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.     

Determination of sensory bristles and pattern formation in Drosophila: I. A model

A model is presented to explain the formation of the pattern of sensory bristles in Drosophila. The model is based on the idea that precision and reproducibility in pattern formation can be achieved by averaging out of many moderately accurate responses to positional cues. According to this model, the determination of bristles in imaginal discs occurs in two steps. First, large numbers of imaginal cells synthesize a freely diffusible inducer, the chaetogen. Second, cells in which the concentration of this chaetogen reaches a threshold are induced to differentiate into a bristle apparatus. Induced cells prevent neighboring cells from being induced too. The synthesis of chaetogen is supposed to be a probabilistic response of cells to positional cues, so that a cell located in one region of the disc is more likely to have its chaetogen gene turned on than a cell located in another region. Various probability distributions are shown to generate the various bristle patterns observed in the adult: precisely located bristles (e.g., thoracic macrochaetes), evenly spaced bristles (e.g., tergal microchaetes), and rows of bristles (e.g., thoracic microchaetes). In the particular case of the precisely located bristles, we show that (i) the distribution of chaetogen concentration in the tissue presents a unique maximum even when a large number of contiguous cells are all engaged in the synthesis of chaetogen; (ii) the position of the maximum is largely unaffected by statistical fluctuations in the decision of each cell to synthesize or not to synthesize the chaetogen; (iii) different maxima can be reproducibly generated even when the corresponding populations of chaetogen-producing cells overlap.     

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.     

A clonal analysis of the relationship of duplicated bristles inDrosophila

Summary Two possible mechanisms are considered for the occurrence of experimentally or genetically induced duplications of bristles: extra cell division of a bristle mother cell versus determination of more than one mother cell. From a clonal analysis it appears that duplications induced by actinomycin-D arise by the latter mechanism, whereas those found in the mutantspl seem to arise by the former mechanism.     

SECRETION AND DEPLOYMENT OF BRISTLES IN MALLOMONAS SPLENDENS (SYNUROPHYCEAE)1

Mallomonas splendens (G. S. West) Playfair has a cell covering of siliceous scales and bristles. Interphase cells bear four anterior and four posterior bristles that each articulate, at their flexed basal ends via a complex of labile fibers (the fibrillar complex), on a specialized body scale (a base-plate scale). Body scales, base-plate scales and bristles are formed independently of each other and at different times in silica deposition vesicles (SDVs) that are associated with one of the two chloroplasts. The fine structure of scale and bristle morphogenesis in M. splendens agrees with that previously described for Synura and Mallomonas. Four new posterior bristles are formed at late interphase with their basal ends towards the cell posterior. The fibrillar complex is formed in situ on the bristle in the SDV. Mature bristles are secreted one by one onto the surface of the protoplast, beneath the layer of body scales, where the basal ends of the bristles adhere to the plasma membrane via the fibrillar complex. The extrusion of posterior bristles and their deployment onto the cell surface was monitored with video. A fine cellular protuberance accompanies the bristles as they are extruded from beneath the scale layer with their basal ends leading. When distant from the cell, the basal ends of the bristles appear attached to the protuberance, possibly by way of their fibrillar complexes. Once bristles are fully extruded, and their tips free in the surrounding environment, the bristle bases are drawn back to the posterior apex of the cell, apparently by the now shortening protuberance. Thus a 180° reorientation of the posterior bristles has been effected outside the cell. Thin-sections of cells that are extruding bristles show a threadlike, cytoplasmic extension of the cell posterior which may be analogous to the protuberance seen in live cells. Four new posterior base-plate scales are secreted after the bristles have reoriented. Scanning electron microscopy indicates that the fibrillar complex is involved in positioning the bristles onto their respective base-plate scales. Anterior bristles are formed in new daughter cells in the same orientation as the posterior bristles; thus they are extruded tip first and no reorientation is required.     

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.     

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.     

Regional differences in the developing foreleg of Drosophila melanogaster

We transplanted imaginal disks of Drosophila melanogaster from larvae of the second half of the third larval instar into prepupae. Disks from the youngest donors differentiated bristles of only the distal segments of the leg. These disks also produced unusually large areas of cuticle that had no bristles. Disks from older donors differentiated bristles of more proximal segments and the area of cuticle with no bristles was reduced. To account for the regional variation in these results, there must be regional differences among the prospective leg cells at some time during the period from the second half of the third larval instar to the end of adult bristle differentiation. We asked whether prospective distal cells were more advanced than prospective proximal cells during bristle differentiation. We estimated when bristle precursor cells undergo their final cell divisions by heavily irradiating prepupae and pupae. We assumed that cells that were insensitive to the radiation had completed their cell divisions. The distal segments were the first to have insensitive bristles. Most leg bristles became insensitive between 12 and 18 hr after pupariation. The tarsus had a larger proportion of its bristles insensitive than the femur at 15 hr after pupariation. We also investigated when bristle-forming cells begin elongating their bristle shafts. We used the length of bristle rudiments as an indicator of when elongation is initiated. At 35 hr after pupariation, bristle rudiments of distal segments were two to three times longer than bristle rudiments of proximal segments. We discuss how these intersegmental differences observed during bristle differentiation can account for the regional variation in response of discs transplanted into older hosts. However, we do not exclude the possibility that regional differences among cells of the leg tissue exist at stages earlier than the time of bristle differentiation.     

Loss of notum macrochaetae as an interspecific hybrid anomaly between Drosophila melanogaster and D. simulans.

With the aim of revealing genetic variation accumulated among closely related species during the course of evolution, this study focuses on loss of macrochaetae on the notum as one of the developmental anomalies seen in interspecific hybrids between Drosophila melanogaster and its closely related species. Interspecific hybrids between a line of D. melanogaster and D. simulans isofemale lines exhibited a wide range in the number of missing bristles. By contrast, D. mauritiana and D. sechellia lines showed almost no reduction in bristle number in hybrids with D. melanogaster. Genetic analysis showed that the D. simulans X chromosome confers a large effect on hybrid bristle loss, although X-autosome interaction may be involved. This suggests that at least one genetic factor contributing to hybrid anomalies arose recently on a D. simulans X chromosome. Moreover, the results indicate sex dependency: the male hybrids were more susceptible to bristle loss than the female hybrids were. Use of cell type markers suggests that the defect does not lie in cell fate decisions during bristle development, but in the maintenance of neural fate and/or differentiation of the descendants of sensory mother cells.     

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.     

The action of mitomycin C on the bristle-forming apparatus ofPhormia regina

Summary Mitomycin C, a known inhibitor of DNA synthesis, was injected into white prepupae ofPhormia regina, Adults which developed from these prepupae showed alterations of the bristle pattern, loss of whole bristle organs, and the formation of bristles without sockets or sockets without bristle shafts. Dose-dependence was found for all modifications. For the abdominal microchaetae, the period of maximum sensitivity to the drug began at 16 h after puparium formation, that is well after all of the macrochaetae and most of the microchaetae of the thorax and the head had grown insensitive. Bristle forming trichogen and tormogen cells developed high degrees of polyteny with distinctly banded chromosomes. Photometric determination of the amount of Feulgen-DNA per nucleus led to estimations of DNA classes ranging from 256C to 2048 C. DNA contents of nuclei from Mitomycin C treated animals were significantly lower during the actual growth of the bristle apparatus, but reached approximately the same level as the controls prior to the time of emergence. Cytological investigations proved that doses of Mitomycin C which yielded bristle organs either without sockets or without shafts do not affect the differential division of the bristle mother cell. Polytene chromosomes damaged by Mitomycin C displayed a diffuse and irregular banding pattern. Possible modes of action of Mitomycin C on replicating polytene chromosomes are discussed.     

The genetic control of tissue polarity in Drosophila.

The cuticular surface of Drosophila is decorated by parallel arrays of polarized structures such as hairs and sensory bristles; for example, on the wing each cell produces a distally pointing hair. These patterns are termed 'tissue polarity'. Several genes are known whose activity is essential for the development of normal tissue polarity. Mutations in these genes alter the orientation of the hair or bristle with respect to neighboring cells and the body as a whole. The phenotypes of mutations in these genes allows them to be placed in three phenotypic groups. Based on their behavior in genetic mosaics, it has proved possible to determine that individual genes are required either for the generation of an intercellular polarity signal and/or the transduction of that signal to the cytoskeleton.     

F-actin bundles in Drosophila bristles are assembled from modules composed of short filaments

The actin bundles in Drosophila bristles run the length of the bristle cell and are accordingly 65 microns (microchaetes) or 400 microns (macrochaetes) in length, depending on the bristle type. Shortly after completion of bristle elongation in pupae, the actin bundles break down as the bristle surface becomes chitinized. The bundles break down in a bizarre way; it is as if each bundle is sawed transversely into pieces that average 3 microns in length. Disassembly of the actin filaments proceeds at the "sawed" surfaces. In all cases, the cuts in adjacent bundles appear in transverse register. From these images, we suspected that each actin bundle is made up of a series of shorter bundles or modules that are attached end-to-end. With fluorescent phalloidin staining and serial thin sections, we show that the modular design is present in nondegenerating bundles. Decoration of the actin filaments in adjacent bundles in the same bristle with subfragment 1 of myosin reveals that the actin filaments in every module have the same polarity. To study how modules form developmentally, we sectioned newly formed and elongating bristles. At the bristle tip are numerous tiny clusters of 6-10 filaments. These clusters become connected together more basally to form filament bundles that are poorly organized, initially, but with time become maximally cross-linked. Additional filaments are then added to the periphery of these organized bundle modules. All these observations make us aware of a new mechanism for the formation and elongation of actin filament bundles, one in which short bundles are assembled and attached end-to-end to other short bundles, as are the vertical girders between the floors of a skyscraper.     

Entwicklungsgeschichtliche Untersuchungen an zentrischen Diatomeen

The marine diatomChaetoceros didymum Ehrenberg has been studied with respect to cell morphology, fine structure of the frustule, and life history. Additional observations were made on otherChaetoceros species grown in culture. Co-existence of linear colonies with both intercellular spaces (foramina) and long stiff bristles is mediated by two morphogenetic peculiarities. After cell division the bristles grow out from the apices of the new valves and pass through preformed “bristle openings”, two of which occur in each girdle. They cross each other and fuse inseparably, thus bridging the foramina bilaterally. The irreversibility of cell junctions established in this manner results in two types of special “colony-separation-divisions”: (1) An equal, but heterovalvate division forming a pair of early dehiscing colony-end-valves; (2) a rare acytokinetic and unequal division. In resting spore formation a heterovalvate division gives rise to a pair of specialized spore-mother-cell hypothecae. Two consecutive acytokineses inside either mother cell follow, the first of which produces the spore epivalvae, the second the spore hypovalvae. In spore germination usually two consecutive acytokineses exchange the specialized valves of the spore by colony-end-valves, thus giving rise to a one-celled colony.C. didymum is monoecious, though most of the sexualized colonies are either completely male or contain vegetative and female cells. The protoplast of the spermatogonangium is usually divided into 8 naked spermatogonia by 3 steps of diploid mitoses, the first of which may or may not be followed by deposition of a pair of rudimentary valves. The spermatocytes undergo a swelling phase, which forces the parental frustule open; thereafter, 2 steps of meiotic nuclear and cell divisions follow; these form 4 uniflagellate and, normally, monoplastidic male gametes (spermia). InC. eibenii with non-dehiscent parental frustules, the spermia are released via bristle openings. The oogonia contain one functioning and two pycnotic nuclei as a result of meiosis. The spermia, however, may have entered the oogonium via bristle openings, possibly already at zygotene, though nuclear fusion is delayed until the egg has become mature. Thereafter, the zygote leaves the oogonial frustule through one of the bristle openings to which it still remains attached. In its isometric phase the zygote is surrounded by the fertilisation membrane. Its later anisometric growth is supported and probably directed by a system of silica rings, the preperizonium, which, in contrast to the otherwise comparable perizonium of pennate diatoms, forms a common layer with the surrounding scale-bearing fertilisation membrane. Finally, two consecutive acytokinetic (metagamic) mitoses induce the deposition of the two valves of the “Erstlingszelle” inside the auxospore envelope, which is ruptured during these last events and thus liberates the new enlarged cell.     

Differences in sensory projections between macro- and microchaetes in Drosophilid flies

From examination of the central axonal projections of sensory bristles on the notum of several species of Drosophilidae, we demonstrate different features that may indicate different functions for macro- and microchaetes. The large macrochaetes have conserved arborizations that correlate with their conserved position. Nevertheless, we find evidence for only two discrete projection patterns for bristles in the dorsocentral (DC) row, even when there may be four or five bristles present. We show that the small microchaetes of Drosophila melanogaster display regional specificity and subsets of contiguous bristles project to a common region in the thoracic ganglion. Interestingly, the axons of each of these subsets also form a specific fasciculation group on the scutum before joining the axon of a particular macrochaete. The positions of microchaetes on the scutum and the shape of the fasciculation groups vary between closely related species. There is no correlation between body size, bristle patterns, and fasciculation patterns. Furthermore, none of these traits correlate with the phylogenetic relationships between the species studied. We discuss the possibility that macro- and microchaetes may have different functions and that these have implications for evolutionary constraints on bristle patterns.     

The development and evolution of bristle patterns in Diptera

The spatial distribution of sensory bristles on the notum of different species of Diptera is compared. Species displaying ancestral features have a simple organization of randomly distributed, but uniformly spaced, bristles, whereas species thought to be more derived bear patterns in which the bristles are aligned into longitudinal rows. The number of rows of large bristles on the scutum was probably restricted to four early on in the evolution of cyclorraphous Brachyceran flies. Most species have stereotyped patterns based on modifications of these four rows. The possible constraints placed upon the patterning mechanisms due to growth and moulting within the Diptera are discussed, as well as within hemimetabolous insects. The holometabolic life cycle and the setting aside of groups of imaginal cells whose function is not required during the growth period, may have provided the freedom necessary for the evolution of elaborate bristle patterns. We briefly review the current state of knowledge concerning the complex genetic pathways regulating achaete-scute gene expression and bristle pattern in Drosophila melanogaster, and consider mechanisms for the genetic regulation of the bristle patterns of other species of Diptera.     

The role actin filaments play in providing the characteristic curved form of Drosophila bristles

Drosophila bristles display a precise orientation and curvature. An asymmetric extension of the socket cell overlies the newly emerging bristle rudiment to provide direction for bristle elongation, a process thought to be orchestrated by the nerve dendrite lying between these cells. Scanning electron microscopic analysis of individual bristles showed that curvature is planar and far greater near the bristle base. Correlated with this, as development proceeds the pupa gradually recedes from the inner pupal case (an extracellular layer that encloses the pupa) leading to less bristle curvature along the shaft. We propose that the inner pupal case induces elongating bristles to bend when they contact this barrier. During elongation the actin cytoskeleton locks in this curvature by grafting together the overlapping modules that comprise the long filament bundles. Because the bristle is curved, the actin bundles on the superior side must be longer than those on the inferior side. This is accomplished during grafting by greater elongation of superior side modules. Poor actin cross-bridging in mutant bristles results in altered curvature. Thus, the pattern of bristle curvature is a product of both extrinsic factors-the socket cell and the inner pupal case--and intrinsic factors--actin cytoskeleton assembly.     

Taxonomic significance of asymmetrical helmet and lance bristles in the genus Mallomonas (Synurophyceae) and their discovery in Eocene lake sediments

Complex bristle types formed by species in the genus Mallomonas include those with helmet or lance-shaped apices. The ornamentation on each side of the helmet has been thought to be equivalent or symmetrical, whereas on a lance-shaped bristle an expanded portion folds over one side of the shaft to form an asymmetrical structure. We describe, for the first time, helmet bristles with a distinctly asymmetrical design, also formed by the folding of a siliceous membrane over one side of the helmet. We postulate that the asymmetrical helmet represents a structure that combines the formation of a symmetrical helmet and a lance-shaped design on the same bristle. Further, we report structurally similar asymmetrical helmet bristles, lance-shaped bristles and scales that are unambiguously assigned to Mallomonas asmundiae in Middle Eocene sediments from a maar lake in northern Canada, supporting the hypothesis that scale and bristle morphology in the Synurophyceae has undergone extensive prolonged evolutionary stasis. Given differences in scale morphology and the presence of asymmetrical helmet bristles, we transfer the North American endemic Mallomonas acaroides var. muskokana to the rank of species. Further, we formally describe Mallomonas dispar and M. lancea, fossil species with asymmetrical helmet bristles and lance-shaped bristles, respectively. The taxonomic and biogeographic significance of asymmetrical and lance-bearing bristles is discussed.