Posterior segmentation of the Drosophila embryo in the absence of a maternal posterior organizer gene

Maternal hunchback activity suppresses the genetic pathway for abdomen formation in the Drosophila embryo. The active component of the posterior group of maternal genes, nanos, acts as a specific repressor of hunchback in the posterior region. Absence of both repressors results in normal embryos, indicating that posterior segmentation may not directly require maternal determinants.     

Krüppel requirement for knirps enhancement reflects overlapping gap gene activities in the Drosophila embryo

Segmental pattern formation in Drosophila proceeds in a hierarchical manner whereby the embryo is stepwise divided into progressively finer regions until it reaches its final metameric form. Maternal genes initiate this process by imparting on the egg a distinct antero-posterior polarity and by directing from the two polar centres the activities of the zygotic genes. The anterior system is strictly dependent on the product of the maternal gene bicoid (bcd), without which all pattern elements in the anterior region of the embryo fail to develop. The posterior system seems to lack such a morphogen. Rather, the known posterior maternal determinants simply define the boundaries within which abdominal segmentation can occur, and the process that actively generates the abdominal body pattern may be entirely due to the interactions between the zygotic genes. The most likely candidates among the zygotic genes that could fulfil the role of initiating the posterior pattern-forming process are the gap genes, as they are the first segmentation genes to be expressed in the embryo. Here we describe the interactions between the gap genes Krüppel (Kr), knirps (kni) and tailless (tll). We show that kni expression is repressed by tll activity, whereas it is directly enhanced by Kr activity. Thus, Kr activity is present throughout the domain of kni expression and forms a long-range protein gradient, which in combination with kni activity is required for abdominal segmentation of the embryo.     

Dynamic control of positional information in the early Drosophila embryo

Morphogen gradients contribute to pattern formation by determining positional information in morphogenetic fields. Interpretation of positional information is thought to rely on direct, concentration-threshold-dependent mechanisms for establishing multiple differential domains of target gene expression. In Drosophila, maternal gradients establish the initial position of boundaries for zygotic gap gene expression, which in turn convey positional information to pair-rule and segment-polarity genes, the latter forming a segmental pre-pattern by the onset of gastrulation. Here we report, on the basis of quantitative gene expression data, substantial anterior shifts in the position of gap domains after their initial establishment. Using a data-driven mathematical modelling approach, we show that these shifts are based on a regulatory mechanism that relies on asymmetric gap-gap cross-repression and does not require the diffusion of gap proteins. Our analysis implies that the threshold-dependent interpretation of maternal morphogen concentration is not sufficient to determine shifting gap domain boundary positions, and suggests that establishing and interpreting positional information are not independent processes in the Drosophila blastoderm.     

Function of torso in determining the terminal anlagen of the Drosophila embryo

The formation of the unsegmented terminal regions of the Drosophila larva, acron and telson requires the function of at least five maternal genes (terminal genes class). In their absence, the telson and acron are not formed. One of them, torso (tor), has gain-of-function alleles which have an opposite phenotype to the lack-of-function (tor-) alleles: the segmented regions of the larval body, thorax and abdomen, are missing, whereas the acron is not affected and the telson is enlarged. In strong gain-of-function mutants, the pair-rule gene fushi tarazu (ftz) is not expressed, demonstrating the suppression of the segmentation process in an early stage of development. The tor gain-of-function effect is neutralized, and segmentation is restored in double mutants with the zygotic gene tailless (tll), which has a phenotype similar (but not identical) to that of tor-. This suggests that tor acts through tll, and that in the gain-of-function alleles of tor, the tll gene product is ectopically expressed at middle positions of the embryo, where it inhibits the expression of segmentation genes like ftz.     

Establishment of developmental precision and proportions in the early Drosophila embryo

During embryonic development, orderly patterns of gene expression eventually assign each cell in the embryo its particular fate. For the anteroposterior axis of the Drosophila embryo, the first step in this process depends on a spatial gradient of the maternal morphogen Bicoid (Bcd). Positional information of this gradient is transmitted to downstream gap genes, each occupying a well defined spatial domain. We determined the precision of the initial process by comparing expression domains in different embryos. Here we show that the Bcd gradient displays a high embryo-to-embryo variability, but that this noise in the positional information is strongly decreased ('filtered') at the level of hunchback (hb) gene expression. In contrast to the Bcd gradient, the hb expression pattern already includes the information about the scale of the embryo. We show that genes known to interact directly with Hb are not responsible for its spatial precision, but that the maternal gene staufen may be crucial.     

A gene complex controlling segmentation in Drosophila.

The bithorax gene complex in Drosophila contains a minimum of eight genes that seem to code for substances controlling levels of thoracic and abdominal development. The state of repression of at least four of these genes is controlled by cis-regulatory elements and a separate locus (Polycomb) seems to code for a repressor of the complex. The wild-type and mutant segmentation patterns are consistent with an antero-posterior gradient in repressor concentration along the embryo and a proximo-distal gradient along the chromosome in the affinities for repressor of each gene's cis-regulatory element.     

Neurotrophin-evoked rapid excitation through TrkB receptors.

Neurotrophins are a family of structurally related proteins that regulate the survival, differentiation and maintenance of function of different populations of peripheral and central neurons. They are also essential for modulating activity-dependent neuronal plasticity. Here we show that neurotrophins elicit action potentials in central neurons. Even at low concentrations, brain-derived neurotrophic factor (BDNF) excited neurons in the hippocampus, cortex and cerebellum. We found that BDNF and neurotrophin-4/5 depolarized neurons just as rapidly as the neurotransmitter glutamate, even at a more than thousand-fold lower concentration. Neurotrophin-3 produced much smaller responses, and nerve growth factor was ineffective. The neurotrophin-induced depolarization resulted from the activation of a sodium ion conductance which was reversibly blocked by K-252a, a protein kinase blocker which prefers tyrosine kinase Trk receptors. Our results demonstrate a very rapid excitatory action of neurotrophins, placing them among the most potent endogenous neuro-excitants in the mammalian central nervous system described so far.     

Determination of spatial domains of zygotic gene expression in the Drosophila embryo by the affinity of binding sites for the bicoid morphogen

The maternal gene bicoid is a key component of the system that determines the pattern of the anterior half of Drosophila embryos. The bicoid protein forms a concentration gradient in early embryos, and is known to bind DNA. Specific binding sites are now shown to confer expression in a region of the embryo that depends on their affinity for bicoid protein: sites of high affinity allow expression further down the bicoid protein gradient than sites of low affinity.     

Targeted misexpression of a Drosophila opsin gene leads to altered visual function

Drosophila mutants transformed with a chimaeric gene that expresses the ocellar visual pigment in the major class of photoreceptor cells of the retina were used to investigate the properties of this minor pigment. The photoreceptor cells in which this opsin was misexpressed showed new spectral characteristics and physiology.     

Clathrin is required for the function of the mitotic spindle

Clathrin has an established function in the generation of vesicles that transfer membrane and proteins around the cell. The formation of clathrin-coated vesicles occurs continuously in non-dividing cells, but is shut down during mitosis, when clathrin concentrates at the spindle apparatus. Here, we show that clathrin stabilizes fibres of the mitotic spindle to aid congression of chromosomes. Clathrin bound to the spindle directly by the amino-terminal domain of clathrin heavy chain. Depletion of clathrin heavy chain using RNA interference prolonged mitosis; kinetochore fibres were destabilized, leading to defective congression of chromosomes to the metaphase plate and persistent activation of the spindle checkpoint. Normal mitosis was rescued by clathrin triskelia but not the N-terminal domain of clathrin heavy chain, indicating that stabilization of kinetochore fibres was dependent on the unique structure of clathrin. The importance of clathrin for normal mitosis may be relevant to understanding human cancers that involve gene fusions of clathrin heavy chain.     

Mediation of meiotic and early mitotic chromosome segregation in Drosophila by a protein related to kinesin.

Contrary to the traditional view that microtubules pull chromosomes polewards during the anaphase stage of meiotic and mitotic cell divisions, new evidence suggests that the chromosome movements are driven by a motor located at the kinetochore. The process of chromosome segregation involves proper arrangement of kinetochores for spindle attachment, followed by spindle attachment and chromosome movement. Mechanisms in Drosophila for chromosome segregation in meiosis differ in males and females, implying the action of different gene products in the two sexes. A product encoded at the claret locus in Drosophila is required for normal chromosome segregation in meiosis in females and in early mitotic divisions of the embryo. Here we show that the predicted amino-acid sequence of this product is related to the heavy chain of kinesin. The conserved region corresponds to the kinesin motor domain and includes the ATP-binding site and a region that can bind microtubules. A second region contains a leucine repeat motif which may mediate protein-subunit interactions necessary for attachment of chromosomes to the spindle. The mutant phenotype of chromosome nondisjunction and loss, and its similarity to the kinesin ATP-binding domain, suggest that the product encoded at claret not only stabilizes chromosome attachments to the spindle, but may also be a motor that drives chromosome segregation in female meiosis.     

Connectivity of chemosensory neurons is controlled by the gene poxn in Drosophila.

The function of the nervous system depends on the formation of a net of appropriate connections, but little is known of the genetic program underlying this process. In Drosophila two genes that specify different types of sense organs have been identified: cut (ct), which specifies the formation of external sense organs as opposed to chordotonal organs, and pox-neuro (poxn), which specifies the formation of poly-innervated (chemosensory) organs as opposed to mono-innervated (mechanosensory) organs. Whether these genes are also involved in specifying the connectivity of the corresponding neurons is not known. The larval sense organs are unsuitable for analysis of the axonal pathway and connections and so we have investigated the effect of poxn on the adult. Here we show that overexpression of poxn induces the morphological transformation of mechanosensory into chemosensory bristles on the legs and that the neurons innervating the morphologically transformed bristles follow pathways and establish connections that are appropriate for chemosensory bristles.     

Specification of limb development in the Drosophila embryo by positional cues from segmentation genes

Limb development in Drosophila requires the activity of a proximo-distal pattern-forming system, in addition to the antero-posterior and dorso-ventral pattern-forming systems that subdivide the embryo. Several lines of genetic evidence indicate that the Distal-less gene plays an important part in specifying proximo-distal positional information. The Distal-less locus encodes a homoeodomain-containing protein, which suggests that Distal-less may exert its activity through differential regulation of subordinate genes. The spatially restricted pattern of Distal-less expression allows direct visualization of the limb primordia during early embryogenesis. Here I report that from their inception, the leg primordia span the parasegment boundary. The segment polarity gene wingless seems to have a key part in defining the positions at which leg primordia will develop along the antero-posterior axis of the embryo. This analysis allows a direct molecular visualization of the compartments that subdivide the limb primordia into discrete developmental domains.