Drosophila headlines

Anterior neurectoderm is progressively induced during gastrulation: the role of the Xenopus homeobox gene orthodenticle

In order to study the regional specification of neural tissue we isolated Xotx2, a Xenopus homolog of the Drosophila orthodenticle gene. Xotx2 is initially expressed in Spemann's organizer and its expression is absent in the ectoderm of early gastrulae. As gastrulation proceeds, Xotx2 expression is induced in the overlying ectoderm and this domain of expression moves anteriorly in register with underlying anterior mesoderm throughout the remainder of gastrulation. The expression pattern of Xotx2 suggests that a wave of Xotx2 expression (marking anterior neurectoderm) travels through the ectoderm of the gastrula with the movement of underlying anterior (prechordal plate) mesoderm. This expression of Xotx2 is reminiscent of the Eyal-Giladi model for neural induction. According to this model, anterior neural-inducing signals emanating from underlying anterior mesoderm transiently induce anterior neural tissues after vertical contact with the overlying ectoderm. Further patterning is achieved when the ectoderm receives caudalizing signals as it comes in contact with more posterior mesoderm during subsequent gastrulation movements. Functional characterization of the Xotx2 protein has revealed its involvement in differentiation of the anterior-most tissue, the cement gland. Ectopic expression of Xotx2 in embryos induces extra cement glands in the skin as well as inducing a cement gland marker (XAG1) in isolated animal cap ectoderm. Microinjection of RNA encoding the organizer-specific homeo-domain protein goosecoid into the ventral marginal zone results in induction of the Xotx2 gene. This result, taken in combination with the indistinguishable expression patterns of Xotx2 and goosecoid in the anterior mesoderm suggests that Xotx2 is a target of goosecoid regulation.

Three maternal coordinate systems cooperate in the patterning of the Drosophila head

In contrast to the segmentation of the embryonic trunk region which has been extensively studied, relatively little is known about the development and segmentation of the Drosophila head. Proper development of the cephalic region requires the informational input of three of the four maternal coordinate systems. Head-specific gene expression is set up in response to a complex interaction between the maternally provided gene products and zygotically expressed genes. Several zygotic genes involved in head development have recently been characterized. A genetic analysis suggests that the segmentation of the head may use a mechanism different from the one acting in the trunk. The two genes of the sloppy paired locus (slp1 and slp2) are also expressed in the embryonic head. slp1 plays a predominant role in head formation while slp2 is largely dispensible. A detailed analysis of the slp head phenotype suggests that slp is important for the development of the mandibular segment as well as two adjacent pregnathal segments (antennal and ocular). Our analysis of regulatory interactions of slp with maternal and zygotic genes suggests that it behaves like a gap gene. Thus, phenotype and regulation of slp support the view that slp acts as a head-specific gap gene in addition to its function as a pair-rule and segment polarity gene in the trunk. We show that all three maternal systems active in the cephalic region are required for proper slp expression and that the different systems cooperate in the patterning of the head. The terminal and anterior patterning system appear to be closely linked. This cooperation is likely to involve a direct interaction between the bcd morphogen and the terminal system. Low levels of terminal system activity seem to potentiate bcd as an activator of slp, whereas high levels down-regulate bcd rendering it inactive. Our analysis suggests that dorsal, the morphogen of the dorsoventral system, and the head-specific gap gene empty spiracles act as repressor and corepressor in the regulation of slp. We discuss how positional information established independently along two axes can act in concert to control gene regulation in two dimensions.

Number, identity, and sequence of the Drosophila head segments as revealed by neural elements and their deletion patterns in mutants

The development of the insect head tagma involves massive rearrangements and secondary fusions of segment anlagen during embryogenesis. Due to the lack of reliable morphological markers, the number, identity, and sequence of the head segments, particularly in the pregnathal region, are still a matter of ongoing debates. We examined the complex array of internal structures of the embryonic Drosophila melanogaster head such as the sensory structures and nerves of the peripheral and stomatogastric nervous systems, and we used embryonic head mutations causing a lack of overlapping segment anlagen to unravel the segmental identity and the sequence of the neural elements. Our results provide evidence for seven distinct segments in the Drosophila head, each characterized by a specific set of sensory neurons, consistent with the proposal that insects, myriapods, and crustaceans share a monophyletic evolutionary tree from a common annelid-like ancestor.

Orthopedia, a novel homeobox-containing gene expressed in the developing CNS of both mouse and Drosophila

A novel homeobox-containing gene has been identified. Its name, Orthopedia (Otp), exemplifies the homology shared by both the orthodenticle and Antennapedia homeodomains. Otp is highly conserved in evolution. In mouse, Otp is expressed only in restricted domains of the developing forebrain, hindbrain, and spinal cord. In Drosophila, otp first appears at gastrulation in the ectodermal proctodeum and later in the hindgut, anal plate, and along the CNS. Here, we compare the Otp-, Distal-less homeobox 1-(DIx1-), Orthodenticle homolog 1-(Otx1-), Otx2-, and Empty spiracles homolog 2-expressing domains. Our results indicate that Otp is expressed along the CNS both in mouse and Drosophila; Otp could specify regional identities in the development of the forebrain and spinal cord; transcription of Otp and DIx1 takes place in alternating hypothalamic regions reminiscent of a segment-like pattern; and the structural and functional conservation could correspond to a conserved function maintained in evolution.

Vertebrate homeobox genes

In the former part of the review the principal available data about Hox genes, their molecular organisation and their expression in vertebrate embryos, with particular emphasis for mammals, are briefly summarized. In the latter part we analysed the expression of four mouse homeobox genes related to two Drosophila genes expressed in the developing head of the fly: Emx1 and Emx2, related to ems, and Otx1 and Otx2, related to otd.

The Drosophila l(2)35Ba/nocA gene encodes a putative Zn finger protein involved in the development of the embryonic brain and the adult ocellar structures

The Drosophila l(2)35Ba/nocA gene is involved in the development of the adult ocelli and the embryonic head. Mutations in this gene lead to at least two distinct phenotypes. Several larva lethal l(2)35Ba alleles cause both hypertrophy and mislocation of the embryonic supraesophageal ganglion (brain) to the dorsal surface of the embryo. A second class of mutant alleles (nocA) is homozygous viable, but the surviving adults either lack or have greatly reduced ocelli and associated bristles. The l(2)35Ba/nocA gene encodes an approximately 3.0-kb transcript doublet; all l(2)35Ba alleles which have been physically mapped delete or disrupt the transcribed region, whereas all of the viable nocA alleles are caused by gross chromosomal aberrations with breakpoints near the 3'-flanking region of the gene. Several nocA breakpoint alleles downregulate the level of l(2)35Ba/nocA transcripts in adults, and their defective ocellar phenotype also fails to be complemented by the lethal alleles, implying that l(2)35Ba and nocA are different phenotypic manifestations of mutations in the same gene. In the l(2)35Ba mutant embryos, cells from the procephalic lobe which normally migrate over and overlie the supraesophageal ganglion during head involution can become incorporated into the supraesophageal ganglion; many of these misplaced cells, which normally form the frontal sac, also adopt a neuronal fate. Sequence analysis of two full-length l(2)35Ba/nocA cDNAs with distinct polyadenylation sites shows that they encode the same deduced protein of 537 amino acids with a serine- and threonine-rich N-terminal region, two putative zinc finger motifs near the carboxyl terminus, and several alanine-rich domains. Consistent with the observed embryonic phenotype, l(2)35Ba/nocA shows a complex embryonic expression pattern which includes the procephalic lobe.

The Drosophila l(2)35Ba/nocA gene encodes a putative Zn finger protein involved in the development of the embryonic brain and the adult ocellar structures

The Drosophila l(2)35Ba/nocA gene is involved in the development of the adult ocelli and the embryonic head. Mutations in this gene lead to at least two distinct phenotypes. Several larva lethal l(2)35Ba alleles cause both hypertrophy and mislocation of the embryonic supraesophageal ganglion (brain) to the dorsal surface of the embryo. A second class of mutant alleles (nocA) is homozygous viable, but the surviving adults either lack or have greatly reduced ocelli and associated bristles. The l(2)35Ba/nocA gene encodes an approximately 3.0-kb transcript doublet; all l(2)35Ba alleles which have been physically mapped delete or disrupt the transcribed region, whereas all of the viable nocA alleles are caused by gross chromosomal aberrations with breakpoints near the 3'-flanking region of the gene. Several nocA breakpoint alleles downregulate the level of l(2)35Ba/nocA transcripts in adults, and their defective ocellar phenotype also fails to be complemented by the lethal alleles, implying that l(2)35Ba and nocA are different phenotypic manifestations of mutations in the same gene. In the l(2)35Ba mutant embryos, cells from the procephalic lobe which normally migrate over and overlie the supraesophageal ganglion during head involution can become incorporated into the supraesophageal ganglion; many of these misplaced cells, which normally form the frontal sac, also adopt a neuronal fate. Sequence analysis of two full-length l(2)35Ba/nocA cDNAs with distinct polyadenylation sites shows that they encode the same deduced protein of 537 amino acids with a serine- and threonine-rich N-terminal region, two putative zinc finger motifs near the carboxyl terminus, and several alanine-rich domains. Consistent with the observed embryonic phenotype, l(2)35Ba/nocA shows a complex embryonic expression pattern which includes the procephalic lobe.

A Drosophila homologue of human Sp1 is a head-specific segmentation gene

Segmentation in Drosophila is based on a cascade of hierarchical gene interactions initiated by maternally deposited morphogens that define the spatially restricted domains of gap gene expression at blastoderm (reviewed in ref. 1). Although segmentation of the embryonic head is morphologically obscured, the repeated patterns of expression of the segment polarity genes reflect the formation of seven head segments; two of these depend on the segmentation and homeotic genes used in the trunk, whereas the others form as a result of the activity of the head-specific genes orthodenticle (otd), empty spiracles (ems) and buttonhead (btd). The genes ems and otd encode homeodomain proteins, suggesting that they may function as transcription factors. They are expressed in overlapping stripes in the early embryonic head of Drosophila, and their vertebrate homologues, otx and emx, are expressed in overlapping domains in the anterior central nervous system of the mouse embryo. We show here that btd is expressed in a stripe covering the head analgen of the segments affected in btd lack-of-function mutants and that btd encodes a zinc-finger-type transcription factor with sequence and functional similarity to the prototype mammalian transcription factor Sp1 (ref. 9). When expressed in the spatial pattern of btd, a transgene providing Sp1 activity can support development of the mandibular segment in the head of btd mutant embryos. A ubiquitous transcription factor from humans can therefore replace an essential component of the genetic circuitry required to specify the development of a particular head segment in the fly.

Emx and Otx homeobox genes in the developing mouse brain

We have analyzed the expression of four mouse homeobox genes related to two Drosophila genes expressed in the developing head of the fly. Two of these genes, Emx1 and Emx2, are related to empty spiracles, and two genes, termed Otx1 and Otx2, are related to orthodenticle. These genes are all expressed in the developing rostral brain of E10 mouse embryos and their expression domains can be compared. Otx2 is expressed in all dorsal and most ventral regions of telencephalon, diencephalon, and mesencephalon. The Otx1 expression domain is similar to that of Otx2, but smaller and contained within it. The Emx2 expression domain is comprised of dorsal telencephalon and small diencephalic regions, both dorsally and ventrally. Finally, Emx1 expression is exclusively confined to the dorsal telencephalon. At the time when regional specification of major brain regions takes place, the expression domains of the four genes appear to be continuous regions contained within each other in the sequence Emx1 < Emx2 < Otx1 < Otx2. The first appearance of transcripts of the four genes is also sequential: Otx2 is expressed first (E5.5), followed by Otx1 and Emx2 (E8-8.5), and finally by Emx1 (E9.5). It is tempting to speculate about a possible role of the four genes in establishing and/or signalling the limits of the various embryonic brain regions in a discrete progressive process with its center in the dorsal telencephalon.

Members of the Dlx- and Nkx2-gene families are regionally expressed in the developing forebrain

Members of the vertebrate Dlx- and Nkx2-homeobox-containing gene families exhibit closely related, complementary areas of gene expression in the developing forebrain. The expression domains and onset of gene transcription indicate that these genes may play a role in forebrain patterning, particularly in the diencephalon. In some cases, gene expression borders coincide with morphological boundaries separating functional and anatomical regions of the forebrain suggesting that the rostral region of the neural tube may indeed arise frm a segmented structure.

Transcriptional regulation and spatial patterning in Drosophila

Pattern formation in Drosophila is initiated by a small set of asymmetrically distributed maternal transcription factors that act as graded morphogens along the anterior-posterior and the dorsal-ventral axes of the embryo. Recent progress in the field provides first insight into the molecular mechanisms by which long-range positional information in the egg causes a series of localized zygotic transcription factors to position the developmental fate along the blastoderm.

Segmentation of the Drosophila embryo

Segmentation in Drosophila is a sequential process directed by at least 30 genes that encode various types of proteins, including: many transcription factors; a putative RNA-binding protein; a membrane-associated receptor kinase; several intracellular protein kinases; a number of secreted signaling molecules; and others of unknown function. Although the detailed molecular reactions used to generate the metameric subdivisions of the embryo are not yet understood, a general outline of the processes involved has been described. The manner in which spatial relations in the developing embryo are established can now be described.

Homeotic genes of Drosophila

Recently, there has been significant progress in advancing understanding of Drosophila homeotic function: including the different mechanisms of activation and maintenance of homeotic gene expression; the phenomenon of phenotypic suppression; and the search for genes downstream of the homeotic genes. Comparison between Drosophila and other species suggests a common functional organization of homeotic complexes in the animal kingdom.

Dynamic Hsp83 RNA localization during Drosophila oogenesis and embryogenesis

Hsp83 is the Drosophila homolog of the mammalian Hsp90 family of regulatory molecular chaperones. We show that maternally synthesized Hsp83 transcripts are localized to the posterior pole of the early Drosophila embryo by a novel mechanism involving a combination of generalized RNA degradation and local protection at the posterior. This protection of Hsp83 RNA occurs in wild-type embryos and embryos produced by females carrying the maternal effect mutations nanos and pumilio, which eliminate components of the posterior polar plasm without disrupting polar granule integrity. In contrast, Hsp83 RNA is not protected at the posterior pole of embryos produced by females carrying maternal mutations that disrupt the posterior polar plasm and the polar granules--cappuccino, oskar, spire, staufen, tudor, valois, and vasa. Mislocalization of oskar RNA to the anterior pole, which has been shown to result in induction of germ cells at the anterior, leads to anterior protection of maternal Hsp83 RNA. These results suggest that Hsp83 RNA is a component of the posterior polar plasm that might be associated with polar granules. In addition, we show that zygotic expression of Hsp83 commences in the anterior third of the embryo at the syncytial blastoderm stage and is regulated by the anterior morphogen, bicoid. We consider the possible developmental significance of this complex control of Hsp83 transcript distribution.

Organization of the commissural fibers in the adult brain of the locust

The brain (supraoesophageal ganglion) is the most complex of the segmental ganglia composing the nerve cord of the locusts Schistocerca gregaria and Locusta migratoria. In this paper, we describe the ground plan of the commissures crossing the midline of the brain and propose a nomenclature with the aim of making a complex neuropil more understandable at the level of individual neurons. For developmental and comparative reasons the neuroarchitecture of the brain is related to the neural axis, not to the body axis. We have identified 73 commissural fiber bundles belonging to the adult brain, and these are named according to their location (ventral, dorsal, anterior, posterior, medial) with respect to the central complex as reference point. Reconstructions of identified neurons from intracellular stainings, cobalt backfills, or immunohistochemical studies demonstrate the various configurations in which fibers cross the brain.

Transcriptional cascades in Drosophila

Genetics and molecular analyses have combined to yield insights into a functional cascade of transcription factors necessary to establish the molecular blueprint of the Drosophila body pattern in response to positional information in the egg. Recent progress in this field raises exciting questions regarding the molecular mechanisms involved, and their conservation in biological pattern-forming processes.

Dynamic Hsp83 RNA localization during Drosophila oogenesis and embryogenesis

Hsp83 is the Drosophila homolog of the mammalian Hsp90 family of regulatory molecular chaperones. We show that maternally synthesized Hsp83 transcripts are localized to the posterior pole of the early Drosophila embryo by a novel mechanism involving a combination of generalized RNA degradation and local protection at the posterior. This protection of Hsp83 RNA occurs in wild-type embryos and embryos produced by females carrying the maternal effect mutations nanos and pumilio, which eliminate components of the posterior polar plasm without disrupting polar granule integrity. In contrast, Hsp83 RNA is not protected at the posterior pole of embryos produced by females carrying maternal mutations that disrupt the posterior polar plasm and the polar granules--cappuccino, oskar, spire, staufen, tudor, valois, and vasa. Mislocalization of oskar RNA to the anterior pole, which has been shown to result in induction of germ cells at the anterior, leads to anterior protection of maternal Hsp83 RNA. These results suggest that Hsp83 RNA is a component of the posterior polar plasm that might be associated with polar granules. In addition, we show that zygotic expression of Hsp83 commences in the anterior third of the embryo at the syncytial blastoderm stage and is regulated by the anterior morphogen, bicoid. We consider the possible developmental significance of this complex control of Hsp83 transcript distribution.

Drosophila hairy pair-rule gene regulates embryonic patterning outside its apparent stripe domains

The hairy (h) segmentation gene of Drosophila regulates segmental patterning of the early embryo, and is expressed in a set of anteroposterior stripes during the blastoderm stage. We have used a set of h gene deletions to study the h promoter and the developmental requirements for individual h stripes. The results confirm upstream regulation of h striping but indicate that expression in the anterodorsal head domain depends on sequences downstream of the two transcription initiation sites. Surprisingly, the two anterior-most h domains appear to be dispensable for head development and embryonic viability. One partial promoter deletion expresses ectopic h, leading to misexpression of other segmentation genes and embryonic pattern defects. We demonstrate that h affects patterning outside its apparent stripe domains, supporting a model in which primary pair-rule genes act as concentration-dependent transcriptional regulators, i.e. as local morphogens.

Secretion and localized transcription suggest a role in positional signaling for products of the segmentation gene hedgehog

The segment polarity genes engrailed and wingless are expressed in neighboring stripes of cells on opposite sides of the Drosophila parasegment boundary. Each gene is mutually required for maintenance of the other's expression; continued expression of both also requires several other segment polarity genes. We show here that one such gene, hedgehog, encodes a protein targeted to the secretory pathway and is expressed coincidently with engrailed in embryos and in imaginal discs; maintenance of the hedgehog expression pattern is itself dependent upon other segment polarity genes including engrailed and wingless. Expression of hedgehog thus functions in, and is sensitive to, positional signaling. These properties are consistent with the non-cell autonomous requirement for hedgehog in cuticular patterning and in maintenance of wingless expression.

Expression of en and wg in the embryonic head and brain of Drosophila indicates a refolded band of seven segment remnants

Based on the expression pattern of the segment polarity genes engrailed and wingless during the embryonic development of the larval head, we found evidence that the head of Drosophila consists of remnants of seven segments (4 pregnathal and 3 gnathal) all of which contribute cells to neuromeres in the central nervous system. Until completion of germ band retraction, the four pregnathal segment remnants and their corresponding neuromeres become arranged in an S-shape. We discuss published evidence for seven head segments and morphogenetic movements during head formation in various insects (and crustaceans).

The role of the teashirt gene in trunk segmental identity in Drosophila

The phenotypes of different mutant combinations of teashirt (tsh) and homeotic genes together with their regulatory interactions are described in order to gain insight into tsh gene function. We show that when tsh, Scr, Antp and BX-C genes are missing, the ventral part of the trunk (or thorax and abdomen) is transformed to anterior head identity showing that tsh is a homeotic gene. These genes act synergistically to suppress the expression of the procephalic gene labial (lab) in subsets of cells in each segment of the trunk. Transcripts from the tsh gene always accumulate in segments destined to acquire trunk identities. tsh gene activity is required for the normal function of the Antp and BX-C genes, which modulate in part the expression of tsh. As a whole, our results suggest that tsh plays an essential dual role, during embryogenesis, for determining segmental identity of the trunk. First, tsh is required critically for the identity of the anterior prothorax. Second, tsh is required globally for segmental identity throughout the entire trunk whereas the “classical” homeotic genes have more specific roles. Our results are consistent with the idea that tsh is defining the ground state of the Drosophila trunk region seen in the absence of the Antp and BX-C genes.

Nested expression domains of four homeobox genes in developing rostral brain

Insight into the genetic control of the identity of specific regions along the body axis of vertebrates has resulted primarily from the study of vertebrate homologues of regulatory genes operating in the Drosophila trunk, but little is known about the development of most anterior regions of the body either in flies or vertebrates. Three Drosophila genes have been identified that are important in controlling the development of the head, two of which, empty spiracles and orthodenticle, have been cloned and shown to contain a homeobox. We previously cloned and characterized Emx1 and Emx2, two mouse genes related to empty spiracles that are expressed in restricted regions of the developing forebrain, including the presumptive cerebral cortex and olfactory bulbs. Here we report the identification of Otx1 and Otx2, which are related to orthodenticle. We have compared the expression domains of the four genes in the developing rostral brain of mouse embryos at a developmental stage, day 10 post coitum, when they are all expressed. Otx2 is expressed in every dorsal and most ventral regions of telencephalon, diencephalon and mesencephalon. The Otx1 expression domain is similar to that of Otx2, but contained within it. The Emx2 expression domain is comprised of dorsal telencephalon and small diencephalic regions, both dorsally and ventrally. Finally, Emx1 expression is exclusively confined to the dorsal telencephalon. Thus at the time when regional specification of major brain regions takes place, the expression domains of the four genes seem to be continuous regions contained within each other in the sequence Emx1 less than Emx2 less than Otx1 less than Otx2.

Drosophila retinal degeneration C (rdgC) encodes a novel serine/threonine protein phosphatase

The Drosophila retinal degeneration C (rdgC) gene is required to prevent light-induced retinal degeneration. Molecular analysis shows that the rdgC transcription unit encodes a novel serine/threonine protein phosphatase. Amino acids 153-393 define a domain that has 30% identity with the catalytic domains of types 1, 2A, and 2B serine/threonine protein phosphatases. A putative regulatory domain is appended that contains multiple potential Ca(2+)-binding sites or "EF hand motifs." Thus, the analysis suggests that the rdgC protein is a novel type of serine/threonine protein phosphatase that is directly regulated by Ca2+. rdgC is expressed in the visual systems of the fly, as well as in the mushroom bodies of the central brain.

Regional expression of the homeobox gene Nkx-2.2 in the developing mammalian forebrain

A novel mouse homeobox-containing gene, Nkx-2.2, has been isolated. Nkx-2.2 is a member of a family of genes whose homeodomains are homologous to that of the Drosophila NK-2 gene. Nkx-2.2 transcripts are found in localized domains of the brain during mouse embryogenesis. Nkx-2.2 expression in the brain abuts and partially overlaps with the expression domains of two other related homeobox-containing genes, TTF-1 and Dlx. The expression domains of the three genes in the developing prosencephalon coincide with anatomical boundaries, particularly apparent in the diencephalon. This result raises the possibility that these genes may specify regional differentiation of the developing diencephalon into its anatomically and functionally defined subregions. Nkx-2.2 may be involved in specifying diencephalic neuromeric boundaries.