Genetic analysis of the reciprocal translocation T2(I;8) of Asperigillus using the technique of mitotic mapping in homozygous translocation diploids

SwoHp, a nucleoside diphosphate kinase, is essential in Aspergillus nidulans

The temperature-sensitive swoH1 mutant of Aspergillus nidulans was previously identified in a screen for mutants with defects in polar growth. In the present work, we found that the swoH1 mutant swelled, lysed, and did not produce conidia during extended incubation at the restrictive temperature. When shifted from the permissive to the restrictive temperature, swoH1 showed the temperature-sensitive swelling phenotype only after 8 h at the higher temperature. The swoH gene was mapped to chromosome II and cloned by complementation of the temperature-sensitive phenotype. The sequence showed that swoH encodes a homologue of nucleoside diphosphate kinases (NDKs) from other organisms. Deletion experiments showed that the swoH gene is essential. A hemagglutinin-SwoHp fusion complemented the mutant phenotype, and the purified fusion protein possessed phosphate transferase activity in thin-layer chromatography assays. Sequencing of the mutant allele showed a predicted V83F change. Structural modeling suggested that the swoH1 mutation would lead to perturbation of the NDK active site. Crude cell extracts from the swoH1 mutant grown at the permissive temperature had approximately 20% of the NDK activity seen in the wild type and did not show any decrease in activity when assayed at higher temperatures. Though the data are not conclusive, the lack of temperature-sensitive NDK activity in the swoH1 mutant raises the intriguing possibility that the SwoH NDK is required for growth at elevated temperatures rather than for polarity maintenance.

The Aspergillus nidulans swoC1 Mutant Shows Defects in Growth and Development

Previous work identified swoC1 as a single-gene mutant with defects in polarity establishment. In this study swoC1 was shown to have defects in endocytosis, compartmentation, nuclear distribution, and conidiation. Temperature-shift experiments showed that the swoC1 mutant establishes multiple random sites of germ tube emergence. Surprisingly, these experiments also showed that even a slight delay in polarity establishment causes defects in later vegetative growth and asexual reproduction. The swoC gene was mapped to the centromere of chromosome III and cloned by complementation of the temperature-sensitive phenotype. The predicted SwoCp is homologous to rRNA pseudouridine synthases of yeast (Cbf5p) and humans (Dkc1p). However, neither rRNA pseudouridine synthesis nor rRNA processing appears to be affected in the swoC1 mutant. The swoC1 mutation occurs in the putative RNA-binding domain upstream of the C terminus, leaving the N-terminal TRUB catalytic domain intact. Interestingly, while deletion of the swoC gene was lethal in A. nidulans, the C terminus, including NLS, microtubule-binding, and coiled-coil domains, was dispensable for growth. SwoCp likely plays an important role in polar growth and nuclear distribution in A. nidulans, functions not yet described for its homologs.

The validity of the Aspergillus nidulans linkage map

The Aspergillus nidulans linkage map is reviewed as a background to physical mapping by cosmid cross-hybridization and genome sequencing. DNA-based methods depend on contiguity, so that the resulting maps are only as good as the weakest link, whereas each new marker added to a linkage map can provide independent confirmation of the positions of its neighbors. For all eight chromosomes of A. nidulans a reliable framework has been provided by analysis of mitotic crossing over, in many cases substantiated by the study of translocation disomics. Building on this framework, there is a backbone of loci linked by reliable three-point meiotic mapping and a second set of less precisely mapped loci. The result is a map with a high degree of self-consistency although some areas of uncertainty or conflict are also noted.

Toxin-deficient mutants from a toxin-sensitive transformant of Cochliobolus heterostrophus

Tox1 is the only genetic element identified which controls production of T-toxin, a linear polyketide involved in the virulence of Cochliobolus heterostrophus to its host plant, corn. Previous attempts to induce toxin-deficient (Tox-) mutants, using conventional mutagenesis and screening procedures, have been unsuccessful. As a strategy to enrich for Tox- mutants, we constructed a Tox1+ strain that carried the corn T-urf13 gene (which confers T-toxin sensitivity) fused to a fungal mitochondrial signal sequence; the fusion was under control of the inducible Aspergillus nidulans pelA promoter which, in both A. nidulans and C. heterostrophus, is repressed by glucose and induced by polygalacturonic acid (PGA). We expected that a transformant carrying this construction would be sensitive to its own toxin when the T-urf13 gene was expressed. Indeed, the strain grew normally on medium containing glucose but was inhibited on medium containing PGA. Conidia of this strain were treated with ethylmethanesulfonate and plated on PGA medium. Among 362 survivors, 9 were defective in T-toxin production. Authenticity of each mutant was established by the presence of the transformation vector, proper mating type, and a restriction fragment length polymorphism tightly linked to the Tox1+ locus. Progeny of each mutant crossed to a Tox1+ tester segregated 1:1 (for wild type toxin production vs. no or reduced toxin production), indicating a single gene mutation in each case. Progeny of each mutant crossed to a Tox1- tester segregated 1:1 (for no toxin production vs. no or reduced toxin production) indicating that each mutation mapped at the Tox1 locus. Availability of Tox- mutants will permit mapping in the Tox1 region without interference from a known Tox1 linked translocation breakpoint.

Translocations in Dictyostelium discoideum

Fourteen translocations of independent origin were identified in Dictyostelium discoideum on the basis of segregation anomalies of diploids heterozygous for these chromosome rearrangements, all of which led to the cosegregation of unlinked markers. Many of these translocations were discovered in strains mutagenized with MNNG or in strains carrying mutations affecting DNA repair; however, spontaneous translocations were also obtained. Haploid mitotic recombinants of the rearranged linkage groups were produced from diploids heterozygous for the translocations at frequencies of up to 5% of viable haploid segregants; this is at least a ten-fold higher frequency than that seen with diploids not heterozygous for translocations (approximately 0.1%). These haploid recombinants included both translocated and nontranslocated strains. The T354(II, VII) translocation and possibly the T357(IV, VII) translocation reduce the chromosome number to n = 6; haploids carrying 11 other translocations all have karyotypes with n = 7. Genetic characterization of the T357(IV, VII) translocation showed that the bwnA and whiC loci normally found on linkage group IV were physically linked to the linkage group VII loci couA, phgA, bsgB and cobA.

Y-chromosome length in sublines of two mouse strains

Differences in intermale aggression have been repeatedly reported for DBA/1 and C57BL/10 mice. The results of rreciprocal crosses combined with cross-fostering procedures suggest an involvement of the Y chromosome. In the present study, the length of the Y chromosome relative to that of chromosome 19 was ascertained in five sublines of DBA/1Bg, three sublines of C57BL/10Bg, and C57BL/10.DBA/1-Y congenic stock of mice, which carries the DBA/1Bg Y chromosome. With respect to the length of the Y chromosome relative to that of chromosome of 19, two of the DBA/1 sublines are shorter than the other three DBA/1 sublines, and all DBA/1 sublines are shorter than the three C57BL/10 sublines. This is attributable primarily to the length of the Y chromosome. The C57BL/10 sublines and the BL10.D1-Y congenic stock tested exhibit the same relative lengths of the Y chromosome, suggesting that its length has changed on the C57BL/10 genetic background. There is a parallel dependence on autosomal background of the effect of the Y chromosome on intermale aggression.

Reciprocal translocations and translocation disomics of aspergillus and their use for genetic mapping

Two new techniques are described for genetic mapping of reciprocal translocations in A. nidulans, which can be used to locate centromeres and meiotically unlinked markers. They both make use of unbalanced disomics from heterozygous translocation crosses. These are mainly hyperhaploids of two classes: either typical-looking n + 1 with a normal chromosome in addition to a haploid set containing the translocation, or translocation disomics. When large chromosome segments are involved, such disomics, as well as stable aneuploids and duplication types, show characteristic phenotypes and can be classified visually. The first method maps translocation breaks qualitatively, since translocated markers can be identified when translocation disomics are analyzed for heterozygous markers. The second method measures meiotic linkage of any marker to the translocation breaks when allele ratios in the balanced haploid sectors of either or both classes of disomics are determined: linked markers show reciprocal deviations from 1:1--In addition, it can be shown that frequencies of nondisjunction and recovery of specific translocation disomics both depend on the relative position of the break within a chromosome arm. Such information can provide a rough estimate of the positions of breaks for a new translocation.--Using these techniques, as well as mitotic mapping in homo- and heterozygous translocation diploids, four reciprocal translocations were mapped. from these results, information on the sequence and orientation of most of the "meiotic fragments" of the current maps (groups III, VI, VII and VIII) was obtained, and the position of the centromeres of groups VI and VII were identified. Translocation disomics are also used to map meiotically unlinked single genes, e.g. oliA of group VII, to specify chromosome segments.