The formation of sense organs in Drosophila: a logical approach

The genetic analysis of development has revealed the importance of small sets of interacting genes in most morphogenetic processes. The results of gene interactions have so far been examined intuitively. This approach is largely sufficient when one deals with simple interactions, a feedback circuit for example. As more components become involved, however, it is difficult to make sure that the intuitive approach gives a comprehensive view of the behaviour of the system. In this paper, we illustrate the use of a logical approach to describe the genetic circuit that underlies the singling out of sense organ precursor cells in Drosophila. We show how to apply logical modelling to a realistic problem, and how this approach allows an easy assessment of the dynamic properties of the system, i.e., of its possible evolutions and of its reactions to fluctuations and perturbations.     

cato encodes a basic helix-loop-helix transcription factor implicated in the correct differentiation of Drosophila sense organs

In Drosophila neurogenesis, proneural genes encode bHLH proteins that are required for neural precursor selection. But many vertebrate homologues are expressed later and are postulated to have multiple roles during neurogenesis. We have isolated a new Drosophila gene, cato, which encodes a protein with a bHLH domain that is closely related to that of the proneural protein Atonal. cato expression is restricted to the developing PNS, where it is expressed in between the stages of precursor selection and terminal differentiation (and therefore later than the proneural genes). We present evidence from loss-of-function and misexpression experiments that cato is involved in sensory neurone morphology. Moreover, in prospero mutants, in which axon and dendrite outgrowth is defective, cato is strongly derepressed in the developing CNS.     

The emergence of sense organs in the wing disc of Drosophila

We have examined the origin of a set of precisely located sense organs in the notum and wing of Drosophila, in transformant flies where lacZ is expressed in the progenitor cells of the sense organs (the sensory mother cells) and in their progeny. Here we describe the temporal pattern of appearance and divisions of the sensory mother cells that will form the eleven macrochaetes and the two trichoid sensilla of the notum, and five campaniform sensilla on the wing blade. The complete pattern of sensory mother cells develops in a strict sequence that extends over most of the third larval instar and the first 10 h after puparium formation. The delay between the onset of lacZ expression and the first differentiative division ranges from 30 h, in the case of the earliest mother cells, to 2 h for the latest mother cells. The first division shows a preferential orientation which is also specific for each sensory mother cell. Up to this stage, there is no marked difference between the three types of mechanosensory organs.     

The determination of sense organs in Drosophila: interaction of scute with daughterless

Summary Several genetic loci have been implicated in the formation of the peripheral nervous system during Drosophila embryogenesis. As a first step towards understanding the functional interrelationships between these genes, we have searched for dominant interactions between deficiencies for the achaete-scute complex (AS-C), daughterless (da) and six other regions necessary for peripheral neurogenesis in the embryo. We have found that adult flies doubly heterozygous for deletions of AS-C and of da, or of AS-C and a small region on the fourth chromosome, exhibit characteristic bristle defects, suggesting that these genes cooperate to form sense organs both in the embryo and in the adult.     

The determination of sense organs in Drosophila: a search for interacting genes.

The determination of sense organs in Drosophila requires the concerted action of a battery of genes, several of which have been identified. Previous experiments revealed that flies doubly heterozygous for mutations in two of these genes have a reduced number of sense organs, suggesting the existence of a direct interaction between the corresponding genes and/or their products. We have now used this observation to search for mutations in additional genes that would show similar interactions. We have detected 10 recessive mutations that show a dominant reduction in the number of bristles when simultaneously heterozygous for either Df(2)J27 or Df(4)M62f. Among these mutations, 3 are homozygous viable and show striking defects in their bristle patterns, confirming that the genes thus identified play a role in the patterning of sense organs. We conclude that the "gene dose titration" method (Botas et al., 1982) is an efficient method for identifying interacting genes involved in a common process, provided one can identify a well-defined phenotype to look at, and at least one mutation that alters the process. Our experience suggests that its efficiency should be substantially improved by the use of insertional mutagenesis.     

The determination of sense organs in Drosophila: effect of the neurogenic mutations in the embryo.

We have examined the early pattern of sensory mother cells in embryos mutant for six different neurogenic loci. Our results show that the neurogenic loci are required to restrict the number of competent cells that will become sensory mother cells, but are not involved in controlling the localization or the position-dependent specification of competent cells. We conclude that these loci are involved in setting up a system of mutual inhibition, which transforms graded differences within the proneural clusters into an all-or-none difference between one cell, which becomes the sense organ progenitor cell, and the other cells, which remain epidermal.     

The paired box gene pox neuro: a determinant of poly-innervated sense organs in Drosophila.

This study describes the structure and function of pox neuro (poxn), a gene previously isolated by virtue of a conserved domain, the paired box, which it shares with the segmentation genes paired and gooseberry. Its expression pattern has been analyzed, particularly during development of the PNS. We propose that poxn is a "neuroblast identity" gene acting in both the PNS and the CNS on the basis of the following evidence. Its expression is restricted to four neuronal precursors in each hemisegment: two neuronal stem cells (neuroblasts) in the CNS, and two sensory mother cells (SMCs) in the PNS. The SMCs that express poxn produce the poly-innervated external sense organs of the larva. In poxn- embryos, poly-innervated sense organs are transformed into mono-innervated. Conversely, ectopic expression of poxn in embryos transformed with a heat-inducible poxn gene can switch mono-innervated to poly-innervated sense organs. Expression of poxn in the wing disc is restricted to the SMCs of the poly-innervated sense organs, suggesting that poxn also determines the lineage of poly-innervated adult sense organs.     

Factors of parallelism in the sense organs

Factors of parallel development of the gravity receptor, traced back in phylogenesis of vertebrates and invertebrates, are presented, and associations of the receptor with the nervous system and eye are considered. The parallel development is due to the similar, hereditary fixed behaviour of animals in relation to such a definite physical factor as the gravitational field of the Earth.     

A pattern formation mechanism to control spatial organization in the embryo of Drosophila melanogaster

It is known that cells are already committed to a particular segment at the cellular blastoderm stage during embryogenesis of Drosophila melanogaster. Recently, several segmentation genes have been observed to be expressed in a sequence of banded spatial patterns in the syncytial blastoderm, prior to the formation of the cellular blastoderm. It is demonstrated in this paper that a two component reaction-diffusion (RD) system with net production functions which are antisymmetric with respect to the uniform steady-state values, is capable of producing a sequence of seven spatial patterns in the syncytial blastoderm. The sequence of patterns obtained exhibit a strong preference for banded or striped patterns. The first pattern is a simple anteroposterior gradient while the second is a gradient in the dorsoventral direction. The next five patterns are a sequence of banded patterns which exhibit frequency doubling, i.e. the number of bands in each pattern tend to be double the number in the previous pattern. The predicted pattern sequence is comparable to that observed in the expression of some segmentation genes. It is suggested that a pattern formation mechanism based on such an RD system may exist in the embryo where it produces a sequence of prepatterns to regulate the expression of various segmentation genes leading ultimately to a segmented embryo. There is sufficient spatial information in the sequence of banded prepatterns for the segments to be unique.     

Juvenile hormone inhibits differentiation of olfactory sense organs during postembryonic development of cockroaches

The density of olfactory sense organs on the antenna of the cockroach, Leucophaea maderae, is relatively constant throughout larval development (average 400 sensilla/mm²), but undergoes a substantial increase at the adult state (to about 620 sensilla/mm²). Experimental manipulations of juvenile hormone (JH) activity result in either supernumerary larval instars (induced by unilaterla antennectomy or addition of exogenous JH), or premature adulthood (induced by allatectomy). The density of antennal sensilla remains at the larval level during the extra instars, but increases to the adult level or surpasses it at the terminal ecdysis following the induction of extra instars. Adultoids resulting from allatectomized sixth instars also have the high density of antennal olfactory sensilla characteristic of the normal adult. These data suggest that an interplay of surface area effects and an inhibitory action of JH controls the pattern of postembryonic development of antennal olfactory sensilla. Limited behavioural observations of the insects resulting from these experiments are consistent with the hypothesis that sex attractant-specific olfactory receptors appear only at the adult stage. However, electrophysiological data will be needed to confirm or negate this hypothesis.     

Type specification and spatial pattern formation of floral organs: A dynamic development model

A mathematical model simulating spatial pattern formation (positioning) of floral organs is proposed. Computer experiment with this model demonstrated the following sequence of spatial pattern formation in a typical cruciferous flower: medial sepals, carpels, lateral sepals, long stamens, petals, and short stamens. The positioning was acropetal for the perianth organs and basipetal for the stamens and carpels. Organ type specification and positioning proceed non-simultaneously in different floral parts and organ type specification goes ahead of organ spatial pattern formation. Computer simulation of flower development in several mutants demonstrated that the AG and AP2 genes determine both organ type specification and formation of the zones for future organ development. The function of the AG gene is to determine the basipetal patterning zones for the development of the reproductive organs, while the AP2 gene maintains proliferative activity of the meristem establishing the acropetal patterning zone for the development of the perianth organs.     

Drosophila olfactory memory: single genes to complex neural circuits

A central goal of neuroscience is to understand how neural circuits encode memory and guide behaviour. Studying simple, genetically tractable organisms, such as Drosophila melanogaster, can illuminate principles of neural circuit organization and function. Early genetic dissection of D. melanogaster olfactory memory focused on individual genes and molecules. These molecular tags subsequently revealed key neural circuits for memory. Recent advances in genetic technology have allowed us to manipulate and observe activity in these circuits, and even individual neurons, in live animals. The studies have transformed D. melanogaster from a useful organism for gene discovery to an ideal model to understand neural circuit function in memory.     

Drosophila Chitinase 2 is expressed in chitin producing organs for cuticle formation

The architecture of the outer body wall cuticle is fundamental to protect arthropods against invading pathogens and numerous other harmful stresses. Such robust cuticles are formed by parallel running chitin microfibrils. Molting and also local wounding leads to dynamic assembly and disassembly of the chitin-matrix throughout development. However, the underlying molecular mechanisms that organize proper chitin-matrix formation are poorly known. Recently we identified a key region for cuticle thickening at the apical cell surface, the cuticle assembly zone, where Obstructor-A (Obst-A) coordinates the formation of the chitin-matrix. Obst-A binds chitin and the deacetylase Serpentine (Serp) in a core complex, which is required for chitin-matrix maturation and preservation. Here we present evidence that Chitinase 2 (Cht2) could be essential for this molecular machinery. We show that Cht2 is expressed in the chitin-matrix of epidermis, trachea, and the digestive system. There, Cht2 is enriched at the apical cell surface and the dense chitin-matrix. We further show that in Cht2 knockdown larvae the assembly zone is rudimentary, preventing normal cuticle formation and pore canal organization. As sequence similarities of Cht2 and the core complex proteins indicate evolutionarily conserved molecular mechanisms, our findings suggest that Cht2 is involved in chitin formation also in other insects.     

Gustatory sense organs in the food canal of aphids

A sensory structure in the anterior region of the food canal of two species of aphid has been examined by light and electron microscopy. The dorsal wall is innervated by a total of 60 neurones which terminate, in groups, at 14 porous papillae on the cuticle. Paired papillae have also been detected in the ventral wall of this region. The fine structure of individual neurones and their grouping around papillae indicates a chemosensory function. The examination of moulting aphids shows that the distal portions of dendrites are shed with the exuviae.     

动物的听觉感受器

对声音的感觉是听觉最广义的定义。在生物进化的过程中 ,生物对声波振动的感觉逐步形成了专一的听觉感受器官。动物界中只有昆虫和脊椎动物具有听觉功能。听觉在很多方面都起着重要的作用 ,例如逃避捕食者、寻觅配偶和相互交流等 ,对人类来说听觉是语言发展的关键。现存的感受器官大体上分为以下几种类型。1　感触毛 (听觉毛 )在直翅目昆虫尾须上及鳞翅目幼虫身体上 ,有很多特殊的刚毛———感触毛 ,刚毛基部有关节腹 ,下联感觉细胞及感觉神经元 ,这套装置除了感受机械刺激外 ,还能感受低频率的音波及气流所给予的压力。这类听觉感受器与感…     

Cupular sense organs in Ciona (Tunicata: Ascidiacea)

The cupular organs of the atrial (exhalent) siphon of Ciona have been studied with scanning and transmission microscopy and are shown to resemble those of the vertebrate acoustico-lateralis system in several respects. The sensory cells are ciliated, and their cilia are apparently non-motile, having a modified inner tubular array. These cells lie amongst supporting cells that probably secrete the cupula, which is composed of polysaccharide and proteins as is the test. Ciona is sensitive to near-field vibrations, even after the brain has been removed; the significance of this observation and of the arrangement of the cupular organs is discussed. It is concluded that the tunicates show a suitable morphological starting point for the vertebrate acoustico-lateralis system.     

Architecture of spatial circuits in the hippocampal region

The hippocampal region contains several principal neuron types, some of which show distinct spatial firing patterns. The region is also known for its diversity in neural circuits and many have attempted to causally relate network architecture within and between these unique circuits to functional outcome. Still, much is unknown about the mechanisms or network properties by which the functionally specific spatial firing profiles of neurons are generated, let alone how they are integrated into a coherently functioning meta-network. In this review, we explore the architecture of local networks and address how they may interact within the context of an overarching space circuit, aiming to provide directions for future successful explorations.