Ultrastructure of the chemosensitive basiconic single-walled wall-pore sensilla on the antennae in adults and embryonic stages of Locusta migratoria L. (Insecta,Orthoptera)

Summary The structure and embryonic development of the two types (A, B) of basiconic sensilla on the antennae of Locusta migratoria were studied in material that had been cryofixed and freeze-substituted, or chemically fixed and dehydrated. Both types are single-walled wall-pore sensilla. Type-A sensilla comprise 20–30 sensory and 7 enveloping cells. One enveloping cell (thecogen cell secretes the dendrite sheath); four are trichogen cells, projections of which form the trichogen process during the 2nd embryonic molt. The trichogen cells form two concentric pairs proximally. Two tormogen cells secrete the cuticular socket of the sensillum. The dendritic outer segments of the sensory cells are branched. Bifurcate type-A sensilla have also been observed. Type-B sensilla comprise three sensory and four enveloping cells (one thecogen, two trichogen and one tormogen). The trichogen process is formed by the two trichogen cells, each of which gives rise to two projections. The trichogen cells are concentrically arranged. The dendritic outer segments of the sensory cells are unbranched. In the fully developed sensillum, all trichogen and tormogen cells border on the outer receptor lymph cavity. It is suggested that the multicellular organization of the type-A sensilla can be regarded as being advanced rather than primitive.Supported by the Dcutschc Forschungsgemeinschaft (SFB 4/G1)     

The cercal sensilla of the blowfly Lucilia cuprina. II. Structure of the enveloping cells and the basal regions of the sensory dendrites

The gustatory, olfactory, touch and stress receptors on the cerci of Lucilia cuprina Wied. (Diptera: Calliphoridae) have either two or three enveloping cells. The gustatory and olfactory sensilla have three enveloping cells: a tormogen, trichogen and thecogen cell. The tormogen and trichogen cells contribute to a sub-cuticular sensillar lumen which divides into two lobes basally. The thecogen cell forms a lumen around the dendrites. Distally the dendrites lie in the contents of the thecogen lumen within the dendritic sheath. Proximally the dendrites embed in the thecogen cell which has an expanded, microlamellate lumen basally. The sensillar lumen of the mechanosensory (trichoid mechanoreceptors and campaniform) sensilla is formed by a single enveloping cell: the presumptive tormogen cell. In trichoid mechanoreceptors the thecogen lumen is restricted to the region of the transitional region of the dendrite whereas the thecogen lumen of campaniform sensilla extends proximally although it is not as well-developed as that of the chemoreceptive sensilla. The dendrites of all sensillum types on the cerci have a granular body in the transitional region: a situation which has not been previously reported in chemoreceptive sensilla although common in the mechanoreceptors of Calliphoridae and Sarcophagidae.     

Development of adult sensilla on the wing and notum of Drosophila melanogaster

We have investigated the temporal pattern of appearance, cell lineage, and cytodifferentiation of selected sensory organs (sensilla) of adult Drosophila. This analysis was facilitated by the discovery that the monoclonal antibody 22C10 labels not only the neuron of the developing sensillum organ, but the accessory cells as well. The precursors of the macrochaetes and the recurved (chemosensory) bristles of the wing margin divide around and shortly after puparium formation, while those of the microchaetes and the stout and slender (mechanosensory) bristles of the wing margin divide between 9 h and 18 h after puparium formation (apf). The onset of sensillum differentiation follows the terminal precursor division within a few hours. Four of the cells in an individual microchaete organ are clonally related: A single first-order precursor cell divides to produce two second-order precursors; one of these divides into the neuron and thecogen cell, the other into the trichogen cell and tormogen cell. Along the anterior wing margin, two rounds of division generate the cells of the mechanosensory sensilla; here, no strict clonal relationship seems to exist between the cells of an individual sensillum. At the time of sensillum precursor division, many other, non-sensillum-producing cells within the notum and wing proliferate as well. This mitotic activity follows a spatially non-random pattern.     

Ultrastructure and ontogeny of the hair sensilla on the funicle of Calliphora erythrocephala (Insecta,Diptera)

Summary Four envelope cells are responsible for the formation of the basiconical sensilla of Calliphora. They are the thecogen, trichogen, and tormogen cells, and envelope cell 4. In early stages of development the still subepithelial sensory cilia are completely enclosed by the innermost thecogen cell. The first formation movements are initiated by a growth thrust of the hair-forming cell into the exuvial space. The sensory cilia only begin to grow into the hair anlage when the hair-forming cell has almost reached its final length. As soon as growth is completed the trichogen cell, tormogen cell, and envelope cell 4 start to excrete cuticular material. The trichogen cell forms the perforated part of the hair shaft and the stimulus-conducting system consisting of the pore tubules. The tormogen cell is responsible for the excretion of the basal non-perforated hair shaft and sheath cell 4 forms the proximal part of the socket region. The thecogen cell only begin to produce dendritic sheath material when the sensory hair is almost complete.Approximately 7–8 days after pupation the tormogen cell degenerates, having, by this time, produced about two-thirds of the sensilla cuticle. The surrounding envelope cells incorporate cell fragments of the tormogen cell. The trichogen cell continues the secretion where the tormogen cell left off. When the secretion of cuticle is finished the sheath cells begin to withdraw towards the proximal direction and to form microvilli on the apical membrane. The resulting outer receptor lymph space is bordered by envelope cell 4 and the trichogen and thecogen cells. The tormogen cell is absent in the sensilla of the imago.Abbreviations DS dendritic sheath - E4 envelope cell 4 - Ex exuvial space - G glial cell - iD inner dendritic segment - iRL inner receptor lymph space - oRL outer receptor lymph space - oD outer dendritic segment - P pore - PT pore tubules - S sensory cell - T thecogen cell - TO tormogen cell - TR trichogen cell Part 1 of a dissertation accepted by the Faculty of Bio- and Geosciences, University of Karlsruhe     

马尾松毛虫雄蛾触角毛状感受器的细微结构

马尾松毛虫Dendrolimus punctagus(Walker)雄蛾有一对羽毛状触角。在触角鞭节的每对侧枝的内侧(迎风面)着生许多毛状感受器。每个毛状感受器由几丁质表皮毛及位于其下的三个感觉神经原和三个呈同心排列的辅助细胞-鞘原细胞、毛原细胞和膜原细胞构成。几丁质表皮毛上有许多孔。毛腔内充满感受器淋巴液。感觉神经原发出的树状突伸入毛腔,浸浴于感受器淋巴液内。这些结构特征表明它是一种司嗅觉的化学感受器。雄蛾终生不取食,推断它的嗅觉感受器主要用以感受雌蛾释放的性外激素,帮助寻找配偶。     

Ultrastructure des sensilles trochanteriens de Campodea sensillifera conde et mathieu (Diplura : Campodeidea)

The fine structure of the basiconica sensilla situated on the posterior part of trochanters in Campodea sensillifera (Diplura : Campodeidea) reveals that they are probably olfactory and mechano-sensitive setae. Each sensillum is composed of one sensory axis made of 3 dendrites ensheathed by 3 cells (thecogen, trichogen and tormogen); one outer segment ends by a tubular corper without connection with the cuticular layer. The setae are generally racket-shaped. The epicuticular layer of the expanded part is perforated by a lattice of numerous slits, which communicate with underlying canals. The ciliary structures and apex of the tormogen cell are eliminated just before ecdysis. The ciliary microtubules are present in the cavity of the new sensillum, but after ecdysis the microtubules persist only at the lower part of the peduncle. An ecdysial canal appears at the tip of the sensillum.     

Fine structure of the pheromone-sensitive sensilla on the antenna of the hawkmoth, Manduca sexta

The flagellar antenna of the male hawkmoth Manduca sexta carries about 42,000 pheromone-sensitive sensilla trichodea, which are arranged in 'baskets' on the single segments. Each sensillum consists of a cuticular hair up to 500 mum long and is innervated by two bipolar sensory neurons. Each neuron sends an unbranched dendrite into the hair shaft. The dendrite is subdivided by a short ciliary region into an inner and an outer segment. The inner segment is especially rich in smooth vesicles, which accumulate beneath the ciliary region where they seem to fuse with the dendritic membrane. The outer dendritic segment often shows conspicuous 'beads' along its length. Three auxiliary, or enveloping, cells belong to each adult sensillum. These are the thecogen, the trichogen, and the 'outer' cell. Most probably, the latter is not homologous with the 'traditional' tormogen cell from a genealogical point of view.     

Diffusion barriers in silkmoth sensory epithelia: application of lanthanum tracer to olfactory sensilla of Antheraea polyphemus and Bombyx mori

The distribution of diffusion barriers in silkmoth olfactory sensilla has been investigated with ionic lanthanum. The tracer was applied from the apical side of the sensory epithelium by first pinching off the hair tips and then dipping the antennal branches into the La(NO(3))(3) solution. The tracer neither passed the apical septate junctions between the dendrite and the thecogen cell nor those between thecogen, trichogen, and tormogen cells, nor the tight contact between the apical membrane of the tormogen cell and the cuticle. After perfusing the hemolymph space with La(NO(3))(3) solution, the tracer was found in the clefts between the thecogen, trichogen, tormogen, and epidermis cells, but not in those between the receptor cells and the thecogen cell, or between the axon and the glial envelope. Lanthanum neither entered the receptor-lymph space nor the subcuticular space. Therefore, (i) receptor-lymph space, subcuticular space, and hemolymph space are isolated from each other, and (ii) the cleft between thecogen and sensory cell is separated from the hemolymph as well as from the receptor-lymph spaces. Furthermore, the results indicate that pleated septate junctions form the diffusion barriers in silkmoth olfactory sensilla.     

Structure of isolated sensilla and sensory neurons in vitro: Observations on developing silkmoth antennae

Summary By combined enzymatic and mechanical treatment, it was possible to dissociate the sensory epithelium of developing antennae of male Antheraea polyphemus and A. pernyi silkmoths from the stage of separation of the antennal branches up to the early stages of cuticle deposition. Large numbers of entire developing trichoid sensilla were isolated. These are characterized by a large trichogen cell with a long apical, hair-forming process and a large nucleus. A cluster of 2–3 sensory neurons, enclosed by the thecogen cell, is situated in the basal region. The dendrites run past the nucleus of the trichogen cell into the apical process from which they protrude laterally. The nuclei of the tormogen and a 4^th enveloping cell can be distinguished near the base of the prospective hair. After further dissociation, only the neuron clusters remain, still enclosed by their thecogen cell and often attached to the antennal branch nerve via their axons. It is finally possible to disrupt the thecogen cells and the axons, leaving the sensory neurons with inner dendritic segments and axon stumps. The majority of these neurons can be expected to be olfactory.     

Immunocytochemical localization of pheromone-binding protein in moth antennae

Summary Odorant-binding proteins are supposed to play an important role in stimulus transport and/or inactivation in olfactory sense organs. In an attempt to precisely localize pheromone-binding protein in the antenna of moths, post-embedding immunocytochemistry was performed using an antiserum against purified pheromone-binding protein of Antheraea polyphemus. In immunoblots of antennal homogenates, the antiserum reacted exclusively with pheromone-binding protein of A. polyphemus, and cross-reacted with homologous proteins of Bombyx mori and Autographa gamma. On sections of antennae of male A. polyphemus and B. mori, exclusively the pheromone-sensitive sensilla trichodea are labelled; in A. gamma, label is restricted to a subpopulation of morphologically similar sensilla trichodea, which indicates that not all pheromone-sensitive sensilla contain the same type of pheromone-binding protein and accounts for a higher specificity of pheromone-binding protein than hitherto assumed. Within the sensilla trichodea, the extracellular sensillum lymph of the hair lumen and of the sensillum-lymph cavities is heavily labelled. Intracellular label is mainly found in the trichogen and tormogen cells: in endoplasmic reticulum, Golgi apparatus, and a variety of dense granules. Endocytotic pits and vesicles, multivesicular bodies and lysosome-like structures are also labelled and can be observed not only in these cells, but also in the thcogen cell and in the receptor cells. Cell membranes are not labelled except the border between thecogen cell and receptor cell and the autojunction of the thecogen cell. The intracellular distribution of label indicates that pheromone-binding protein is synthesized in the tormogen and trichogen cell along typical pathways of protein secretion, whereas its turnover and decomposition does not appear to be restricted to these cells but may also occur in the thecogen and receptor cells. The immunocytochemical findings are discussed with respect to current concepts of the function of pheromone-binding protein.     

Morphogenesis of the antenna of the male silkmoth. Antheraea polyphemus, III. Development of olfactory sensilla and the properties of hair-forming cells

During adult development of the male silkmoth Antheraea polyphemus, the anlagen of olfactory sensilla arise within the first 2 days post-apolysis in the antennal epidermis (stage 1-3). Approximately on the second day, the primary dendrites as well as the axons grow out from the sensory neurons (stage 4). The trichogen cells start to grow apical processes approximately on the third day, and these hair-forming 'sprouts' reach their definite length around the ninth day (stages 5-6). Then the secretion of cuticle begins, the cuticulin layer having formed on day 10 (stage 7a). The primary dendrites are shed, the inner dendritic segments as well as the thecogen cells retract from the prospective hair bases, and the inner tormogen cells degenerate around days 10/11 (stage 7b). The hair shafts of the basiconic sensilla are completed around days 12/13 (stage 7c), and those of the trichoid sensilla around days 14/15 (stage 7d). The trichogen sprouts retract from the hairs after having finished cuticle formation, and the outer dendritic segments grow out into the hairs: in the basiconic sensilla directly through, and in the trichoid sensilla alongside, the sprouts. The trichogen sprouts contain numerous parallel-running microtubules. Besides their cytoskeletal function, these are most probably involved in the transport of membrane vesicles. During the phase of cuticle deposition, large numbers of vesicles are transported anterogradely from the cell bodies into the sprouts, where they fuse with the apical cell membrane and release their electron-dense contents (most probably cuticle precursors) to the outside. As the cuticle grows in thickness, the surface area of the sprouts is reduced by endocytosis of coated vesicles. When finally the sprouts retract from the completed hairs, the number of endocytotic vesicles is further increased and numerous membrane cisterns seem to be transported retrogradely along the microtubules to the cell bodies. Here the membrane material will most probably be used again in the formation of the sensillum lymph cavities. Thus, the trichogen cells are characterized by an intensive membrane recycling. The sensillum lymph cavities develop between days 16-20 (stage 8), mainly via apical invaginations of the trichogen cells. The imago emerges on day 21.     

Cellular organization of the dome sensillum: A presumed chemoreceptor in Gammarus setosus (Gammaridea: Amphipoda)

A previously unknown type of sensillum with a thin cuticular dome and two pairs of pores is described in the amphipod Gammarus setosus. There is only one dome sensillum on each interantennal lobe of the head. The receptor is innervated by two sensory dendrites that bifurcate into two pairs of 9 + 0 cilia, concentrically enclosed by four auxiliary cells—two thecogen, one trichogen, and one tormogen and surrounded by a cluster of accessory cells. The ciliary regions are contained in small inner lymph cavities. The outer segments are sheathed by the apical extensions of the thecogen cells, are looped inside the outer lymph cavity, and come in close contact with lipid spheroids inside the dome. The basal bodies consist of microtubule doublets, which extend into the distal segments where they are interspersed with singlets. The nodal inner dendritic segments join the ventral suspension cord of the organ of Bellonci and enter its ganglion. The application of colloidal lanthanum resulted in intraciliary lanthanum deposits. The dome sensilla are presumed to be chemosensory because their cellular plan has similarities to that of some known olfactory and pheromone-sensitive sensilla in decapod crustaceans and insects. © 1994 Wiley-Liss, Inc.     

Morphogenesis of the antenna of the male silkmoth, Antheraea polyphemus. II. Differential mitoses of 'dark' precursor cells create the Anlagen of sensilla

The antenna of the male silkmoth Antheraea polyphemus develops from a one-layered, flattened epidermal sac during the pupal phase. Within the first day post-apolysis (developmental stages 1 and 2), this epithelium differentiates into 'sensillogenic' and 'nonsensillogenic' regions, while numerous slender 'dark cells' interpreted as the precursor cells of sensilla arise in the former. Approximately between the first and second day post-apolysis (developmental stage 3), the dark cells retract to the apical pole of the epidermis, assume a round shape, and undergo a series of differential mitoses with spindles usually oriented parallel to the epidermal surface. These mitoses finally yield the Anlagen of the olfactory sensilla trichodea, each consisting of mostly 6-7 dark cells arranged side by side. In most of the Anlagen, 3-4 of these cells are situated more basally, each giving off a slender apical process which together are arranged in a fascicle. These are the prospective 2-3 sensory neurons plus the thecogen cell, which most probably is a sister cell of the former. Three additional cells are arranged more apically and partly enclose the fascicle of presumed sensory and thecogen cell processes. These are interpreted as the trichogen plus 2 tormogen cells, one of the latter degenerating later during development. In the basal region of the sensillogenic epidermis, massive signs of cell degeneration have been found. At stage 3, the basal epidermal feet in the non-sensillogenic regions have assumed a more uniform orientation as compared with the preceding stages.     

Volume and surface of receptor and auxiliary cells in hygro-/thermoreceptive sensilla of moths (Bombyx mori,Antheraea pernyi,and A. polyphemus)

Summary The thermo/hygroreceptive sensilla styloconica of the silkmoths Bombyx mori, Antheraea pernyi, and A. polyphemus were reconstructed from serial sections of cryofixed and chemically fixed specimens. The volume and surface area of the different sensillar cells were calculated from the area and circumference of consecutive section profiles. In addition, data are provided on the length and diameter of the outer and inner dendritic segments of the receptor cells. The morphometric data obtained from the three species are highly consistent and significantly different from those of olfactory sensilla trichodea of the same species. In each sensillum two type-1 receptor cells (hygroreceptors) are associated with one type-2 cell with a lamellated outer dendritic segment, a comparatively thick inner dendritic segment, and a particularly large soma (thermoreceptor). In contrast to olfactory sensilla, the thecogen cell is the largest auxiliary cell forming an extensive apical labyrinth bordering the inner sensillum-lymph space, whereas an inconspicuous trichogen cell and a medium-sized tormogen cell border a comparatively small outer sensillum-lymph cavity. Moreover, both sensillum-lymph spaces are separated from each other not only by the dendrite sheath, but also by the trichogen cell. The results are discussed with regard to recent electrophysiological observations and current hypotheses on the function of sensilla.     

Pheromone receptors in Bombyx mori and Antheraea pernyi

Summary The cellular organization of freeze-substituted antennal sensilla trichodea, which contain the sex pheromone receptors, was studied in male silkmoths of two species (Bombyx mori, Bombycidae; Antheraea pernyi, Saturniidae). The cellular architecture of these sensilla is complex, but very similar in both species. A three-dimensional reconstruction of a sensillum trichodeum of B. mori is presented. Two receptor cells (in A. pernyi 1–3) and three auxiliary cells are present. Of the latter, only the thecogen cell forms a true sheath around the receptor cells. A unique thecogen-receptor cell junction extends over the entire area of contact. Septate junctions occur between all sensillar cells apically, and in the region of the axonal origin basally. Gap junctions are also found between all cells except the receptor cells. The trichogen and tormogen cells show many structural indications of secretory activity and are thought to secrete the receptor lymph. Their apical membrane bordering the receptor-lymph space is enlarged by microvilli and microlamellae, but only those of the trichogen cell show regularly arranged membrane particles (portasomes), indicating secretory specialization among the auxiliary cells. Epidermal cells are found as slender pillars between sensilla, but extend apically along the non-sensillar cuticle and basally along the basal lamina.     

Morphology and ontogeny of single-walled multiporous sensilla of hemimetabolous insects

Comparative morphological investigations were made to determine the common organization plan of single-walled multiporous sensilla. The development of multiporous chemoreceptive sensilla of Gryllus, Oncopeltus and Lepisma follows the same path. Each chemoreceptive sensillum is associated with four types of enveloping cell. During ontogeny, enveloping cell 1 secretes the dendritic sheath. Enveloping cell 4 builds the connection of the hair base with the antennal cuticle. In Gryllus and Oncopehus, enveloping cells 2 and 3 build the hair shaft, the wall pores and pore tubules in nearly equal parts. Enveloping cells 2 and 3 lie side by side in the hair process, in which enveloping cell 2 produces the inner part, enveloping cell 3 the outer part of the hair shaft. In Lepisma the predominant part of the hair shaft with the wall pores is formed by the doubled enveloping cells 3. Interpreting our findings and the literature data, a new proposal is given for the homology of the enveloping cells. In singlewalled chemoreceptors, enveloping cell 1 is considered as thecogen and enveloping cell 4 as tormogen cell. Enveloping cell 2 is interpreted as inner trichogcn cell and enveloping cell 3 as outer trichogen cell.     

Ultrastructure and receptor cell responses of the antennal grooved peg sensilla of Triatoma infestans (Hemiptera: Reduviidae)

Ultrastructural examination of grooved-peg (GP) sensilla on the antenna of fifth instar Triatoma infestans nymphs by scanning electron microscopy and transmission electron microscopy reveal that they are 8–18 μm long with a diameter of about 2–2.8 μm at the non-articulated base. Some pegs have a terminal pore. These double-walled wall-pore (dw-wp) sensilla have an outer cuticular wall with 13–18 longitudinal grooves at the distal part of the peg. Groove channels are present at the bottom of the grooves from which radial spoke channels lead into the inner sensillum-lymph cavity. A dendrite sheath connects the tip of the thecogen cell to the inner cuticular wall thus forming separated outer and inner sensillum-lymph cavities. Four or five bipolar receptor cells are ensheathed successively within the GP sensilla by the thecogen cell, trichogen and tormogen cells. The inner dendritic segments of each sensory cell give rise at the ciliary constriction to an unbranched outer dendritic segment which can reach the tip of the sensillum.Electrophysiological recordings from the GP sensilla indicate that they house NH₃, short-chain carboxylic acid and short-chain aliphatic amine receptor cells and can be divided into three functional sub-types (GP 1–3). All GP sensilla carry a receptor cell excited by aliphatic amines, such as isobutylamine, a compound associated with vertebrate odour. GP type 1 and 2 sensilla house, in addition, an NH₃-excited cell whereas the type 2 sensilla also contains a short-chain carboxylic acid receptor. No cell particularly sensitive to either NH₃ or carboxylic acids was found in the grooved-peg type 3 sensilla. GP types 1, 2 and 3 represent ca. 36, 10 and 43% of the GP sensilla, respectively, whereas the remaining 11% contain receptor cells that manifest normal spontaneous activity but do not respond to any of the afore mentioned stimuli.     

The coelocapitular sensillum,an antennal hygro- and thermoreceptive sensillum of the honey bee,Apis mellifera L.

Summary The sensillum coelocapitulum, a hygro- and thermoreceptive sensillum of the honey bee, Apis mellifera, was investigated by electron microscopy. The cuticular apparatus of the sensillum is a mushroomshaped protrusion, devoid of pores, set in a narrow cylindrical pit positioned centrally within a cuticular, shallow depression. There may be three or four receptor cells. Three receptor cells have unbranched sensory cilia, containing densely packed microtubules, which extend distally into the cuticular apparatus and completely fill its cavity. These connecting cilia are of the usual 9+0 type. The fourth receptor, if present, has a thin sensory cilium which terminates beneath the cuticular apparatus. Its connecting cilium has armed outer doublets. The outer cavity is formed by two enveloping cells and is completely sealed off. Lipid deposits are present within the cavity and the tormogen cell. The thecogen cell has scolopale rod-like structures around the inner cavity. Features common to the insect hygro- and thermoreceptive sensilla are discussed in comparison with those of other insects.     

Three odorant-binding proteins are co-expressed in sensilla trichodea of Drosophila melanogaster

Odorant-binding proteins (OBPs) are small soluble proteins present in the aqueous medium surrounding olfactory receptor neurones. In this study we examine the expression patterns of three Drosophila OBPs (LUSH=OBP76a, OS-E=OBP83b and OS-F=OBP83a), using post-embedding immunocytochemistry. All three OBPs are co-expressed in sensilla trichodea whereas sensilla intermedia show co-expression of OS-E and OS-F only, but not of LUSH. Thus, it is confirmed that an individual sensillum can contain more than one OBP, even if it comprises only a single receptor neurone, such as the subtype T-1. In s. trichodea of lush⁻ mutants, expression of OS-E and OS-F is not impaired. No other sensillum type on antenna or maxillary palp (e.g. sensilla basiconica, sensilla coeloconica) expresses LUSH, OS-E or OS-F. Within the s. trichodea the three OBPs show the same labelling pattern: the extracellular sensillum lymph in the hair lumen and the sensillum-lymph cavities are heavily labelled. Intracellularly, the three OBPs are co-localised in a variety of dense granules in all auxiliary cells, and also in the receptor neurones. Immunocytochemical data from antennal sections of flies where lush gene expression has been tagged with the reporter gene lacZ suggest that LUSH is synthesised only in the trichogen and the thecogen cells. Thus, LUSH OBP is produced and secreted by two auxiliary cells, whereas its turnover and decomposition does not appear to be restricted to these auxiliary cells but may also occur in the tormogen and receptor cells. The immunocytochemical results are discussed with respect to current concepts of the function of odorant-binding proteins.     

Prospero distinguishes sibling cell fate without asymmetric localization in the Drosophila adult external sense organ lineage

The adult external sense organ precursor (SOP) lineage is a model system for studying asymmetric cell division. Adult SOPs divide asymmetrically to produce IIa and IIb daughter cells; IIa generates the external socket (tormogen) and hair (trichogen) cells, while IIb generates the internal neuron and sheath (thecogen) cells. Here we investigate the expression and function of prospero in the adult SOP lineage. Although Prospero is asymmetrically localized in embryonic SOP lineage, this is not observed in the adult SOP lineage: Prospero is first detected in the IIb nucleus and, during IIb division, it is cytoplasmic and inherited by both neuron and sheath cells. Subsequently, Prospero is downregulated in the neuron but maintained in the sheath cell. Loss of prospero function leads to 'double bristle' sense organs (reflecting a IIb-to-IIa transformation) or 'single bristle' sense organs with abnormal neuronal differentiation (reflecting defective IIb development). Conversely, ectopic prospero expression results in duplicate neurons and sheath cells and a complete absence of hair/socket cells (reflecting a IIa-to-IIb transformation). We conclude that (1) despite the absence of asymmetric protein localization, prospero expression is restricted to the IIb cell but not its IIa sibling, (2) prospero promotes IIb cell fate and inhibits IIa cell fate, and (3) prospero is required for proper axon and dendrite morphology of the neuron derived from the IIb cell. Thus, prospero plays a fundamental role in establishing binary IIa/IIb sibling cell fates without being asymmetrically localized during SOP division. Finally, in contrast to previous studies, we find that the IIb cell divides prior to the IIa cell in the SOP lineage.