Genetic instability in Drosophila melanogaster: Transpositions of the white gene and their role in the phenotypic expression of the zeste gene

Summary Seven independent transpositions of the w ⁺ gene have been recovered as derivatives of two separate direct tandem duplications of the white locus. The transpositions map to discrete sites on both major autosomes. Five transpositions were employed to study the role of w ⁺ gene dos-age on zeste (z) gene expression. Each transposition generates a unique zeste phenotype; one transposition is not predictive for another. A functional allele of zeste, z ^77h, responds to w ⁺ gene dosage contrary to the z response.Supported by NIH grant GM22221     

Trithorax: A new homoeotic mutation of Drosophila melanogaster causing transformations of abdominal and thoracic imaginal segments

Summary A new homoeotic mutation, trithorax ¹ (trx ¹), of Drosophila melanogaster is described which causes intersegmental transformations in both the adult thorax and abdomen. Specifically, the metathorax and ventral prothorax are partially transformed to mesothroax, whilst abdominal segments are partially transformed towards the first abdominal segment. Three variables are shown to influence the expressivity of the mutant phenotype: (1) presence of wild type copies of the gene in the female parent; (2) culture temperature during the first four hours of embryogenesis; (3) exposure of embryos to ether vapour at the blastoderm stage. In addition it is shown that phenocopies of the mutant phenotype can be induced by ether treatment of heterozygous trx ¹ embryos with a frequency higher than that of bithorax phenocopies in wild type embryos.These findings are together taken to imply that the mutated gene functions early during embryogenests, and that a common mechanism underlies the determination of all the thoracic and abdominal segments.     

Genetic instability at the cut locus of Drosophila melanogaster induced by the MR-h12 chromosome

Summary Unstable mutations were generated at the cut locus by the MR-h12 factor which induces male recombination. The unstable allele ct ^MR2, containing the MR-transposon in the cut locus is a very powerful mutator producing a number of different viable and lethal mutations both in the cut locus and outside it.I describe several types of mutations: stable reversion to wild type, which were sometimes associated with the appearance of unstable mutations in other loci; of stable deficiencies at the cut locus (lethals); new unstable mutations at different loci with the ct ^MR2 allele conserved; new unstable cut alleles with a phenotype other than that of ct ^MR2. The possible mechanisms of these mutational events are discussed. The genetic system constructed in the present work affords an opportunity for molecular studies of the cut locus and the MR-transposon, as a sequence from the cut locus has recently been cloned (Tchurikov et al. 1981).     

Phenocopies ofBithorax mutants

Summary Exposure to ether of wild-type embryos of different strains ofDrosophila melanogaster causes phenocopies of different alleles of thebithorax system. Clonal analysis of the phenocopy spots has shown that the transformation caused by the treatment is maintained by cell heredity. Embryos heterozygous for several recessive mutant alleles ofbithorax show the same frequency of phenocopies as wild-type homozygous sib controls. The same holds for embryos heterozygous for the dominant mutant allelesCbx andUbx ¹ which are point mutants in thecis-regulatory region of the system. However, for dominant mutants which have breakpoints in this region (Ubx ⁸⁰,Ubx ¹³⁰ andHm) the frequency of phenocopies is about twice that of their sib controls. Embryos with increasing numbers of copies (from 1 to 4) of thebithorax system show a decreasing frequency of phenocopies. A model is proposed that explains bithorax phenocopies as resulting from disturbances in the distribution of positional information signals for segments (inductor molecules) which compete with the product of a regulator gene (repressor) and thecis-regulatory region of thebithorax system. On this model, the initiation of a metathoracic developmental pathway would result from the derepression of thebithorax system.     

Genetic instability in Drosophila melanogaster

Summary An unstable long tandem duplication which includes the white locus twice, marked with w ^sp in the left and w ^17G in the right locus, when kept in males has been found to produce red-eyed sons which have lost the long duplication and with it the w ^sp and w ^17G mutants. Such exceptions were produced also when w ^17G had been exchanged for w ^a.Stocks originating from these exceptions are unstable, producing: 1) zeste males, also unstable, 2) w ^- deletions, stable, 3) transpositions of the white locus to sites in other chromosomes.The instability is interpreted as the effect of an IS element, within or adjacent to the white locus, which is supposed to retain a duplication of the proximal zeste interacting part of this locus. According to the orientation of the IS element the duplicated part can be active or inactive, giving a zeste or red eye phenotype.The frequency of exceptional offspring after X-ray treatment of the red and zeste unstable stocks have been compared to stable stocks with corresponding genotypes.     

A mutable white-inversion in Drosophila melanogaster

Summary From a zeste mutant stock with a mutable white locus a new mutant (z w ^w ) was isolated. It has a white-eyed phenotype and a short X-chromosome inversion (In(1)w ^w ) which extends from salivary chromosome bands 3B2-C1 to 4B4-C1. In giant chromosomes of heterozygotes the inversion is unusually tightly paired. Probably because of this intimate pairing the recombination frequencies for regions near the inversion are not decreased in comparison to those for structurally normal chromosomes. The inversion chromosome is mutable. The mutations which arise have pigmented eyes and can be subdivided into two groups. One group is characterized by a re-inversion to normal chromosome structure. The mutability of the white locus appears to be independent of the inversion and reinversion. The process of reinversion is discussed.     

Identification of alleles at the gliadin loci Gli-U1 and Gli-Mb1 in Aegilops biuncialis Vis.

The diversity of alleles at the gliadin loci Gli-U1 and Gli-M ^b 1 was studied in the tetraploid species Aegilops biuncialis (UUM^bM^b). The collection of 41 Ae. biuncialis accessions and F₂ seeds obtained from five crosses served as the material used in this study. Gliadins were separated by acid polyacrylamide gel electrophoresis. To determine genomic affiliation (U or M^b) of components of Ae. biuncialis gliadin pattern, accessions of Ae. umbellulata and Ae. comosa were analyzed. In Ae. biuncialis accessions, 14 alleles were identified at the locus Gli-U1 and 12 alleles, at the locus Gli-M ^b 1. The results testify to a high degree of allele diversity at major gliadin-coding loci of homeologous group 1 chromosomes of Ae. biuncialis.     

Isolation and properties of Tn10 insertions in the rac locus of Escherichia coli

Summary Two Tn10 insertions that are in the rac locus of the chromosome of Escherichia coli have been isolated and characterized. These insertions are located at min 29.7 and min 30.0. The insertions are stable when an F123 rac::Tn10 episome is transferred to an F^- rac ⁺ recipient, but they are lost at a high frequency when transferred to an F^- rac ^- recipient. This latter condition has been previously, demonstrated to cause the excision of the rac locus. The Tn10 insertions are also lost at a high frequency when strains containing them are lysogenized with reverse. If the lysogens that have lost the Tn10 insertion are subsequently cured of reverse, the cells no longer contain sequences homologous with rac locus DNA. These strains were rac ^- when tested for recombination activation (Low 1973), and this procedure consequently provides a simple means to make isogenic rac ^- and rac ^- strains.     

The dominant Drop eye mutations of Drosophila melanogaster define two loci implicated in normal eye development

The three existing dominant gain-of-function Drop alleles, Dr ¹, Dr ^Mio and Dr ^We , previously assumed to define a single locus, severely disrupt eye development. Genetic analysis of ethylmethanesulphonate (EMS) and irradiation-induced revertants revealed that the Drop mutations define two loci: the Drop locus, which is defined by the Dr ¹ and Dr ^Mio mutants, and a separate locus defined by the Dr ^We mutation, which has been renamed Wedge. The majority of the Dr ¹ and Dr ^Mio revertants are embryonic lethal in trans, mutant embryos exhibiting trachea that fail to join the Filzkörper, thus revealing a role for the Drop gene in embryogenesis. Clonal analysis of lethal revertant alleles suggests a role for both genes in eye development. In the Drop homozygous mutant clones, the outer photoreceptor cells R1–R6 develop aberrantly. Wedge, however, is not required by the developing photoreceptor cells but its absence does disrupt normal ommatidial alignment. Although the Drop and nearby string loci were shown to be genetically distinct, both Dr ¹ and Dr ^Mio were found to interact in trans with lesions at the string locus, causing loss and derangement of bristles and loss of neuromuscular coordination.     

Changes of DNA replication behavior associated with intragenic changes of thewhite region inDrosophila melanogaster

The pattern of late labeling spots in the X chromosome ofDrosophila melanogaster has been studied by H³-thymidine autoradiography. The pattern has been found to be identical with that of the “weak spots”, or places of “ectopic pairing”. The late replicating spot in region 3C has been found to lie close to the right of the locus ofwhite. A triplication and a deficiency involving the right half of thewhite region and exhibiting changes in their interaction with the mutantzeste have been found to be associated with changes in the frequency and intensity of labeling of the late material in 3 C. Twoz ⁺ revertants derived from the triplication by X irradiation again show concomitant changes in labeling behavior at 3C.     

Properties of transposon SCTn1 of Streptomyces coelicolor A3(2)

Summary Chloramphenicol resistance (Cml^r) of Streptomyces coelicolor A3(2) behaves like a transposon locus, not being localisable in any region of the map and yet being transferable in crosses at a rate comparable to that of chromosomal markers. It can, also be transposed onto a plasmid (SCP1) and back to the chromosome. Some traits, such as arginino-succinate synthase production (ArgG), aerial mycelium formation (AmyA), resistance to tetracycline and to rifamycin C appear to be joined to Cml in three processes: co-mutation, i.e. simultaneous loss, post-mutation, i.e. spontaneous loss at high, frequency in subclones from Cml^s strains, co-transfer, i.e. joint transfer with the cml locus in crosses or during infection by the aggregate SCP1::SCTn1 plasmid. All these processes have been consistently observed with special attention to the argG locus.     

Polyhomeotic: A gene of Drosophila melanogaster required for correct expression of segmental identity

Summary A new locus in Drosophila melanogaster that is required for the correct expression of segmental identity has been discovered. The new locus, termed polyhomeotic (ph), is X-linked and maps cytologically to bands 2D2-3. Homozygous ph flies have homeotic transformations similar to those of known dominant gain of function mutants in the Antennapedia and bithorax complexes (ANT-C, BX-C), and in addition show loss of the humerus. ph interacts with three other similar mutations: Polycomb (Pc), Polycomblike (Pcl), and extra sex comb (esc), and acts as a dominant enhancer of Pc. The expression of ph depends on the ANT-C and BX-C dosage. ph has no embryonic phenotype, but temperature shift studies on ph ² show that the ph ⁺ product is required during embryogenesis and larval development. We propose that ph mutants in some way disrupt the normal expression of the ANT-C and BX-C, and, therefore, that ph ⁺ is needed for maintenance of segmental identity.     

The Pik m gene, conferring stable resistance to isolates of Magnaporthe oryzae , was finely mapped in a crossover-cold region on rice chromosome 11

The Pik ^m gene in rice confers a high and stable resistance to many isolates of Magnaporthe oryzae collected from southern China. This gene locus was roughly mapped to the long arm of rice chromosome 11 with restriction fragment length polymorphic (RFLP) markers in the previous study. To effectively utilize the resistance, a linkage analysis was performed in a mapping population consisting of 659 highly susceptible plants collected from four F₂ populations using the publicly available simple sequence repeat (SSR) markers. The result showed that the locus was linked to the six SSR markers and defined by RM254 and RM144 with ≈13.4 and ≈1.2 cM, respectively. To fine map this locus, additional 10 PCR-based markers were developed in a region flanked by RM254 and RM144 through bioinformatics analysis (BIA) using the reference sequence of cv. Nipponbare. The linkage analysis with these 10 markers showed that the locus was further delimited to a 0.3-cM region flanked by K34 and K10, in which three markers, K27, K28, and K33, completely co-segregated with the locus. To physically map the locus, the Pik ^m -linked markers were anchored to bacterial artificial chromosome clones of the reference cv. Nipponbare by BIA. A physical map spanning ≈278 kb in length was constructed by alignment of sequences of the clones anchored by BIA, in which only six candidate genes having the R gene conserved structure, protein kinase, were further identified in an 84-kb segment.     

Autonomy of the wingless mutation in Drosophila melanogaster

Summary T(Y;2) translocations were used to cytologically localise the wingless locus of Drosophila melanogaster. We found that an existing T(Y;2), which is an insertion of a segment of 2L into the Y chromosome, has wg ⁺ within this insert. This Y chromosome was used to generate an attached XY chromosome containing wg ⁺. The mutation claret-nondisjunctional (ca ^nd) was used to induce the loss of this XY chromosome and thus generate gynandromorphs with wg ¹/wg ¹ male tissue and wg ⁺/wg ¹/wg ¹ female tissue. Analysis of these gynanders demonstrated that a genotypically wingless mutant hemithorax is usually also phenotypically mutant in these half body mosaics; thus wg ¹ is discautonomous. This observation is of interest as it is known that wg is not cell autonomous.     

Interaction of non-allelic loci in expression of the extra-sexcomb phenotype inDrosophila melanogaster

Summary The expression of the extra-sexcomb phenotype is enhanced in males that are doubly heterozygous for mutants from two non-allelic loci,extra-sexcomb (Polycomb, Extra-sexcomb) andbithorax (bithorax, bithoraxoid, Ultrabithorax) as compared to the expression of the extra-sexcomb mutants when no bithorax mutants are present in the genotype. If the bithorax mutants are recessive, the expression is usually greater when the non-allelic mutants are cis-hererozygotes than when they are trans-heterozygotes, but there is no difference between cis-and trans-heterozygotes in extra-sexcomb andUltrabithorax compounds.This work, begun while the author was a member of the staff of the Zoology Dept. of the University of California at Berkeley, was completed, with the assistance of a National Institutes of Health research grant (RG-6754), in the Dept. of Genetics of the University of Turku.     

Genetics of gliding motility in Myxococcus xanthus: Molecular cloning of the mgl locus

Summary Wild-type Myxococcus xanthus cells move across solid surfaces by gliding. However no locomotory organelles for gliding have as yet been identified. Two sets of genes are required for gliding in M. xanthus: Gene System A is necessary for the gliding of isolated cells and Gene System S comes into play when cells are close together. The product of the mgl locus is required for both types of gliding and therefore may be a structural component of the gliding organelle. To begin to investigate the function of mgl in gliding a 12 kb segment of M. xanthus DNA containing the locus was cloned in Escherichia coli and returned to Myxococcus by specialized transduction with coliphage P1. In M. xanthus the chimeric plasmid integrates into the chromosome by recombination between the cloned segment and its homolog in the recipient chromosome forming a tandem duplication of the cloned segment with the vector sequences at the novel joint. The construction of partial diploids in this manner facilitated dominance tests and interallelic crosses with ten mgl alleles. We also describe a method for the analysis of tandem duplications that precisely maps alleles to a specific copy of the duplicated sequences. This method provides evidence for the dominance of mgl ⁺ over the mgl ^- alleles. It also reveals what appears to be gene conversion at this locus during recombination between a cloned mgl sequence and its homolog in the chromosome.     

A previously unrecognized glutamine synthetase expressed in Klebsiella pneumoniae from the glnT locus of Rhizobium leguminosarum

Summary Using glnT DNA of Rhizobium meliloti as a hybridization probe we identified a R. leguminosarum biovar phaseoli (R. l. phaseoli) locus (glnT) expressing a glutamine synthetase activity in Klebsiella pneumoniae. A 2.2 kb DNA fragment from R. l. phaseoli was cloned to give plasmid pMW5a, which shows interspecific complementation of a K. pneumoniae glnA mutant. The cloned sequence did not show cross-hybridization to glnA or glnII, the genes coding for two glutamine synthetase isozymes of Rhizobium spp. While in previous reports on glnT of R. meliloti and Agrobacterium tumefaciens no glutamine synthetase activity was detected, we do find activity with the glnT locus of R. l. phaseoli. The glutamine synthetase (GSIII) activity expressed in a K. pneumoniae glnA strain from pMW5a shows a ratio of biosynthetic to transferase activity 10³-fold higher than that observed for GSI or GSII. GSIII is similar in molecular weight and heat stability to GSI.     

A lethal allele at the putative regulatory locus, cpc-1, of cross-pathway control in Neurospora crassa

Summary A lethal allele at the putative regulatory locus, cpc-1, of cross-pathway control in Neurospora crassa was discovered by genetic analysis. cpc-1 ^j-5 is viable only in the presence of a second mutation, slo, causing slow growth. The detection of a lethal allele at a regulatory locus is a rare event and points to the physiological importance of the regulatory circuit concerned, namely the cross-pathway or general control of amino acid biosynthetic enzymes in lower eukaryotes.