Drosophila segmentation: after the first three hours

Gene expression boundary scaling and organ size regulation in the Drosophila embryo

How the shape and size of tissues and organs is regulated during development is a major question in developmental biology. Such regulation relies upon both intrinsic cues (such as signaling networks) and extrinsic inputs (such as from neighboring tissues). Here, we focus on pattern formation and organ development during Drosophila embryogenesis. In particular, we outline the importance of both biochemical and mechanical tissue-tissue interactions in size regulation. We describe how the Drosophila embryo can potentially provide novel insights into how shape and size are regulated during development. We focus on gene expression boundary scaling in the early embryo and how size is regulated in three organs (hindgut, trachea, and ventral nerve cord) later in development, with particular focus on the role of tissue-tissue interactions. Overall, we demonstrate that Drosophila embryogenesis provides a suitable model system for studying spatial and temporal scaling and size control in vivo.

Design and constraints of the Drosophila segment polarity module: robust spatial patterning emerges from intertwined cell state switches

The Drosophila segment polarity genes constitute the last tier in the segmentation cascade; their job is to maintain the boundaries between parasegments and provide positional "read-outs" within each parasegment for the entire developmental history of the animal. These genes constitute a relatively well-defined network with a relatively well-understood patterning task. In a previous publication (von Dassow et al. 2000. Nature 406:188-192) we showed that a computer model predicts the segment polarity network to be a robust boundary-making device. Here we elaborate those findings. First, we explore the constraints among parameters that govern the network model. Second, we test architectural variants of the core network, and show that the network tolerates a wide variety of adjustments in design. Third, we evaluate several topologically identical models that incorporate more or less molecular detail, finding that more-complex models perform noticeably better than simplified ones. Fourth, we discuss two instances in which the failure of the network model to behave in a life-like fashion highlights mechanistic details that need further experimental investigation. We conclude with an explanation of how the segment polarity network can be understood as an interwoven conspiracy of simple dynamical elements, several bistable switches and a homeostat. The robustness with which the network as a whole maintains a spatial regime of stable cell state emerges from generic dynamical properties of these simple elements.

Homeotic transformation of legs to mouthparts by proboscipedia expression in Drosophila imaginal discs

The Drosophila homeotic gene proboscipedia (pb) specifies labial identify and directs formation of the adult distiproboscis from the labial imaginal discs. pb null alleles result in the homeotic transformation of the distiproboscis into prothoracic (T1) legs [Kaufman (1978) Genetics 90, 579-596; Pultz et al. (1988) Genes Dev. 2, 901-920]. Homology with other transcription factors, localization to the nucleus, and restricted embryonic and imaginal expression implicate the pb protein (PB) as a transcription factor. In order to examine the possible roles that PB may play in the specification of adult mouthparts, we have expressed PB in cells of wing, leg and eye-antennal imaginal discs and assayed for effects on the development of adult structures. We report here that the ectopic expression of PB in the imaginal discs under the control of the inducible GAL4 system [Brand and Perrimon (1993) Development 118, 401-415] alters the developmental program of adult legs into maxillary or labial palps. These homeotic transformations have an equal effect on all three sets of legs, indicating an activity that is not solely dependent upon the unique combinations of other homeotic genes present in each of the leg discs. Segment polarity genes required for establishing the AP compartment boundary were found to be undisturbed by ectopic PB. Furthermore, normal patterns of apoptosis are observed in animals expressing ectopic PB, indicating that PB does not alter or affect cell death. These results suggest that molecular events occurring downstream of the establishment of the compartment boundary are affected by ectopic PB expression in imaginal discs and point to a general role in 'palp' formation in addition to the specification of labial identity.

msh may play a conserved role in dorsoventral patterning of the neuroectoderm and mesoderm

Many of the mechanisms that govern the patterning of the Drosophila neuroectoderm and mesoderm are still unknown. Here we report the sequence, expression, and regulation of the homeobox gene msh, which is likely to play an important role in the early patterning events of these two tissue primordia. msh expression is first observed in late blastoderm embryos and occurs in longitudinal bands of cells that are fated to become lateral neuroectoderm. This expression is under the control of dorsoventral axis-determination genes and depends on dpp-mediated repression in the dorsal half of the embryo and on fib-(EGF-) mediated repression ventrally. The bands of msh expression define the cells that will form the lateral columns of proneural gene expression and give rise to the lateral row of SI neuroblasts. This suggests that msh may be one of the upstream regulators of the achaete-scute (AS-C) genes and may play a role that is analogous to that of the homeobox gene vnd/NK2 in the medial sector of the neuroectoderm. During neuroblast segregation, msh expression is maintained in a subset of neuroblasts, indicating that msh, like vnd/NK2, could function in both dorsoventral patterning of the neuroectoderm and neuroblast specification. The later phase of msh expression that occurs after the first wave of neuroblast segregation in defined ectodermal and mesodermal clusters of cells points to similar roles of msh in patterning and cell fate specification of the peripheral nervous system, dorsal musculature, and the fat body. A comparison of the expression patterns of the vertebrate homologs of msh, vnd/NK2, and AS-C genes reveals striking similarities in dorsoventral patterning of the Drosophila and vertebrate neuroectoderm and indicates that genetic circuitries in neural patterning are evolutionarily conserved.

decapentaplegic, a target gene of the wingless signalling pathway in the Drosophila midgut

dishevelled, shaggy/zeste-white 3 and armadillo are required for transmission of the wingless signal in the Drosophila epidermis. We show that these genes act in the same epistatic order in the embryonic midgut to transmit the wingless signal. In addition to mediating transcriptional stimulation of the homeotic genes Ultrabithorax and labial, they are also required for transcriptional repression of labial by high wingless levels. Efficient labial expression thus only occurs within a window of intermediate wingless pathway activity. Finally, the shaggy/zeste-white 3 mutants revealed that wingless signalling can stimulate decapentaplegic transcription in the absence of Ultrabithorax, identifying decapentaplegic as a target gene of wingless. As decapentaplegic itself is required for wingless expression in the midgut, this represents a positive feed-back loop between two cell groups signalling to each other to stimulate each other's signal production.

Determination events in the nervous system of the vertebrate embryo

Molecular and functional data suggest that the regionalization of the caudal portion of the vertebrate embryonic brain (hindbrain) is set up through a process of segmentation. In contrast, the more rostrally located met-mesencephalic domain (mid-hindbrain junction) appears to follow a mode of specification that relies on long-range inducing and organizing activities originating from the central region of the domain. Recent studies addressing this mode of anterior/posterior determination point to Wnt-1 as a key player in this process.

Axes, boundaries and coordinates: the ABCs of fly leg development

Recent studies of gene expression in the developing fruitfly leg support a model--Meinhardt's Boundary Model--which seems to contradict the prevailing paradigm for pattern formation in the imaginal discs of Drosophila--the Polar Coordinate Model. Reasoning from geometric first principles, this article examines the strengths and weaknesses of these hypotheses, plus some baffling phenomena that neither model can comfortably explain. The deeper question at issue is: how does the fly's genome encode the three-dimensional anatomy of the adult? Does it demarcate territories and boundaries (as in a geopolitical map) and then use those boundaries and their points of intersection as a scaffolding on which to erect the anatomy (the Boundary Model)? Or does it assign cellular fates within a relatively seamless coordinate system (the Polar Coordinate Model)? The existence of hybrid Cartesian-polar models shows that the alternatives may not be so clear-cut: a single organ might utilize different systems that are spatially superimposed or temporally sequential.

Expression of a novel Toll-like gene spans the parasegment boundary and contributes to hedgehog function in the adult eye of Drosophila

Many proteins involved in signal transduction and cell adhesion are characterized by the presence of an extracellular domain with repeated copies of a leucine-rich motif (LRR). Here we report the isolation and characterization of a novel gene, tlr (for Toll-like receptor), which encodes a protein containing multiple LRRs in its presumed extracellular domain, a single transmembrane segment and homology to the cytoplasmic domain of the interleukin 1 receptor in its presumed intracellular domain. The pattern of tlr expression at the extended germ band stage is characterized by 15 transverse stripes in the gnathal and trunk segments, with four patches of expression corresponding to head segments and an additional patch of expression in the presumptive hindgut. The segmentally repeated tlr stripes in the trunk overlap both the wingless and engrailed stripes and thus span the parasegment boundary. The tlr stripes require pair rule gene function for their establishment and later become dependent upon segment-polarity gene function for their maintenance. Segmental modulation of tlr expression later in the tracheal system is dependent upon the function of the homeotic genes of the bithorax complex. The tlr gene also is prominently expressed in the imaginal discs. In the eye disc, this expression occurs in two stripes at the anterior and posterior margins of the morphogenetic furrow; this expression is consistent with a genetic interaction between a tlr mutation and an eye-specific allele of hedgehog. All of these data combine to suggest a role for tlr in interactions between cells at critical boundaries during development.

Positional information and pattern formation in development

A widely used mechanism for pattern formation is based on positional information: cells acquire positional identities as in a coordinate system and then interpret this information according to their genetic constitution and developmental history. In Drosophila maternal factors establish the axes and set up a maternal system of positional information on which further patterning is built. There is a cascade of gene activity which leads both to the development of periodic structures, the segments, and to their acquiring a unique identity. This involves the binding of transcription factors to regulatory regions of genes to produce sharp thresholds. Many of the genes involved in these processes, particularly the Hox complex, are also involved in specifying the body axis and limbs of vertebrates. There are striking similarities in the mechanisms for specifying and recording positional identity in Drosophila and vertebrates.