Wall ultrastructure ofScenedesmus culture N 46

Bristles radiating from openings were detected on colonies and unicells ofScenedesmus culture N 46, when examined with transmission and scanning electron microscopes. Although narrower, they correspond in gross appearance and ultrastructure to previously describedScenedesmus bristles. Openings, bordered by a series of props, are unlike those ofScenedesmus culture 614. Additional props are observed scattered independently on the cell wall; ridges are composed of a linear row of props. Sections of cells, or cell walls, reveal an additional prop, situated inside the openings; these props are composed of several tubules. Possible extrusion of bristles through these tubules, as well as the origin of the bristle from the cavity and vesicles immediately under the opening are discussed.     

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.     

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.     

Is There a Gene Regulating the Scute Locus on the Third Chromosome of D. MELANOGASTER?

A section of the third chromosome of D. melanogaster some 25 to 40 centimorgans long including sr was transferred from a wild-type stock selected by Latter for high scutellar bristle number into a scute stock with a large number of scutellar bristles. This segment is shown to have a large effect on the bristle numbers of wild-type flies, to reduce the strength of canalization of the scute phenotype at 4 bristles, to have little, if any, effect on bristle numbers of scute flies with less than 4 bristles but to increase the number of flies with 5 and 6 scutellar bristles in scute stocks that normally have a large number of flies with 4 bristles. It is suggested that this segment in unselected chromosomes contains a gene that regulates bristle number by repressing the scute locus and that Latter has selected a mutant of the regulator which fails to repress the action of the scute locus.     

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 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.     

Flamingo controls the planar polarity of sensory bristles and asymmetric division of sensory organ precursors in Drosophila.

The sensory bristles of the fruit fly Drosophila are organized in a polarized fashion such that bristles on the thorax point posteriorly. These bristles are derived from asymmetric division of sensory organ precursors (SOPs). The Numb protein, which is localized asymmetrically in a cortical crescent in each SOP, segregates into only one of the two daughter cells during cell division, thereby conferring distinct fates to the daughter cells [1] [2]. In neuroblasts, establishment of apical-basal polarity by the protein Inscuteable is crucial for orienting asymmetric division, but this is not the case for division of SOPs [3]. Instead, the Frizzled (Fz) protein mediates a planar polarity signal that controls the anteroposteriorly oriented first division (pl) of SOPs [4]. Here, we report that Flamingo (Fmi), a seven-transmembrane cadherin [5], controls the planar polarity of sensory bristles and the orientation of the SOP pl division. Both the loss of function and overexpression of fmi disrupted bristle polarity. During mitosis of the SOP, the axis of the pl division and the positioning of the Numb crescent were randomized in the absence of Fmi activity. Overexpression of Fmi and Fz caused similar effects. The dependence of proper Fmi localization on Fz activity suggests that Fmi functions downstream of Fz in controlling planar polarity. We also present evidence suggesting that Fz also functions in the Wingless pathway to pattern sensory organs.     

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.     

SEXUAL REPRODUCTION AND MEIOSIS IN PERIDINIUM INCONSPICUUM LEMMERMANN (DINOPHYCEAE)

In Peridinium inconspicuum Lemmermann, sexual reproduction occurs in both nitrogen-enriched and nitrogen-deficient media. In this homothallic strain, protoplasmic fusion begins between two thecate gametes; but zygote formation is completed in a space outside the fusing pair. This diploid cell can form a plated theca which is shed as the cell enlarges. This spherical zygote then forms a new non-plated theca. The process of ecdysis and the formation of a new non-plated theca is repeated several times. During this process the zygote gradually elongates and by cytoplasmic infurrowing becomes peanut-shaped. Eventually two cells are formed. The first and second meiotic divisions are greatly separated in time. The first meiotic division occurs in the spherical non-thecate zygote. The second meiotic division can occur in the peanut-shaped zygote before it completes cytokinesis. This meiotic division may not be synchronous, occasionally resulting in a trinucleate stage. Eventually four flagellated, haploid products are produced.     

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.     

Restoration of wing margins in Lyra mutants of Drosophila melanogaster

Lyra is a dominant, homozygous lethal mutation of Drosophila melanogaster; in heterozygotes the wings lack portions of the anterior and posterior margins including the characteristic bristles. We have found that, in addition to the loss of bristle forming cells, there is a decrease in the number of wing surface cells that varies between 10 and 20%. However, we observed no histological evidence of excessive cell death in either the larval discs or the pupal wing precursors in Lyra flies. Restoration of all or part of the normal wing margins occurs in some, but not all, cases of morphogenetic mosaics, in which there were patches of wild-type cells in Lyra wing margins due to irradiation-induced mitotic recombination. Analysis of these restorations, using margin bristles as indicators, shows that the Lyra wild-type gene is not involved in bristle formation per se and further that its expression is not cell autonomous. Instead the effect of the Lyra mutation appears to be associated with development of a margin forming subpopulation of cells and to influence the characteristic pattern of cells and bristles in the wing margin via an inductive interaction. The dorsal-ventral boundary can be demonstrated in the de facto wing margins of Lyra mutants suggesting that its origin is independent of any function Lyra might have in normal wing margin morphogenesis. In wing margin restorations the dorsal-ventral boundary is clearly delimited by trichomes and somewhat less rigorously shown by the margin bristles. Further, in these restorations ventral clones induce dorsal bristles, as well as ventral ones, and vice versa, indicating that the influence of Lyra is not restricted by the dorsal-ventral boundary.     

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.     

A Plant Germline-Specific Integrator of Sperm Specification and Cell Cycle Progression

The unique double fertilisation mechanism in flowering plants depends upon a pair of functional sperm cells. During male gametogenesis, each haploid microspore undergoes an asymmetric division to produce a large, non-germline vegetative cell and a single germ cell that divides once to produce the sperm cell pair. Despite the importance of sperm cells in plant reproduction, relatively little is known about the molecular mechanisms controlling germ cell proliferation and specification. Here, we investigate the role of the Arabidopsis male germline-specific Myb protein DUO POLLEN1, DUO1, as a positive regulator of male germline development. We show that DUO1 is required for correct male germ cell differentiation including the expression of key genes required for fertilisation. DUO1 is also necessary for male germ cell division, and we show that DUO1 is required for the germline expression of the G2/M regulator AtCycB1;1 and that AtCycB1:1 can partially rescue defective germ cell division in duo1. We further show that the male germline-restricted expression of DUO1 depends upon positive promoter elements and not upon a proposed repressor binding site. Thus, DUO1 is a key regulator in the production of functional sperm cells in flowering plants that has a novel integrative role linking gametic cell specification and cell cycle progression.     

Bristle patterning in Drosophila.

The 5000 bristles that protrude from the cuticle of a Drosophila adult function as either mechanosensors or chemosensors, and they are arranged in surprisingly intricate patterns. Development of the patterns appears to involve five stages: (1) establishment of a coordinate system of 'positional information'; (2) partitioning of the epidermis into areas where bristles either can or cannot originate; (3) selection of one or more bristle mother cells within each permissible area; (4) suppression of bristle development in the neighborhood of each mother cell; and (5) differentiation of the mother cell to produce four or more descendant cells, each of which forms part of the bristle apparatus. Some of the genes that control these events participate in more than one stage, and others play key roles in seemingly unrelated developmental pathways, including embryonic neurogenesis, body segmentation, and sex determination.     

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.     

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.     

DISTRIBUTION AND INNERVATION OF FACIAL BRISTLES AND HAIRS IN THE FLORIDA MANATEE (TRICHECHUS MANATUS LATIROSTRIS)

The distribution and anatomy of sirenian perioral bristles (modified vibrissae) and facial hairs are of interest because of their use during feeding and tactile exploration. In the present study we have identified six fields of perioral bristles on the face of the Florida manatee (T. manatus latirostris), four (U1-U4) on each side of the upper lips and oral cavity, and two (L1-L2) on each side of the lower lip pad, inside the oral cavity and rostral to the horny mandibular pad. Each field has a characteristic location, number of bristles, and range of bristle length and diameter. There is a mean of 110 (± 19) bristles per side, with no left-right differences. Branches of the infraorbital nerve innervate the bases of the largest bristles (U2 group) on the upper bristle pad, and the inferior alveolar nerve supplies the bristles of the lower bristle pad. The dorsal and ventral buccal branches of the facial nerve innervate the superficial facial musculature, which is likely to be involved in bristle eversion and other movements which constitute feeding behavior. Hair is denser in the facial region than on the remainder of the body. Within the face, hair is denser on the oral disk than on the supradisk. The oral disk contains bristle-like hair, whereas the supradisk region possesses hair that is similar in length and diameter to that on the postcranial body. The mean total of bristles and hairs per face was 1,942. Means for the subregions were 220 (± 39) bristles on the perioral bristle pads, 601 (± 115) bristlelike hairs in the oral disk region, 710 (± 229) typical hairs in the supradisk region, and 411 (± 108) typical hairs on the chin. There were no significant differences between left and right side counts. Facial hair density was inversely correlated with facial area and body size. These data provide new information on the anatomical basis of the exceptional orofacial activities characteristic of manatees during feeding and tactile exploration.     

Dusky-like functions as a Rab11 effector for the deposition of cuticle during Drosophila bristle development

The morphogenesis of Drosophila sensory bristles is dependent on the function of their actin and microtubule cytoskeleton. Actin filaments are important for bristle shape and elongation, while microtubules are thought to mediate protein and membrane trafficking to promote growth. We have identified an essential role for the bristle cuticle in the maintenance of bristle structure and shape at late stages of bristle development. We show that the small GTPase Rab11 mediates the organized deposition of chitin, a major cuticle component in bristles, and disrupting Rab11 function leads to phenotypes that result from bristle collapse rather than a failure to elongate. We further establish that Rab11 is required for the plasma membrane localization of the ZP domain-containing Dusky-like (Dyl) protein and that Dyl is also required for cuticle formation in bristles. Our data argue that Dyl functions as a Rab11 effector for mediating the attachment of the bristle cell membrane to chitin to establish a stable cuticle. Our studies also implicate the exocyst as a Rab11 effector in this process and that Rab11 trafficking along the bristle shaft is mediated by microtubules.     

On auxospore formation inCaloneis and the nature ofAmphiraphia (Bacillariophyta)

Amphiraphia Chen & Zhu, together with theAmphiraphiaceae andAmphiraphidales, should be abandoned, sinceAmphiraphia cells are the heterovalvar initial cells ofCaloneis. WhenCaloneis reproduces sexually two cells pair and become surrounded by a two-layered capsule of mucilage. Each cell produces two gametes and these become rearranged within the gametangial frustule before plasmogamy. The gametes are amoeboid and fusion is isogamous. Following plasmogamy the zygote contracts, becomes ellipsoidal, and lays down a primary perizonial band. This is a complete, wide hoop, while subsequent perizonial bands are narrow and open.