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.     

Genetic analysis of chromosomal region 97D2-9 of Drosophila melanogaster

The rough homeobox gene of D. melanogaster is required for the correct patterning of the developing eye. The locus maps to cytological location 97D2-5, a region which has not been extensively characterised. As part of our genetic and molecular characterization of rough we carried out an EMS mutagenesis to generate mutants that map to the surrounding region, 97D2-9 which is deleted in Df(3R)ro-XB3. We have generated 1 visible and 13 lethal mutations which, together with the previously described Toll and ms(3)K10 loci, and other unpublished lethals, define nine complementation groups — four lethal, three semi-lethal, one visible and one male-sterile. In addition to rough, one other locus within this region, 1(3)97De, was shown to be required for formation of the normal pattern of photoreceptor cells in the compound eye.     

Developmental genetic analysis of lethal alleles at the ewg locus and their effects on muscle development in Drosophila melanogaster

The recessive X-linked mutation erect wing (ewg), in Drosophila melanogaster, was characterized as a flightless behavioral mutant which specifically lacked the dorsal longitudinal flight muscles [1]. This mutation was mapped distal to the X chromosomal locus yellow, and further to the cytological segment 1 A 1 to 1 B2-3 [2]. Several lethal complementation groups have been mapped to this interval [3]. Our complementation tests show that ewg is allelic to one lethal complementation group in the region 1 A 1 to 1 B2-3. A further analysis of ewg and several lethal alleles isolated at this locus was undertaken in the present investigation. Most of the lethal alleles at this locus lead to a late embryonic or early larval lethal phase, indicating that the ewg⁺ gene product is necessary for the development of more than just the dorsal longitudinal flight muscles. Intragenic complementation was observed for some of the ewg lethal alleles. Genetic mosaics with ewg lethal alleles showed that mutant cell clones in cuticular structures are viable. Mosaic analysis is consistent with a mesodermal defect associated with the locus.     

Retrotransposon-Induced Ectopic Expression of Cut Causes the Om(1a) Mutant in Drosophila Ananassae

Optic morphology (Om) mutations in Drosophila ananassae map to at least 22 loci scattered throughout the genome. They are semidominant, neomorphic, nonpleiotropic, and are associated with the insertion of a retrotransposon, tom. The Om(1A) gene, which is cytogenetically linked to the cut locus, was cloned using a DNA fragment of the cut locus of Drosophila melanogaster as a probe. Three of the eight alleles of Om(1A) examined have insertion of the tom element within a putative cut region. The γ-ray-induced revertants of Om(1A) are accompanied with cut lethal mutations and rearrangements within the cut coding region. In the eye imaginal discs of the Om(1A) mutants, differentiation of photoreceptor clusters is suppressed, abnormal cell death occurs in the center and the cut protein is expressed ectopically. D. melanogaster flies transformed with a chimeric cut gene under the control of a heat-inducible promoter show excessive cell death in the region anterior to the morphogenetic furrow, suppressed differentiation to photoreceptor clusters and defect in the imaginal eye morphology when subjected to temperature elevation. These findings suggest that the tom element inserted within the Om(1A) region induces ectopic cut expression in the eye imaginal discs, thus resulting in the Om(1A) mutant phenotype.     

Revertants of Dominant Mutations Associated with the Antennapedia Gene Complex of DROSOPHILA MELANOGASTER: Cytology and Genetics

Using X-ray mutagenesis we have induced and recovered phenotypic revertants of four dominant mutations thought to be associated with the Antennapedia complex of Drosophila melanogaster. These include seven revertants of Antennapedia-73b (Antp^73b), six of Extra Sex Combs of Wakimoto (Scx^w), three of Deformed (Dfd) and one of Humeral (Hu). Fifteen of the 17 revertants are associated with chromosomal aberrations and localize Antp^73b, Scx ^w and Hu to polytene chromosome bands 84B1,2. The Dfd lesion is apparently located in or adjacent to bands 84A4,5. Since all of the dominants are reverted by events that delete their respective chromosomal loci, we conclude that all four are the result of a gain-of-function lesions. Complementation analysis of the various revertant chromosomes has shown that Scx^w and Hu are dominant allelic variants of the Antp locus. The Dfd lesion represents a dominant mutation at a locus just proximal to Antp and previously only occupied by recessive lethal mutations. Characterization of the revertants of Scx^w and a comparison with the properties of the original mutation has revealed that the original lesion has effects on both the Antp and Sex Combs Reduced (Scr) loci and that these defects are in some cases separable by the reverting event.     

Temperature-Sensitive Mutations of the Notch Locus in DROSOPHILA MELANOGASTER

Temperature-conditional mutations of the Notch locus were characterized in an attempt to understand the organization of a "complex locus" and the control of its function in development. Among 21 newly induced Notch alleles, about one-half are temperature-conditional for some effects, and three are temperature-sensitive for viability. One temperature-sensitive lethal, l(1)N^ts1, is functionally non-complementing for all known effects of Notch locus mutations and maps at a single site within the locus. Among the existing alleles involved in complex patterns of interallelic complementation, Ax^59d5 is found to be temperature-sensitive, while fa ^g, spl, and l(1)N are temperature-independent. Whereas temperature-sensitive alleles map predominantly to the right-most fifth of the locus, fa^g, spl, and l(1)N are known to map to the left of this region. Temperature-shift experiments demonstrate that fa^g, spl, and l(1)N cause defects at specific, non-overlapping times in development.—We conclude (1) that the Notch locus is a single cistron (responsible for a single functional molecule, presumably a polypeptide); (2) that the right-most fifth of the locus is, at least in part, the region involved in coding for the Notch product; (3) that the complexity of interallelic complementation is a developmental effect of mutations that cause defects at selected times and spaces, and that complementation occurs because the mutant defects are temporally and spatially non-overlapping; and (4) that mutants express selected defects due to critical temporal and spatial differences in the chemical conditions controlling the synthesis or function of the Notch product. The complexity of the locus appears to reside in controlling the expression (synthesis or function) of the Notch product in development.     

The effect of eye reduction on White position-effect variegation in Drosophila melanogaster

Position-effect variegation for the white locus was studied in normally shaped eyes and in reduced eyes of Bar (B) and Drop (Dr) flies. The average number of spots per eye is successively lower in +, B, and Dr eyes; moreover, B eyes show a relatively strong pigmentation. No simple relation seems to be present between the degree of pigmentation and the number of facets, either between +, B, and Dr eyes or within classes of Dr eyes that have been analysed.The chance that ommatidia will become pigmented follows a gradient across mottled eyes of wild-type shape that seems fixed early in development. The gradient is less clear or absent in B eyes.The results are best interpreted on the basis of the cell-lineage theory and an early one-sided action of B on the developing eye disc after fixation of the gradient.     

Multiple Allele Approach to the Study of Genes in DROSOPHILA MELANOGASTER That Are Involved in Imaginal Disc Development

The phenotypes of five different lethal mutants of Drosophila melanogaster that have small imaginal discs were analyzed in detail. From these results, we inferred whether or not the observed imaginal disc phenotype resulted exclusively from a primary imaginal disc defect in each mutant. To examine the validity of these inferences, we employed a multiple-allele method. Lethal alleles of the five third-chromosome mutations were identified by screening EMS-treated chromosomes for those which fail to complement with a chromosome containing all five reference mutations. Twenty-four mutants were isolated from 13,197 treated chromosomes. Each of the 24 was then tested for complementation with each of the five reference mutants. There was no significant difference in the mutation frequencies at these five loci. The stage of lethality and the imaginal disc morphology of each mutant allele were compared to those of its reference allele in order to examine the range of defects to be found among lethal alleles of each locus. In addition, hybrids of the alleles were examined for intracistronic complementation. For two of the five loci, we detected no significant phenotypic variation among lethal alleles. We infer that each of the mutant alleles at these two loci cause expression of the null activity phenotype. However, for the three other loci, we did detect significant phenotypic variation among lethal alleles. In fact, one of the mutant alleles at each of these three loci causes no detectable imaginal disc defect. This demonstrates that attempting to assess the developmental role of a gene by studying a single mutant allele may lead to erroneous conclusions. As a byproduct of the mutagenesis procedure, we have isolated two dominant, cold-sensitive mutants.     

Molecular cloning of the white locus region of Drosophila melanogaster using a large transposable element

We report the molecular cloning of a chromosome segment including the white locus of Drosophila melanogaster. This region was isolated using a deficiency extending from the previously cloned heat-shock puff sequences at 87A7 to a large transposable element containing the loci white and roughest.FB-NOF, a 7.5 kb element with partial homology to a family of inverted repeat sequences (Potter et al., 1980), is found very near the deficiency breakpoint, and is followed by DNA originating from the white locus region. Sequences totalling ˜60 kb surrounding this initial entry point were obtained by the cloning of successively overlapping fragments from a wild-type strain. Several rearrangement breakpoints have been mapped relative to the cloned DNA; these define the limits of the white locus and further differentiate the “white proximal region”, thought to function in gene regulation, from the remainder of the locus. Insertion of the dispersed repetitive element copia into the white locus is observed in strains carrying the white-apricot allele. Analysis of several white-apricot revertants suggests that copia insertion is responsible for the apricot eye color phenotype.     

Specificity of embryonic lethal mutations in Drosophila analyzed in germ line clones

Summary Only a small fraction of the known mutations causing death to homozygous Drosophila produce gross morphological defects during embryogenesis. We have examined fourteen such loci on the X-chromosome to determine: 1) whether the requirement for their respective activities is restricted to embryogenesis; and 2) whether the embryonic phenotype in mutant embryos is affected by the dosage of wild-type alleles in the mother. For two alleles per locus germ line clones were produced during larval development by irradiating females heterozygous for the lethal mutation and a dominant female sterile (ovo^D). Only one of the 14 loci (armadillo) is required during development of the germ cell to make morphologically normal eggs. Mutations at two other loci, (bazooka and Notch), allow normal oogenesis but cause major reductions in the viability of genetically normal (i.e., heterozygous) progeny. The majority of the loci (11/14) are not required in the germ line for either oogenesis or embryogenesis. However, in three cases (extradenticle, faintoid and lethal myospheroid), germ line homozygosity results in a readily detectible enhancement of embryonic phenotype over that observed in embryos derived from heterozygous mothers still possessing one wild type allele. The same six loci which show the most substantial effects on germ line homozygosity (arm, baz, N, exd, ftd and mys) also show an amelioration of the mutant phenotypes when maternal dosage is increased to wild type levels by using attached-X females. Four of these same loci (arm, baz, N and exd were cell lethal in imaginal discs.     

Recombination between genes located on nonhomologous chromosomes in Saccharomyces cerevisiae

We constructed strains of Saccharomyces cerevisiae that contained two different mutant alleles of either the leu2 gene or the ura3 gene. These repeated genes were located on nonhomologous chromosomes; the two ura3^- alleles were located on chromosomes V and XII and the two leu2^- alleles were located on chromosomes III and XII. Genetic interactions between the two mutant copies of a gene were detected by the generation of either Leu⁺ or Ura⁺ revertants. Both spontaneous and ultraviolet irradiation-induced revertants were examined. By genetic and physical analysis, we have shown that Leu⁺ or Ura⁺ revertants can arise by a variety of different genetic interactions. The most common type of genetic interaction is the nonreciprocal transfer of information from one repeat to the other. We also detected reciprocal recombination between repeated genes, resulting in reciprocally translocated chromosomes.     

Superunstable alleles at the cut locus in Drosophila melanogaster

Summary This is a detailed study of the reversions of the ct ^MR2 allele putatively carrying á mobile element (MR-transposon) in the cut locus. Stable, unstable and superunstable revertants have been identified. Besides, a series of multiple unstable visible and lethal ct mutations derived from the ct ^MR2 allele have been obtained. They are shown to include supermutable alleles. The results suggest that the MR-transposon is connected with at least three functions: excision; change of orientation; and change of position within the cut locus, these functions being disturbed in different ways in different unstable ct ⁺ and ct alleles. In some cases the mutant transitions are somehow strongly stimulated leading to superinstability, reaching the rate of 0.5.     

Characterisation of a new tumorous-head mutant of Drosophila melanogaster

Summary A new homoeotic mutant, I127, showing abnormal growths in the head region including homoeotic transformation of eye to genitalia and antenna to leg, was isolated in a screen designed to find new alleles of the tumorous head (tuh-3), mutation. Similarities in the phenotype and genetics of the mutant, and complementation studies with tuh-1; tuh-3, suggest that I127 is indeed an allele of tuh-3. In combination with the first chromosome modifier tuh-1, the mutant is temperature-sensitive during the third larval instar, giving an increased penetrance of the tumorous head phenotype when reared at 25° C as opposed to 18° C. The isolation of further alleles at the tumorous-head locus are essential. The types of morphological defects which can result from mutations at this locus would enable us to establish if this is a complex locus, and if null mutations are lethal during development. The interactions of the tumorous-head gene with first chromosome modifiers and other homoeotic mutations will only be understood if we able to induce a number of mutations at this locus, and as a consequence begin to elucidate the role of the wild-type gene product in normal development.     

Negative Complementation at the Notch Locus of DROSOPHILA MELANOGASTER

Four Abruptex alleles (Ax^E1, Ax^E₂, Ax^₉B₂, and Ax¹⁶¹⁷²) have been mapped within the Notch locus. Based on their visible phenotypes and their interactions with one another and with N mutations, the Ax alleles can be divided into two groups. Heterozygous combinations of members of the same group are intermediate in phenotype compared to the respective homozygotes, whereas heterozygotes of Ax alleles from different groups exhibit negative heterosis, being much less viable and more extremely mutant than either homozygote. It is suggested that the Notch locus is a multi-functional regulator ("integrator") gene, whose product possesses both "repressor" and "activator" functions for the processes it regulates.     

Mobilization of the gypsy and copia retrotransposons in Drosophila melanogaster induces reversion of the ovo dominant female-sterile mutations: molecular analysis of revertant alleles

The ovo locus is required for the maintenance of the female germ line in Drosophila melanogaster. In the absence of an ovo⁺ gene, males are completely normal but females have no germ-line stem cells. Three dominant mutations at the ovo locus, called ovo^D, were observed to revert towards recessive alleles at high frequency when ovo^D males were crossed to females of the strain y v f mal. We have found that this strain contains an inordinately high number of gypsy transposable elements, and crossing it with the ovo^D strains results in the mobilization of both gypsy and copia, with high-frequency insertions into the ovo locus: of 16 revertants examined 12 have gypsy and four have copia inserted at 4E, the ovo cytological site. Using gypsy DNA as a tag we have cloned 32 kb of wild-type DNA sequences surrounding a gypsy insertion and characterized molecular rearrangements in several independent revertants: in 10 of them gypsy appears to be inserted into the same site. The orientation of gypsy is strictly correlated with whether the neighbouring lozenge-like mutation appears in the revertants. A distal limit of the ovo locus was molecularly determined from the breakpoint of a deletion affecting closely flanking regions.     

Analysis of Pleiotropism at the Dominant White-Spotting (W) Locus of the House Mouse: A Description of Ten New W Alleles

Characterization of the pleiotropic effects of ten new putative W locus mutations, nine co-isogenic and one highly congenic with the C57BL/6J strain, reveals a wide variety of influences upon pigmentation, blood formation and gametogenesis. None of the putative alleles, each of which is closely linked to Ph, a gene 0.1 cM from W, gave evidence of complementation with W³⁹, a new allele previously shown to be allelic to W^v. All W*/W³⁹ genotypes resulted in black-eyed-white anemics with reduced gametogenic activity.¹ Homozygotes for seven of these mutations are lethal during perinatal life; anemic embryos have been identified in litters produced by intercross matings involving each of these alleles.—Phenotypes of mice of several mutant genotypes provide exceptions to the frequent observation that a double dose of dominant W alleles (e.g., W/W^v or W/W) results in defects of corresponding severity in each of the three affected tissues. One viable homozygote has little or no defect in blood formation, and another appears to have normal fertility. The phenotypes of these homozygotes support the conclusion that the three tissue defects are not dependent on each other for their appearance and probably do not result from a single physiological disturbance during the development of the embryo.—Although homozygosity for members of this series results in a wide range of phenotypes, the absence of complementation of any allele with W³⁹, the close proximity of each mutant to Ph, and the fact that all alleles produce detectable (though sometimes marginal) defects in the same tissues affected by W and W^v, support the hypothesis that each new mutant gene is a W allele.     

N-Ethyl-N-nitrosourea-induced prenatally lethal mutations define at least two complementation groups within the embryonic ectoderm development (eed) locus in mouse Chromosome 7

Two loci [l(7)5Rn and l(7)6Rn] defined by N-ethyl-N-nitrosourea (ENU)-induced, prenatally lethal mutations were mapped by means of trans complementation crosses to mice carrying lethal deletions of the albino (c) locus in Chromosome (Chr) 7. Both loci were found to map to the subregion of the Mod-2-sh-1 interval that contains the eed (embryonic ectoderm development) locus. eed has been defined by the inability of embryos homozygous for certain c deletions to develop beyond the early stages of gastrulation. Evidence for at least two loci necessary for normal prenatal development, rather than one locus, that map within the eed interval came from the observation that two prenatally lethal mutations, 3354SB [l(7)5Rn ^3354SB ] and 4234SB [l(7)6Rn ^4234SB ], could complement each other in trans, but could not each be complemented individually by c deletions known to include the eed locus. A somewhat leaky allele of l(7)5Rn [l(7)5Rn ^1989SB ] was also recovered, in which hemizygotes are often stillborn and homozygotes exhibit variable fitness and survival. The mapping of the loci defined by these mutations is likely to be useful for genetic, molecular, and phenotypic characterization of the eed region, and mutations at either locus (or both loci) may contribute to the eed phenotype.     

Genetic Studies of Membrane Excitability in Drosophila: Lethal Interaction between Two Temperature-Sensitive Paralytic Mutations

Two mutants of Drosophila melanogaster, para ^ts1 (1-53.9) and nap^ts (2-56.2) both display similar temperature-sensitive paralysis associated with blockage in the conduction of nerve action potentials, suggesting that the two gene products have a similar function. This idea is supported by the observation that the double mutant is unconditionally lethal. Genetic analysis of this synergistic interaction has revealed the following: 1) it specifically involves the para and nap loci; (2) all para alleles interact with nap^ts, but the strength of the interaction varies in an allele-dependent fashion; (3) lethality of the double mutant occurs during the first larval instar with para^ts1 but differs with other para alleles; (4) hypodosage of para ⁺ causes lethality in a nap^ts background. These results together with previous electrophysiological, behavioral and pharmacological studies of these mutants suggest that both para and nap affect sodium channels and possibly encode different subunits.