Bristle patterns,clones and cell competition along the anterior margin ofNotch wings ofDrosophila hydei

Summary The bristle pattern along the anterior margin ofNotch (N1-22.3) wings ofDrosophila hydei and the occurrence ofyellow (y 1–38.8) marked clones induced by X-ray irradiation during various larval stages are described. UnirradiatedN/N ⁺ wings show gaps (notches) in the longitudinal bristle rows along the 1st longitudinal vein, with tufts of bristles particularly near gaps. X-ray irradiation increases the number and total length of the gaps. The patterning of bristles along the margin depends on theN ⁽⁺⁾ genotype of the induced clones. RecombinantN ⁺/N ⁺ clones from irradiated wings show excessive growth with an autonomous wildtype bristle pattern. Characteristically, these clones do not respect the dorso-ventral compartment boundary along the wing margin, do not follow an exponential (2ⁿ) growth pattern, tend to fill the gaps with bristles and theiryellow medial row bristles are less often interspersed withy ⁺ bristles than described forN ⁺/N ⁺ wings. HomozygousN appears to be a cell lethal condition inD. hydei as it is inD. melanogaster. When y clones were kept phenotypicallyNotch (viz.,N/N/N ⁺) as the background cells, we found a lower number ofy bristles, a lower percentage of mosaic wings but also a reltive deficiency ofy ⁺ interspersions. The latter is discussed in relation to a possible clonal originof the notches.     

A high-resolution morphogenetic map of the second-leg basitarsus inDrosophila melanogaster

Summary The lineages of cells on the second-leg basitarsus ofDrosophila melanogaster were analyzed by examining gynandromorphs andMinute mosaics. Bracts lie proximal to bristles on the adult basitarsus, yet bract precursor cells were found to originate lateral to bristle precursor cells. In 6 of the 8 longitudinal rows of bristles on this segment, the bract cells arise ventral to the bristle cells; in the others they arise dorsally. The lateral cell origins are interpreted as reflecting a pattern of lateral cell movements associated with evagination of the leg disc. An unusual discrepancy was observed in the relative frequencies of male vs. female bracts and bristles in gynandromorphs. The discrepancy suggests that there is a cell-autonomous sexual difference in either the time at which cells begin moving during evagination or the speed with which they move.On the basis of the results, it is reasoned that the bristle pattern of the basitarsus does not originate in its final form. Prior to evagination, the bristle cells of each row are apparently closer together than in the final pattern, and the rows are farther apart. Evidence is presented which suggests that the bristle cells of each row may originally be arranged in a jagged line which is later straightened by cell movements.The two locations where the anterior/posterior compartment boundary of the second leg passes through the basitarsus were found to vary relative to the bristle pattern. If this boundary is assumed to be a fixed line of positional values, then the extent of the observed variability — which is estimated to be ± 1 or 2 cell diameters — provides a measure of the precision of patterning around the circumference.     

Establishment of compartments in the developing leg imaginal discs ofDrosophila melanogaster

Summary By X-irradiation ofM/M ⁺ embryos and larvae to induce mitotic recombination, clones ofM ⁺/M⁺ genotype were obtained (Fig. 1). Since such cells grow faster than the surroundingM/M ⁺-cells they can fill large areas within the compartments of an imaginal disc.The present studies concentrated mainly on the three leg discs. Clones were induced by doses of 1000 r at ages ranging from 3±0.5 h after oviposition to 144 h.All clones induced later than the blastoderm stage were absolutely restricted to either the anterior or the posterior compartment of a disc. The border between the anterior and posterior compartment runs as a straight line along the entire leg and at the distal end separates the two claws (Figs. 5, 6, 7). A further subdivision of the anterior compartment is indicated by clones initiated in the second larval instar (Fig. 11). A parallel subdivision could not be detected in the posterior compartment. Irradiation in the early third instar led to clones which were restricted to single longitudinal bristle rows and leg segments. But no clear-cut compartment borders could be found; in particular a proximo-distal separation appears to be absent.Among the 318 clones induced at the blastoderm stage eleven extended from the wing into the second leg (Fig. 8), or from the haltere into the third leg.With the exception of 3 clones that apparently occupied the anterior as well as the posterior compartment of a wing or a leg, all clones remained confined to either the anterior or the posterior compartment.Frequently clones overlapped left and right forelegs (Fig. 9). Intersegmental overlaps were not observed.The results show that the earliest compartment borders appear in all thoracic discs. This suggests that compartmentalization is a fundamental process common to all discs.Supported bySchweizerischer Nationalfonds Gesuch Nr. 3.480-0.75     

Pattern as a function of cell number and cell size on the second-leg basitarsus ofDrosophila

Summary The bristle pattern of the second-leg basitarsus inDrosophila melanogaster was studied as a function of the number and size of the cells on this segment in well-fed and starved wild-type flies, in triploid flies, and in two mutants (dachs andfour-jointed) that have abnormally short basitarsi. The second-leg basitarsi of well-fed, wild-type flies from 22 otherDrosophila species were studied in a similar manner. There are typically 8 longitudinal rows of evenly-spaced bristles on the second-leg basitarsus, and in each row the number of bristles was consistently found to vary in proportion to the estimated number of cells along the segment, and the interval between bristles was found to vary in proportion to the average cell diameter on the segment. These correlations are interpreted to mean that the spacing of the bristles within each row is controlled developmentally, whereas the number of bristles is not. The interval between bristles is evidently measured either as a fixed number of cells or as a distance which indirectly depends upon cell diameter.     

Correlation between adult transformation and aldehyde oxidase staining pattern in wing discs ofApterous genotypes inDrosophila melanogaster

Summary The aldehyde oxidase staining pattern in wing discs ofDrosophila melanogaster bearing the genotypesap ^blt /ap ^blt andap ^blt andap ^blt /ap ⁷³ⁿ showns changes from the wild-type pattern. Extensive areas of the presumptive dorsal posterior wing blade, which are normally unstained, have enzyme activity in these mutants. In wings of these genotypes, dorsal posterior structures are replaced by dorsal anterior wing structures. A strong correlation has been found between the frequencies of various staining patterns in the discs and the extent of transformation in the cuticular structures of the wing, which is consistent with the idea that aldehyde oxidase activity can be used as an indicator in the wing disc of this transformation. Unlike the homoeotic mutationengrailed, apterous has not been interpreted as a selector gene yet the work reported here shows thatapterous alleles can cause changes resembling those of theengrailed phenotype both in aldehyde oxidase staining behaviour and in the cuticular transformation.     

Homoeotische flügelbildung durch dasloboid-allel “ophthalmoptera” (ld oph ) beiDrosophila melanogaster

In awhite-apricot (w^a) stock, a new mutant was found which reduces the eyes and produces wing-structures in the eyes. This homeotic effect can occur with a high frequency, depending on temperature and genetic background. The mutant is an allele ofloboid (ld, 3–102±) and ofeyesreduced (eyr, 3–103±); the designationloboid-ophthalmoptera (ld^oph) is proposed. The varying contours of the reduced eye inld ^oph fit into a pattern which is quite similar to the cell-lineage pattern established byBecker (1957) for the normal eye. This indicates that inld ^oph-eyes one or more cell-lineage sectors are missing because the corresponding cell families are not competent to differentiate into ommatidia. Genal and orbital bristle rows are frequently doubled, and isolated clusters of bristles occur on the gena. In sectors devoid of ommatidia, “allotypic” wing structures are differentiated. These consist of wing membrance and fragments of anterior and posterior wing border in haphazard combination, and together form a balloon filled with hemolymph. The striking similarity between the homeotic mutation and transdetermination of cultured imaginaldisks (Hadorn, 1966) is discussed.      

Pattern formation inDrosophila melanogaster: The effects of mutations on polarity in the developing leg

Summary The effects of the mutations eyeless dominant (ey ^D) and shibire (shi) on bristle pattern in the legs ofDrosophila melanogaster were examined. Both mutations cause gaps in the intersegmental membranes which separate leg segments and often alter the position of these membranes. It was observed that pattern disturbances including reversed bristle polarity and duplication of structures such as sex combs and transverse rows were associated with defects in the intersegmental membranes. The alterations in bristle polarity and most of the duplication of structures could be accounted for by a segmentally reiterated gradient in the legs which controls bristle polarity and which requires the integrity of the intersegmental membrane. A computer simulation of this gradient model was devised which accounted for the observed results. The possible role of cell death as a cause of the gaps in the intersegmental membrane and of some of the pattern disturbances was examined.     

Accuracy of bristle placement on a leg segment in Drosophila melanogaster

Bristle positions in two rows of bristles on the basitarsus of the second leg of the fruitfly Drosophila melanogaster were analyzed in order to determine the accuracy of bristle placement within these rows. Within each row the positions of the two terminal bristles were found to be approximately equally variable, and positional variability was found to increase toward the middle of each row. Rows having fewer bristles manifested more positional variability in their midsection. These results are interpreted in terms of a possible bristle spacing mechanism involving repulsive forces between mobile bristle cells.     

Aldehyde oxidase distribution in the imaginal discs of some diptera

Summary In the imaginal discs ofMusca domestica, Drosophila melanogaster, D. simulans, D. hydei, andZaprionus spec. the enzyme aldehyde oxidase (AO) appeared in a clear-cut pattern. In the leg and eye-antennal discs of these species this pattern shows a high degree of conformity, while that of the wing and haltere discs is species-specific.No aldehyde oxidase activity was detected in the imaginal discs ofCalliphora erythrocephala, Phormia regina orLucilia cuprina, but the discs of these species are characterized by grossly similar patterns of 5-nucleotidase. Since the other species studied lack this enzyme, the two enzymes may perform similar functions in the morphogenesis of the discs.The coincidence of the sharp boundary of the AO pattern in the leg and wing discs ofD. melanogaster with the boundary between the anterior and posterior disc compartments gives a strong indication for the existence of analogous compartments in other discs showing a similar sharply bounded AO pattern. Compartmentalization may be considered a general phenomenon which occurs in discs of all segments and is not restricted toD. melanogaster. From the changes in the AO pattern during disc development it can be deduced that the localisation of this enzyme is regulated by supracellular determination involving positional information.     

Some observations on variegation inDrosophila

Attempts to modify experimentally the variegated phenotypes of the mutantsz inDrosophila melanogaster andw ^m2 inD. hydei, were largely unsuccessful. Transplanted eyes ofz were found to be more intensely pigmented, but this effect was not influenced by treatment of the imaginal disks or by the host's genotype. Inw ^m2, increased pigmentation was observed after treatment with 5-bromo-uracil.Thew ^m2 phenotype is strongly modified by supernumerary Y chromosomes. One Y shifts the eye colour from yellow to brown, two Y's, to dark red. In the Malpighian tubules, two Y's have a similarly strong effect, but one Y a slight effect only. Malpighian tubules ofw ^m2 larvae were used for a quantitative analysis of mottling. An average clonal cluster size of 1.1 was found. This indicates that the state of pigmentation is determined around the last cell division in the embryo.     

Effects of the “sexcombless” chromosome (In(1)sx) on males and females ofDrosophila melanogaster

Summary The chromosome which carries the mutationsexcombless (In(1)sx) affects males and females ofD. melanogaster. In the male foreleg basitarsi the number of sexcomb teeth is dramatically reduced from 10 to 0.7 and the number of transverse rows of bristles is increased from 6 to 8. Females homozygous forIn(1)sx show a normal bristle pattern in the foreleg basitarsus. The genital disc derivatives of both male and femaleIn(1)sx flies are strongly affected. While the external genitalia show a duplicated or a reduced bristle pattern, the internal genitalia are mostly absent. However, the sexually dimorphic tergites and sternites of the abdomen remain unaffected. The male-specific effect on the basitarsus and the general effects on the genital disc derivatives are proposed to represent two different phenotypic effects ofIn(1)sx which may derive from mutations at different gene loci in the inverted chromosome.     

BrdU incorporation reveals DNA replication in non dividing glial cells in the larval abdominal CNS ofDrosophila

Glial cells are of significant importance for central nervous system development and function. In insects, knowledge of the types and development of CNS glia is rather low. This is especially true for postembryonic glial development. Using bromodeoxyuridine incorporation and enhancer trap lines we identified a reproducible spatial and temporal pattern of DNA replicating cells in the abdominal larval CNS (A3-7 neuromeres) ofDrosophila melanogaster. These cells correspond to embryonically established glial cells in that region. Except for a specific subfraction, these cells apparently do not divide during larval life. Similar patterns were found in two otherDrosophila species,D. virilis andD. hydei.     

Transdetermination in leg imaginal discs ofDrosophila melanogaster undDrosophila nigromelanica

Summary The transdetermination capacities of leg discs ofDrosophila melanogaster were examined by mechanically disrupting and kneading whole discs from late third instar larvae and by culturing the resulting tissue mass for 10–14 days in adult female abdomens where the cells continued to divide. The grown implants were then dissected from the abdomens and injected into third instar larvae to undergo metamorphosis.After this treatment, prothoracic leg discs ofDrosophila melanogaster transdetermined with a high frequency (59% of all implants) to wing. Mesothoracic leg discs also transdetermined to wing, but at a very low frequency (4%). Metathoracic leg discs exhibited the same low frequency of transdetermination (4%), but in this case the direction of transdetermination was to haltere (Table 1,D. melanogaster).Very similar results were obtained with leg discs ofDrosophila nigromelanica (Table 1,D. nigromelanica), showing that the peculiar behavior of the three leg discs is not unique forDrosophila melanogaster.The homeotic mutation Polycomb (Pc ³) which partially transforms meso- and metathoracic legs into prothoracic legs did not significantly increase the frequencies of transdetermination in these leg dises and had clearly no effect on the direction of transdetermination (Table 1).We dedicate this publication to the memory of our teacher and advisor, the late Professor Ernst Hadorn, whose enthusiasm and interest stimulated our work     

The distribution of the transposable elementBari-1 in theDrosophila melanogaster andDrosophila simulans genomes

The distribution of the transposable elementBari-1 inD. melanogaster andD. simulans was examined by Southern blot analysis and byin situ hybridization in a large number of strains of different geographical origins and established at different times.Bari-1 copies mostly homogeneous in size and physical map are detected in all strains tested. Both inD. melanogaster and inD. simulans a relatively high level of intraspecific insertion site polymorphism is detectable, suggesting that in both speciesBari-1 is or has been actively transposing. The main difference between the two sibling species is the presence of a large tadem array of the element in a well-defined heterochromatic location of theD. melanogaster genome, whereas such a cluster is absent inD. simulans. The presence ofBari-1 elements with apparently identical physical maps in allD. melanogaster andD. simulans strains examined suggests thatBari-1 is not a recent introduction in the genome of themelanogaster complex. Structural analysis reveals unusual features that distinguish it from other inverted repeat transposons, whereas many aspects are similar to the widely distributedTc1 element ofC. elegans.     

Abnormal-abdomen beiDrosophila hydei

Several strains ofDrosophila hydei exhibiting an abnormal-abdomen (aa) phenotype were investigated. They proved to be mutant at one and the same major gene, located on chromosome 2 (homologous to 3R ofD. melanogaster).This mutant gene,aa, causes skeletal abnormalities of the adult abdomen. Sternites and, less frequently, tergites are imperfectly sclerotized. They may be dissolved into separate areas, or may be missing altogether. Often they are normal in shape, while the number of hairs is reduced. Compound sclerites originating from parts of adjoining segments were never observed. Small hypoderm spherules with internal hairs occur frequently under the ventral skin. In aa larvae, segmentation is normal, in contrast to the situation in abnormal-abdomen mutants ofD. melanogaster. Even where theaa mutant expression is extremely strong, puparia are perfectly normal. Theaa gene thus appears to act exclusively during metamorphosis. This was confirmed by temperature experiments. The expression of theaa gene is much reduced at low temperature, but low temperature is effective during metamorphosis only.The seven aa strains compared, showed considerable differences in penetrance and expressivity as well as in the amount of dominance in crosses. Reciprocal crosses yield in the F₁ values either intermediate, or lower or higher, compared to those of the parent strains. The F₁ values show a nonlinear correlation between penetrance and expressivity.On the basis of chromosomal location and phenotypic effect, it is unlikely thataa ofD. hydei is homologous to any of the abnormal-abdomen mutants ofD. melanogaster described so far. However, a new dominant autosomal mutant was found inD. simulans, which seems to be a homologue.     

Arrangement of bristles as a function of bristle number on a leg segment inDrosophila melanogaster

Summary The arrangement of bristles on a leg segment of the fruitflyDrosophila melanogaster was studied in various mutants that have abnormal numbers of bristles on this segment. Eighteen mutations at six different genetic loci were analyzed, plus five double or triple mutant combinations. Recessive mutations at theachaete-scute locus were found to affect distinct groups of bristles:achaete mutations remove mechanosensory bristles, whereasscute mutations remove mainly chemosensory bristles. Mechanosensory bristles remain uniformly spaced along the longitudinal axis unless their number decreases below a certain threshold, suggesting that spacing is controlled by cell interactions that cannot function when bristle cells are too far apart. Above a certain threshold, bristle spacing and alignment both become irregular, perhaps due to excessive force from these same interactions. Chemosensory bristles occupy definite positions that are virtually unaffected by removal of individual bristles from the array. Extra chemosensory bristles develop only near the six normal sites. At two of the six sites the multiple bristles tend to exhibit uniform longitudinal spacing — a property confined to mechanosensory bristles in wild-type flies. To explain the various mutant phenotypes the following scheme is proposed, with different mutations directly or indirectly affecting each step: (1) spots and stripes are demarcated within the pattern area, (2) one bristle cell normally arises within each spot, multiple bristle cells within each stripe, (3) incipient bristle cells inhibit neighboring cells from becoming bristle cells, and (4) the bristle cells within each stripe become aligned to form rows and then repel one another to generate uniform spacing.     

The evolutionary genetics of thehobo transposable element in theDrosophila melanogaster complex

Hobo elements are a family of transposable elements found inDrosophila melanogaster and its three sibling species:D. simulans, D. mauritiana andD. sechellia. Studies inD. melanogaster have shown thathobo may be mobilized, and that the genetic effects of such mobilizations included the general features of hybrid dysgenesis: mutations, chromosomal rearrangements and gonadal dysgenis in F1 individuals. At the evolutionary level somehobo-hybridizing sequences have also been found in the other members of themelanogaster subgroup and in many members of the relatedmontium subgroup. Surveys of older collected strains ofD. melanogaster suggest that completehobo elements were absent prior to 50 years ago and that they have recently been introduced into this species by horizontal transfer. In this paper we review our findings and those of others, in order to precisely describe the geographical distribution and the evolutionary history ofhobo in theD. melanogaster complex. Studies of the DNA sequences reveal a different level of divergence between the groupD. melanogaster, D. simulans andD. mauritiana and the fourth speciesD. sechellia. The hypothesis of multiple transfers in the recent past into theD. melanogaster complex from a common outside source is discussed.     

Sensitive periods for abnormal patterning on a leg segment inDrosophila melanogaster

Summary The development of a leg segment of the fruitflyDrosophila melanogaster was analyzed in order to determine whether the orderliness of the segment's bristle pattern originates via waves of cellular interactions, such as those that organize the retina. Fly development was perturbed at specific times by either teratogenic agents (gamma rays, heat shock, or the drug mitomycin C) or temperature-sensitive mutations (l(1)63, l(1) Notch^ts1, orl(1) shibire ^ts1 ), and the resulting abnormalities (e.g., missing or extra structures) were mapped within the pattern area. If bristles develop in a linear sequence across the pattern, then they should show sensitivity to perturbations in the same order, and wavefronts of cuticular defects should result. Contrary to this prediction, the maps reveal no evidence for any directional waves of sensitivity. Nevertheless, other clues were uncovered as to the nature and timing of patterning events. Chemosensory bristles show earlier sensitivities than mechanosensory bristles, and longer bristles precede shorter ones. The types and sequence of cuticular abnormalities imply the following stages of bristle pattern development: (1) scattered inception of bristle mother cells, each surrounded by an inhibitory field, (2) alignment of the mother cells into rows, (3) differential mitoses, (4) assignment of cuticular fates to the mitotic progeny, (5) polytenization of the bristle cells, (6) fine-tuning adjustments in bristle spacing, and (7) signalling from bristle cells to adjacent epidermal cells, inducing them to form bracts.     

The Structure and Development of the Dorsal Bristle Complex of Oxytricha fallax and Stylonychia pustulata*

SYNOPSIS. Oxytricha fallax and Stylonychia pustulata possess 6 rows of dorsal bristle units. Each dorsal bristle unit consists of a pair of kinetosomes; the anterior kinetosome has a cilium and the posterior kinetosome a ciliary stub. The kinetosome pair, located at the bottom of a cortical pit surrounding the cilium and ciliary stub, is surrounded by an asymmetrical fibrillar mass. Future rows 1-4 are formed from 2 sets of primordia originating within mature dorsal rows 1-3. Rows 5 and 6 originate from the anterior regions of both right marginal cirral primordia. Old dorsal bristle units utilized in formation of primordia are presumably maintained in the new rows of the proter and opisthe; those outside the primordia are resorbed. The morphogenetic pattern of the Oxytrichidae is similar to those of the Urostylidae and Holostichidae, but quite different from that of the Euplotidae.     

The LTR retrotransposon micropia in the cardini group of Drosophila (Diptera: Drosophilidae): a possible case of horizontal transfer

The presence of the micropia retroelement from the Ty1-copia family of LTR retroelements was investigated in three species of the Drosophila cardini group. Southern blot analysis suggested the existence of at least four micropia copies in the genomes of D. cardinoides, D. neocardini and D. polymorpha populations. The high sequence similarity between dhMiF2 and Dm11 clones (micropia retroelements isolated from D. hydei and D. melanogaster, respectively) with micropia sequences amplified from D. cardini group genome supports the hypothesis that this retroelement plays an active role in horizontal transfer events between D. hydei and the D. cardini group. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.