The segment polarity gene costal-2 in Drosophila. II. The origin of imaginal pattern duplications

Imaginal pattern duplications caused by hypomorphic expression of the segment polarity gene costal-2 are described. These affect the anteroposterior coordinate of the imaginal disc. A very small part of the pattern is deleted and a large number of additional pattern elements arise in a progressive order, anterior-most first followed by more and more posterior structures. Mosaic analyses show that the duplications arise nonautonomously in the larval stages but that the costal-2 gene is not required after early embryogenesis. Arguments that the duplications are the result of cell interactions and intercalary growth that themselves arise from an abnormal polarity of the embryonic segment are presented.     

Segmental Polarity in Drosophila Melanogaster: Genetic Dissection of Fused in a Suppressor of Fused Background Reveals Interaction with Costal-2

fused (fu) is a segment polarity gene that encodes a putative serine/threonine kinase. A complete suppressor of the embryonic and adult phenotypes of fu mutants, Suppressor of fused (Su(fu)), was previously described. The amorphic Su(fu) mutation is viable and displays no phenotype by itself. We have used this suppressor as a tool to perform a genetic dissection of the fu gene. Analysis of the interaction between Su(fu) and 33 fu alleles shows that they belong to three different classes. Defects due to class I fu alleles are fully suppressed by Su(fu). Class II fu alleles lead to a new segment polarity phenotype in interaction with Su(fu). This phenotype corresponds to embryonic and adult anomalies similar to those displayed by the segment polarity mutant costal-2 (cos-2). Class II alleles are recessive to class I alleles in a fu[I]/fu[II];Su(fu)/Su(fu) combination. Class 0 alleles, like class I alleles, confer a normal segmentation phenotype in interaction with Su(fu). However class II alleles are dominant over class 0 alleles in a fu[0]/fu[II];Su(fu)/Su(fu) combination. Alleles of class I and II correspond to small molecular events, which may leave part of the Fu protein intact. On the contrary, class 0 alleles correspond to large deletions. Several class I and class II fu mutations have been mapped, and three mutant alleles were sequenced. These data suggest that class I mutations affect the catalytic domain of the putative Fu kinase and leave the carboxy terminal domain intact, whereas predicted class II proteins have an abnormal carboxy terminal domain. Su(fu) enhances the cos-2 phenotype and cos-2 mutations interact with fu in a way similar to Su(fu). All together these results suggest that a close relationship might exist between fu, Su(fu) and cos-2 throughout development. We thus propose a model where the Fu(+) kinase is a posterior inhibitor of Costal-2(+) while Su(fu)(+) is an activator of Costal-2(+). The expression pattern of wingless and engrailed in fu and fu;Su(fu) embryos is in accordance with this interpretation.     

hunchback, a gene required for segmentation of an anterior and posterior region of the Drosophila embryo

The locus hunchback (hb) is a member of the gap class of segmentation genes of Drosophila. A number of X-ray-induced deletions locate the hb locus at the chromosomal site 85A3-B1, to the right of the pink locus, which maps in the same interval. A total of 14 EMS and 3 X-ray-induced hb alleles have been studied. Homozygous mutant embryos show deletions of segments in two separate regions. In the six strong alleles, the labium and all three thoracic segments are deleted anteriorly while posteriorly the 8th abdominal segment and adjacent parts of the 7th abdominal segment are lacking. The eight weak alleles show smaller deletions both in the thoracic and posterior abdominal region. In the weakest allele only part of the mesothorax is deleted. Three hb alleles produce a homoeotic transformation: superimposed on a strong or weak deletion phenotype, head or thoracic segments are transformed into abdominal segments, respectively. This suggests that hb might also be involved in the regulation of genes in the Bithorax complex (BX-C). Fate mapping of the normal-appearing segments in strong mutant embryos using the UV-laser beam ablation technique (Lohs-Schardin et al., 1979) shows that these segments arise from the normal blastoderm regions. The mutant phenotype can be recognized soon after the onset of gastrulation in a failure to fully extend the germ band. In 6-hr-old mutant embryos, two clusters of dead cells are observed in the thoracic and posterior abdominal region. These observations indicate region specific requirement of hb gene function. The analysis of germ line chimeras by transplantation of homozygous mutant pole cells shows that hb is already expressed during oogenesis. Homozygous mutant embryos derived from a homozygous mutant germ line have a novel phenotype. The anterior affected region is enlarged, including all three gnathal segments and the anterior three abdominal segments. In addition three abdominal segments with reversed polarity are formed between the remaining head structures and the posterior abdomen. Heterozygous mutant embryos derived from a homozygous mutant germ line develop normally, indicating that maternal gene expression is not required for normal development.     

Genetic and Developmental Analysis of the Locus vnd in DROSOPHILA MELANOGASTER

Genetic and developmental analysis of an X-linked vital locus vnd was undertaken. Embryos hemizygous for the original allele vnd did not hatch and exhibited a disorganized ventral nervous system (VNS). The mutation maps in the region 1B6-7 to 1B9-10, a subregion of an area previously shown to be essential to normal neural development. In this paper, we report isolation of five new alleles at the locus vnd. Genetic complementation analysis of all mutations at the vnd locus, with lethal alleles at adjacent loci, indicates that all lesions at the locus vnd affect only one vital gene function in the region. Four of the five alleles are embryonic lethal; one allele is subvital and behaves like an hypomorphic mutation. Hemizygous embryos for three of the four embryonic lethal alleles were inspected in histological sections; all exhibited disorganized VNS similar to the original allele. The developmental analysis in gynandromorphic genetic mosaics shows that (1) vnd+ gene function is not essential in most imaginal-disc cell derivatives, (2) only about 30% of the mosaic zygotes survive as adults, (3) mosaic zygotes with mutant tissue close to the head cuticle are least likely to survive, and (4) mutant tissue in the thoracic ganglion in the adult is not necessarily lethal. The mosaic data are consistent with the vnd+ gene function being necessary in neural cells derived from the anterioventral region of the blastoderm.     

The Drosophila EGF receptor homolog (DER) gene is allelic to faint little ball, a locus essential for embryonic development

Recessive lethal mutations in the genetic locus of the Drosophila EGF receptor homolog (DER) were isolated. Identification of mutations in the gene is based on assays of DER protein autophosphorylation activity. Most DER alleles show little or no in vivo autophosphorylation. The ability to monitor these activities in vivo and in vitro offers a preliminary insight into the functional defects in the different mutant proteins. The identification of the DER locus was also confirmed by partial rescue of the mutant phenotype with a DER P-element construct. Homozygous DER mutants display a complex embryonic phenotype. Most notably, the anterior structures deteriorate, ventral denticle bands are missing, the germ band does not retract, and the central nervous system shows a collapse of commissure and midline pattern. Mutations in DER were shown to be allelic to the previously described locus faint little ball.     

Butyrate sensitive suppressor of position-effect variegation mutations in drosophila melanogaster

Summary Mutations at a locus on chromosome II of D. melanogaster suppressing position-effect variegation mutations have been identified which display recessive butyrate sensitivity. Survival of homozygous mutant flies is significantly reduced on medium containing sodium n-butyrate. The butyrate sensitive suppressor mutations are further characterized by recessive female sterility and reduced survival of homozygotes. Complementation analysis showed their allelism. The locus of these mutations, Su-var (2) 1, has been localized to 40.5±0.2 and, by using interstitial duplications, to region 31CD on the cytogenetic map. Moreover, the mutant alleles of the Su-var (2) 1 locus display a lethal interaction with the heterochromatic Y chromosome. The presence or absence of a Y chromosome in males or females has a strong influence on the viability of homozygous or transheterozygous suppressor flies. All the genetic properties of Su-var (2) 1 mutants suggest strongly that this locus affects chromosome condensation.     

Krüppel, a gene whose activity is required early in the zygotic genome for normal embryonic segmentation

Embryos homozygous for Krüppel die as late embryos with an altered segmentation pattern. In strong alleles the normal thoracic and anterior abdominal segments are replaced by a partial mirror image duplication of the posterior abdomen. Weak alleles cause smaller pattern deletions in the thorax and abdomen and are not associated with mirror image duplications. The altered segmentation pattern can be traced back to 12 min after the onset of gastrulation, when the shorter germ bands in homozygous Kr embryos provide a first indication of abnormal patterning. The mutant was mapped to position 107.6 at the tip of the right arm of the second chromosome, cytologically to bands 60F2-5. Analysis of homozygous deficiency embryos indicate that the phenotype produced by strong point mutations probably represents the amorphic condition. The requirement for Kr+ gene activity is strictly zygotic. Maternal dosage of Kr+ has no effect on the embryonic phenotype, nor does homozygosity for Kr prevent germ cells from making normal eggs capable of normal embryonic development when fertilized by wild-type sperm. The requirement for Kr+ seems restricted to embryogenesis. Homozygous clones induced in imaginal discs during larval development survive and develop normally and in vivo cultures established from homozygous embryos proliferate normally and metamorphose into adult structures of normal morphology.     

Cloning sequences from the hairy gene of Drosophila.

A series of mutations that alter the pattern of segmentation in Drosophila embryos has been identified. Mutations in one of these loci, hairy, delete the posterior part of each odd-numbered segment and the anterior part of each even-numbered segment; although the amount deleted depends on the allele. Weak alleles delete less than an entire segment and do not always eliminate structures in every other segment. Strong alleles show the same periodicity in the pattern defect, but delete regions greater than one segment. In such cases the remaining parts of the pattern duplicate with mirror-image symmetry. To study the function of this gene at a molecular level, sequences from the hairy locus were cloned. This was facilitated by the hairy1 (h1) mutation, which is caused by the insertion of the transposable element, gypsy.     

Characterization of Mutant Alleles of Myospheroid, the Gene Encoding the β Subunit of the Drosophila PS Integrins

This paper presents the characterization of nine alleles of myospheroid, which encodes the beta PS subunit of the Drosophila PS integrins. On Southern blots, the mysXB87, mysXN101 and mysXR04 genes yield restriction digest patterns similar to that seen for wild-type chromosomes, however the mys1 and mysXG43 genes contain detectable deletions. mys1, mysXB87 and mysXG43 make little or no stable protein product, and genetically behave as strong lethal alleles. For the mysXN101 mutation, protein product is seen on immunoblots and a reduced amount of beta PS protein is seen at muscle attachment sites of embryos; this mutant protein retains some wild-type function, as revealed by complementation tests with weak alleles. Protein is also seen on immunoblots from mysXR04 embryos, and this allele behaves as an antimorph, being more deleterious in some crosses than the complete deficiency for the locus. mysts2 and mysnj42 are typically lethal in various combinations with other alleles at high temperatures only, but even at high physiological temperatures, neither appears to eliminate gene function completely. The complementation behaviors of mysts1 and mysts3 are quite unusual and suggest that these mutations involve regulatory phenomena. For mysts3, the data are most easily explained by postulating transvection effects at the locus. The results for mysts1 are less straightforward, but point to the possibility of a chromosome pairing-dependent negative interaction.     

Analysis of maxillopedia expression pattern and larval cuticular phenotype in wild-type and mutant tribolium

The Tribolium castaneum homeotic gene maxillopedia (mxp) is the ortholog of Drosophila proboscipedia (pb). Here we describe and classify available mxp alleles. Larvae lacking all mxp function die soon after hatching, exhibiting strong transformations of maxillary and labial palps to legs. Hypomorphic mxp alleles produce less severe transformations to leg. RNA interference with maxillopedia double-stranded RNA results in phenocopies of mxp mutant phenotypes ranging from partial to complete transformations. A number of gain-of-function (GOF) mxp alleles have been isolated based on transformations of adult antennae and/or legs toward palps. Finally, we have characterized the mxp expression pattern in wild-type and mutant embryos. In normal embryos, mxp is expressed in the maxillary and labial segments, whereas ectopic expression is observed in some GOF variants. Although mxp and Pb display very similar expression patterns, pb null embryos develop normally. The mxp mutant larval phenotype in Tribolium is consistent with the hypothesis that an ancestral pb-like gene had an embryonic function that was lost in the lineage leading to Drosophila.     

A pupal lethal mutation with a paternally influenced maternal effect on embryonic development in Drosophila melanogaster

The maternal effect and zygotic phenotype of l(1)pole hole (l(1)ph) is described. l(1)ph is a zygotic lethal mutation which affects cell division of adult precursor cells in Drosophila larvae. The locus is located in 2F6 on the salivary gland chromosome map and four alleles have been characterized. Germ-line clonal analysis of amorphic alleles indicates that l(1)ph has a maternal effect lethal phenotype. Two lethal phenotypes are observed among embryos derived from female germ-line clones homozygous for amorphic alleles dependent upon the zygotic activity of l(1)ph⁺ introduced via the sperm. Class 1: If no wild-type dose of the gene is introduced, embryos form abnormal blastoderms in which nuclear migration and cell formation is disrupted leading to an ill-defined cuticular pattern. Class 2: If a wild-type copy of the gene is introduced, blastoderm cells do not form beneath the pole cells (the pole hole phenotype); subsequently such embryos are missing cuticular structures posterior to the seventh abdominal segment (the torso phenotype). When the zygotic activity l(1)ph⁺ is modulated using position effect variegation a new phenotype is observed among class 2 embryos in which torso embryos are twisted along their longitudinal axis.     

Homoeosis in Drosophila: anterior and posterior transformations of Polycomb lethal embryos

Lethal embryos homozygous for Polycomb (Pc) mutations show transformations of segment-specific cuticular features to those of more anterior or posterior segments; the frequency and extent of such changes show differences which depend on the genotype and the region. The mesothorax of Polycomb lethal embryos often shows posterior transformations of the anterior- and posterior-most portions of the segment, and anterior transformations of the medial portion. A comparison of Polycomb embryos also bearing various genetic lesions of the bithorax gene complex (BX-C) shows that the penetrance of anterior transformation and the extent of posterior transformation in the appears independent of posterior transformation, even though cells undergoing each of these changes lie in close proximity in the developing embryo. It has been shown previously that in Polycomb lethal embryos posterior transformations require the normal function of the BX-C. We show here that anterior transformations of the mesothorax and other segments require the normal function of the Sex combs reduced (Scr) locus, also necessary for the normal development of the prothorax and some head segments. Similar observations are also presented for a Polycomblike mutation. We suggest that in Polycomb embryos there are errors in the clonal transmission of determined states resulting in expression of the BX-C and Scr+ loci at abnormal locations, and that such events are probabilistic in nature and show marked regional differences in frequency.     

The genetics of a small autosomal region of Drosophila melanogaster containing the structural gene for alcohol dehydrogenase. III. Hypomorphic and hypermorphic mutations affecting the expression of hairless

A lethal locus (l(2)br7;35B6-10), near Adh on chromosome arm 2L of D. melanogaster, is identified with Plunkett's dominant suppressor of Hairless (H). Of eight new alleles, seven act as dominant suppressors of H, the eighth is a dominant enhancer of H. One of the suppressor alleles is both a leaky lethal and a weak suppressor of H. Confirming Nash (1970), deletions of l(2)br7 are dominant suppressors, and duplications are dominant enhancers of H. A simple model is proposed to account for the interaction of l(2)br7 and H, assuming that amorphic (or hypomorphic) alleles of l(2)br7 suppress H and that hypermorphic alleles enhance H.     

l(1)hopscotch,a larval-pupal zygotic lethal with a specific maternal effect on segmentation in Drosophila

The maternal and zygotic effect phenotypes of mutations at the l(1)hopscotch (l(1)hop) locus are described. l(1)hop is located in 10B6-8 on the salivary gland chromosome map and 17 alleles have been characterized. A complex complementation pattern is observed among the 17 alleles. The lethal phase of null alleles of l(1)hop occurs at the larval-pupal interface associated with a small disc phenotype. Embryos produced from homozygous l(1)hop germline clones show segment specific defects. The extent of these defects depends upon both the strength of the allele and the paternal contribution. In the most extreme case embryos exhibit defects associated with five segments T2, T3, A4, A5, and A8. In the less extreme phenotype defects are only associated with A5. Thus, activity of l(1)hop⁺ is required both for the maintenance and continued cell division of diploid imaginal precursors and for the establishment of the full array of segments.     

A genetic and developmental analysis of mutations in the Deformed locus in Drosophila melanogaster

Individuals expressing recessive mutations in the Deformed (Dfd) locus of Drosophila melanogaster were examined for embryonic and adult defects. Mutant embryos were examined in both scanning electron microscope and light microscope preparations. The adult Dfd recessive mutant phenotype was assessed in somatic clones and in survivors homozygous for hypomorphic alleles of the gene. The time of Dfd+ action was determined by studying a temperature conditional allele. Dfd+ is required in three embryonic cephalic segments to form a normal head. Mutant embryos of Dfd display defects in derivatives of the maxillary segment, of the mandibular segment, and of some more anterior segments. In the adult fly, defects are seen in the posterior aspect of the head when the gene is mutant. A transformation from head to thoracic-like tissue is seen dorsally and a deletion of structures is seen ventrally. Shift studies utilizing a temperature conditional allele have shown that the gene product is necessary during at least two periods of development, during embryonic segmentation and head involution and during the late larval and pupal stages. From these studies we conclude that Dfd is a homeotic gene necessary for proper specification of both the embryonic and the adult head.     

The evolutionary fate of heterogeneous gene duplications: A precarious overdominant equilibrium between environment,sublethality and complementation

Gene duplications occur at a high rate. Although most appear detrimental, some homogeneous duplications (identical gene copies) can be selected for beneficial increase in produced proteins. Heterogeneous duplications, which combine divergent alleles of a single locus, are seldom studied due to the paucity of empirical data. We investigated their role in an ongoing adaptive process at the ace‐1 locus in Culex pipiens mosquitoes. We assessed the worldwide diversity of the ace‐1 alleles (single‐copy, susceptible S and insecticide‐resistant R, and duplicated D that pair one S and one R copy), analysed their phylogeography and measured their fitness to understand their early dynamics using population genetics models. It provides a coherent and comprehensive evolutionary scenario. We show that D alleles are present in most resistant populations and display a higher diversity than R alleles (27 vs. 4). Most appear to result from independent unequal crossing‐overs between local single‐copy alleles, suggesting a recurrent process. Most duplicated alleles have a limited geographic distribution, probably resulting from their homozygous sublethality (HS phenotype). In addition, heterozygotes carrying different HS D alleles showed complementation, indicating different recessive lethal mutations. Due to mosaic insecticide control practices, balancing selection (overdominance) plays a key role in the early dynamics of heterogeneous duplicated alleles; it also favours a high local polymorphism of HS D alleles in natural populations (overdominance reinforced by complementation). Overall, our study shows that the evolutionary fate of heterogeneous duplications (and their long‐term role) depends on finely balanced selective pressures due to the environment and to their genomic structure.     

Characterization of New Punch Mutations: Identification of Two Additional Mutant Classes

The Punch locus of Drosophila melanogaster which encodes the pteridine biosynthetic enzyme, GTP cyclohydrolase, is genetically complex. Lethal alleles of the locus resolve into an array of interallelic complementation groups, and at least one class of mutations is developmentally specific, affecting GTP cyclohydrolase activity only in the heads of adults. All previously isolated Punch alleles were identified on the basis of a mutant eye color phenotype. By screening mutagenized chromosomes over Punch region deficiencies, we have now isolated new alleles on the basis of lethal and visible phenotypes. Most of these alleles fall into previously identified genetic classes, but two new classes of mutations were also found. One class contains two alleles that behave as dominant lethal mutations in some genetic backgrounds. The other class represents a second developmentally specific set of alleles that affect the function of the Punch locus only during embryogenesis.     

Analysis of the Cut Locus of DROSOPHILA MELANOGASTER

Mutants of the cut (ct) locus can be divided into two classes: viable and lethal. Most of the viable alleles are characterized by varying degrees of scalloping and notching of the wings. One mutant, kinked femur, exhibits kinking of the femurs and failure of wing expansion, but no other changes in wing structure. In heterozygous combination with the other viable alleles, it exhibits complete complementation, but it fails to complement with lethal ct alleles with respect to its viable phenotype. Similarly, all of the other viable ct alleles express a mutant wing phenotype when heterozygous with lethal ct alleles.-Mapping experiments indicate that the lethal alleles, which comprise the majority of all ct mutations recovered, are confined to a small region at the right end of the locus. That this restriction is real and not an artifact imposed by the limited number of lethal mutations mapped in the locus is supported by an examination of the mutant ct(JC20), a presumptive deficiency for the left-most third of the locus. Despite its behavior as a deletion, ct(JC20) is viable, though mutant, in combination with the lethal alleles. The restriction of the noncomplementary lethals to a small part of the locus, distinct from the other ct mutants, suggests a polarity that may define a segment that functions only in cis within the complex.-Based on the comparison of the data with the prediction of several models, we suggest that the left portion of the locus, which contains the viable alleles, defines a regulatory region controlling the expression of the locus, while the segment encoding a polypeptide product is at the right end and only it is capable of mutating to a lethal state.     

Mutations in a newly identified Drosophila melanogaster gene, mago nashi, disrupt germ cell formation and result in the formation of mirror-image symmetrical double abdomen embryos.

The mago nashi (mago) locus is a newly identified strict maternal effect, grandchildless-like, gene in Drosophila melanogaster. In homozygous mutant mago females reared at 17 degrees C, mago+ function is reduced, the inviable embryos lack abdominal segments and 84-98% of the embryos die. In contrast, at 25 degrees C, some mago alleles produce a novel gene product capable of inducing the formation of symmetrical double abdomen embryos. Reciprocal temperature-shift experiments indicate that the temperature-sensitive period is during oogenetic stages 7-14. Furthermore, embryos collected from mago1 homozygous females contain no apparent functional posterior determinants in the posterior pole. In viable F1 progeny from mago mutant females, regardless of genotype and temperature, polar granules are reduced or absent and germ cells fail to form (the grandchildless-like phenotype). Thus, we propose that the mago+ product is a component of the posterior determinative system, required during oogenesis, both for germ cell determination and delineation of the longitudinal axis of the embryo.