-S. The het-s encoded protein is a prion propagating as a self-perpetuating amyloid aggregate. M eiotic drive is a process mediated by genetic elements, called segregation distorters, that actively bias Mendelian segregation in their favor. In metazoans, classic examples of meiotic drive include the t haplotype in mice and Segregation Distorter (SD) in Drosophila (1). In both these autosomal drive systems, heterozygous males produce gametes of the driver genotype in excess. The molecular mechanisms of meiotic drive remain largely elusive, except in the case of SD in Drosophila, where distortion involves a truncated form of a RanGAP protein and its mislocalization to the nucleus (2). The best studied examples of segregation distortion in fungi are the spore killer systems in the ascomycetes Neurospora crassa (3-6) and Podospora anserina (7). In these haploid organisms, the sexual progeny, the ascospores, are linearly arranged after meiosis. Ascomycetes therefore provide excellent opportunities to investigate the behavior of meiotic drive elements (8). Presence of a killer gene can readily be detected by directly analyzing the pattern of ascospore abortion. Spore killer genes exist as a killer and a sensitive allele or haplotype. In a killer ϫ sensitive cross, the ascospores harboring only the sensitive genotype degenerate. Heterokaryotic spores, containing both a sensitive and a killer nucleus, escape abortion. Both in Neurospora and in Podospora, several spore killer loci have been genetically identified, but so far the mechanism of spore killing is unknown.In 1965, Bernet (9, 10) described properties of the het-s gene in Podospora, now recognized to be analogous to a spore killer locus. The discovery that het-s encodes a prion protein (11, 12) places this observation into a new perspective. The het-s locus, together with at least eight other loci, determines vegetative incompatibility in P. anserina (13,14). Two alleles are found at this locus, termed het-s During the sexual cycle, a single mating-type culture of P. anserina differentiates both male (microconidia) and female (protoperithecia) reproductive structures. The protoperithecium emits a specialized hypha called the trichogyne. Fertilization occurs when a microconidium fuses with a trichogyne of opposite mating type. After migration of the male nucleus down the trichogyne into the female ascogonium, individualized male and female nuclei divide synchronously in a syncytial structure. Because the male gamete contributes very little cytoplasm to this heterokaryon, the cytoplasm of the zygote is essentially of maternal origin. Nuclei of opposite mating type then pair up, and karyogamy takes place in specialized cells. Importantly, at this stage there is a transition from a syncytial to a cellular state (21). Meiotic progeny are linearly arranged as ...

A homologous transformation system for the filamentous fungus Aspergillus niger has been developed, based on the orotidine-5'-phosphate-decarboxylase gene. A. niger Pyr- mutants have been selected from 5-fluoro-orotic acid resistant mutants. These mutants were found to comprise two complementation groups, pyrA and pyrB. The A. niger OMP-decarboxylase gene was isolated from a gene library by heterologous hybridization with the Neurospora crassa pyr4 gene. The cloned gene is capable to transform A. nidulans pyrG mutants at high frequencies. Transformation of A. niger pyrA mutants occurs with moderate frequencies (about 50 transformants/micrograms DNA) whereas the pyrB mutants cannot be complemented with the cloned OMP-decarboxylase gene. Analysis of the DNA of the A. niger PyrA+ transformants showed that transformation resulted in integration of the vector DNA into the genome by homologous recombination. Both gene replacements and integration of one or more copies of the complete vector have been observed.

Candida albicans organisms isolated from the oral cavities of healthy carriers (26 individuals) and compromised hosts (40 human immunodeficiency virus [HIV]-seropositive patients, all showing symptomatic oral candidiasis) were compared by resolving chromosome-sized DNA molecules into electrophoretic karyotypes.Seven-to 10-band electrophoretic patterns were obtained, with significant and reproducible differences in the distributions of the DNA bands. Seven distinct classes were identified and were designated type a (8 bands), type b (8 bands), type c (7 bands), type d (9 bands), type x (10 bands), type y (10 bands), and type z (9 bands). Four of these (types a to d) were the most representative within all of the isolated strains (95.5%), and the other three (types x to z) were observed only once in three HIV-seropositive individuals (4.5%). Only types b and c were isolated from healthy carriers, with the percentage of their isolation being 61.5 and 38.5%, respectively, while all the described karyotypes were isolated from HIV-seropositive patients, with type b being the most frequent (45%); this was followed by types c (25%), a (15%), and d (7.5%). The prevalence of type b and c karyotypes in HIV-infected individuals, as well as in healthy carriers, suggests that commensal strains in the oral cavities of healthy individuals may become the prevalent agents of subsequent oral candidiasis in compromised hosts. However, replacement of the original, commensal strain, if there is one, cannot be excluded in a compromised host, although strain replacement may be more reasonably hypothesized for types a and d, since only these types were isolated at a relative high percentage from the oral lesions of HIV-infected individuals.