Cell lineage: compartments and Capricious.

Until recently, little was known about the mechanisms that prevent cell migration across compartment boundaries in Drosophila. A new report suggests that the lineage restriction between the dorsal and ventral compartments of the developing wing relies in part on the transmembrane proteins, Capricious and Tartan.

Wnts, signaling and sulfates.

Questions remain about the signaling pathways that control pattern formation during development. Blair describes how sulfated glycosaminoglycans affect several developmentally important signaling pathways, including Wnt-Wingless, Fibroblast growth factor, Hedgehog, and Bone morphogenetic protein-4 signaling. A new secreted sulfatase, Qsulf1, regulates the sensitivity of vertebrate cells to Wnts, possibly by modifying the sulfation of glycosaminoglycans.

Notch signaling: Fringe really is a glycosyltransferase.

Fringe modifies the ligand-selectivity of Notch in ways that are crucial for a number of Notch's developmental functions. Recent results have confirmed the suspicion that Fringe is a glycosyltransferase that works in the Golgi complex by modifying Notch's glycosylation state.

Crossveinless 2 contains cysteine-rich domains and is required for high levels of BMP-like activity during the formation of the cross veins in Drosophila.

The BMP-like signaling mediated by the ligands Dpp and Gbb is required to reinforce the development of most veins in the Drosophila wing. However, the formation of the cross veins is especially sensitive to reductions in BMP-like signaling. We show here that the formation of the definitive cross veins occurs after the initial specification of the longitudinal veins in a process that requires localized BMP-like activity. Since Dpp and Gbb levels are not detectably higher in the early phases of cross vein development, other factors apparently account for this localized activity. Our evidence suggests that the product of the crossveinless 2 gene is a novel member of the BMP-like signaling pathway required to potentiate Gbb of Dpp signaling in the cross veins. crossveinless 2 is expressed at higher levels in the developing cross veins and is necessary for local BMP-like activity. The Crossveinless 2 protein contains a putative signal or transmembrane sequence, and a partial Von Willebrand Factor D domain similar to those known to regulate the formation of intramolecular and intermolecular bonds. It also contains five cysteine-rich domains, similar to the cysteine-rich domains found in Chordin, Short Gastrulation and Procollagen that are known to bind BMP-like ligands. These features strongly suggest that Crossveinless 2 acts extracelluarly or in the secretory pathway to directly potentiate Dpp or Gbb signaling.

Dorsoventral lineage restriction in wing imaginal discs requires Notch.

The formation of boundaries that prevent the intermixing of cells is an important developmental patterning mechanism. The compartmental lineage restrictions that appear in the developing imaginal discs of Drosophila are striking examples of such boundaries. However, little is known about the cellular mechanism underlying compartmental lineage restrictions. The dorsoventral (D/V) lineage restriction that arises late in the developing wing imaginal disc requires the dorsal expression of the transcription factor Apterous and it has been hypothesized that apterous (ap) maintains compartmentalization by directly regulating the expression of molecules that modify cell adhesion or affinity. However, ap expression also regulates signalling between dorsal and ventral compartments, resulting in high levels of Notch signalling at the D/V boundary. Here we show that the formation of Notch-dependent boundary cells is required for the D/V lineage restriction.

Eye development: Notch lends a handedness.

The arrangement of photoreceptors in the ommatidia of the Drosophila compound eye is polarized, having a handedness or chirality. Notch signalling helps determine this handedness, first by establishing a signalling center at the eye equator, and second by mediating a choice between two photoreceptor fates

Smoothened-mediated Hedgehog signalling is required for the maintenance of the anterior-posterior lineage restriction in the developing wing of Drosophila.

It is thought that the posterior expression of the 'selector' genes engrailed and invected control the subdivision of the growing wing imaginal disc of Drosophila into anterior and posterior lineage compartments. At present, the cellular mechanisms by which separate lineage compartments are maintained are not known. Most models have assumed that the presence or absence of selector gene expression autonomously drives the expression of compartment-specific adhesion or recognition molecules that inhibit intermixing between compartments. However, our present understanding of Hedgehog signalling from posterior to anterior cells raises some interesting alternative models based on a cell's response to signalling. We show here that anterior cells that lack smoothened, and thus the ability to receive the Hedgehog signal, no longer obey a lineage restriction in the normal position of the anterior-posterior boundary. Rather these clones extend into anatomically posterior territory, without any changes in engrailed/invected gene expression. We have also examined clones lacking both en and inv; these too show complex behaviors near the normal site of the compartment boundary, and do not always cross entirely into anatomically anterior territory. Our results suggest that compartmentalization is a complex process involving intercompartmental signalling; models based on changes in affinity or growth will be discussed.

The function and regulation of cut expression on the wing margin of Drosophila: Notch, Wingless and a dominant negative role for Delta and Serrate.

We have investigated the role of the Notch and Wingless signaling pathways in the maintenance of wing margin identity through the study of cut, a homeobox-containing transcription factor and a late-arising margin-specific marker. By late third instar, a tripartite domain of gene expression can be identified about the dorsoventral compartment boundary, which marks the presumptive wing margin. A central domain of cut- and wingless-expressing cells are flanked on the dorsal and ventral side by domains of cells expressing elevated levels of the Notch ligands Delta and Serrate. We show first that cut acts to maintain margin wingless expression, providing a potential explanation of the cut mutant phenotype. Next, we examined the regulation of cut expression. Our results indicate that Notch, but not Wingless signaling, is autonomously required for cut expression. Rather, Wingless is required indirectly for cut expression; our results suggest this requirement is due to the regulation by wingless of Delta and Serrate expression in cells flanking the cut and wingless expression domains. Finally, we show that Delta and Serrate play a dual role in the regulation of cut and wingless expression. Normal, high levels of Delta and Serrate can trigger cut and wingless expression in adjacent cells lacking Delta and Serrate. However, high levels of Delta and Serrate also act in a dominant negative fashion, since cells expressing such levels cannot themselves express cut or wingless. We propose that the boundary of Notch ligand along the normal margin plays a similar role as part of a dynamic feedback loop that maintains the tripartite pattern of margin gene expression.

Limb development: marginal fringe benefits.

Recent results show that in the developing Drosophila wing, the secreted pioneer protein Fringe regulates the sensitivity of the Notch signaling pathway to different ligands. This provides a likely mechanism by which Fringe-like molecules may control patterning in both Drosophila and vertebrates.

wingless refines its own expression domain on the Drosophila wing margin.

The imaginal discs of Drosophila, which give rise to the adult appendages, are patterned during a period of intense cell proliferation. The specification of differing regions occurs in some cases by subdividing the disc epithelium into lineage compartments. However, in most cases precise boundaries are formed between different cell types without early compartmentalization. One such boundary occurs between the wingless (wg)-expressing cells of the wing margin and the adjacent proneural cells, which give rise to margin sensory bristles. Here we show that this boundary arises in part by a mechanism of 'self-refinement', by which wingless protein (Wg) represses wg expression in adjacent cells. Cells unable to receive the Wg signal do not resolve the boundary between wg-expressing and proneural cells.

Notch regulates wingless expression and is not required for reception of the paracrine wingless signal during wing margin neurogenesis in Drosophila.

In the developing wing margin of Drosophila, wingless is normally expressed in a narrow stripe of cells adjacent to the proneural cells that form the sensory bristles of the margin. Previous work has shown that this wingless is required for the expression of the proneural achaete-scute complex genes and the subsequent formation of the sensory bristles along the margin; recently, it has been proposed that the proneural cells require the Notch protein to properly receive the wingless signal. We have used clonal analysis of a null allele of Notch to test this idea directly. We found that Notch was not required by prospective proneural margin cells for the expression of scute or the formation of sensory precursors, indicating Notch is not required for the reception of wingless signal. Loss of Notch from proneural cells produced cell-autonomous neurogenic phenotypes and precocious differentiation of sensory cells, as would be expected if Notch had a role in lateral inhibition within the proneural regions. However, loss of scute expression and of sensory precursors was observed if clones substantially included the normal region of wingless expression. These 'anti-proneural' phenotypes were associated with the loss of wingless expression; this loss may be partially or wholly responsible for the anti-proneural phenotype. Curiously, Notch- clones limited to the dorsal or ventral compartments could disrupt wingless expression and proneural development in the adjacent compartment. Analysis using the temperature-sensitive Notch allele indicated that the role of Notch in the regulation of wingless expression precedes the requirement for lateral inhibition in proneural cells. Furthermore, overexpression of wingless with a heat shock-wingless construct rescued the loss of sensory precursors associated with the early loss of Notch.

Compartments and appendage development in Drosophila.

The appendages of Drosophila develop from the imaginal discs. During the extensive growth of these discs cell lineages are for the most part unfixed, suggesting a strong role for cell-cell interactions in controlling the final pattern of differentiation. However, during early and middle stages of development, discs are subdivided by strict lineage restrictions into a small number of spatially distinct compartments. These compartments appear to be maintained by stably inheriting states of gene expression; the compartment-specific expression of two such 'selector'-like genes, engrailed and apterous, are critical for anterior-posterior and dorso-ventral compartmentalization, respectively. Recent work suggests that one purpose of compartmentalization is to establish regions of specialized cells near compartment boundaries via intercompartmental induction, using molecules like the hedgehog protein. Thus, compartments can act as organizing centers for patterning within compartments. Evidence for non-compartmental patterning mechanisms will also be discussed.

The role of apterous in the control of dorsoventral compartmentalization and PS integrin gene expression in the developing wing of Drosophila.

During the development of Drosophila appendages from imaginal discs lineage restrictions appear that prevent dividing cells from crossing between regionally distinct compartments. These compartments correspond not only to regions of cell lineage restrictions but also to regions of specific gene expression. When compartments were first discovered, it was proposed that their formation relied on compartment-specific 'selector' gene activity; engrailed is thought to play such a role for the early-arising anterior-posterior restriction. Recent results suggest that the dorsally expressed transcription factor encoded by apterous may control dorsoventral identity in the wing. In this study we use mosaic analysis to show that apterous maintains the late-arising dorsoventral lineage restriction in a manner that strongly supports the selector gene hypothesis: loss of apterous function from dorsal cells after the formation of the boundary causes them to cross into the ventral compartment. Moreover, we show that apterous plays a role controlling patterns of gene expression in the developing wing disc. The PS1 and PS2 integrins are normally expressed in primarily dorsal-specific and ventral-specific patterns, respectively. We show that ectopic expression of apterous induces ectopic ventral expression of PS1 integrin and alpha PS1 mRNA, while loss of apterous can induce the ectopic dorsal expression of PS2 integrin. Thus, apterous plays a selector-like role both in terms of the control of lineage restrictions and the regulation of downstream gene expression.

A role for the segment polarity gene shaggy-zeste white 3 in the specification of regional identity in the developing wing of Drosophila.

The Drosophila segment polarity gene known as shaggy or zeste white 3 encodes a ubiquitously expressed serine-threonine protein kinase which is critical for a number of important developmental processes. In the developing wing blade, clones of cells lacking the normal shaggy-zeste white 3 product form dense tufts of margin-like bristles and bristle precursors. In a previous study I hypothesized that this phenotype could be best explained as a transformation in the regional identity of wing blade cells to one resembling that found along the normal wing margin. A number of genes have recently been identified which are expressed exclusively or at higher levels along the normal wing margin; in this study I will show that two of these genes, vestigial and scalloped, are overexpressed at margin-like levels in shaggy-zeste white 3 clones. This phenotype does not depend upon the formation of ectopic bristle precursors and occurs in clones lacking both shaggy-zeste white 3 and the entire achaete-scute complex. As vestigial and scalloped are both involved in early patterning events prior to the stages of bristle specification, these results strongly suggest that shaggy-zeste white 3 is required for the normal specification or maintenance of regional identity in the developing wing blade. The margin-like transformation is, however, partial, since the expression of apterous (in pupal wings) and wingless and cut (at late third instar) was not reliably altered in shaggy-zeste white 3 clones. It has been suggested that shaggy-zeste white 3 is involved in a wingless signaling pathway in the embryo; a model is discussed in which shaggy-zeste white 3 acts downstream of localized apterous and wingless expression to specify or maintain margin identity in the wing.

Mechanisms of compartment formation: evidence that non-proliferating cells do not play a critical role in defining the D/V lineage restriction in the developing wing of Drosophila.

The dorsoventral (D/V) lineage boundary in the developing wing disc of Drosophila restricts growing cells to the prospective dorsal or ventral compartments of the wing blade. This restriction appears along the prospective margin of the wing some time during the middle to late stages of wing disc growth. It has been proposed that the restriction is established and maintained by the formation of a zone of non-proliferating cells that acts as a barrier between cells in the dorsal and ventral compartments (O'Brochta and Bryant, Nature 313, 138-141, 1985). In the adult, however, no group of barrier cells has been identified between the compartments. This study will show the following. (1) A group of cells does exist that lies between the dorsal and ventral rows of margin bristle precursors; these cells, which express cut in the late third instar wing disc, are thus in an ideal position to act as barrier cells. (2) This cut-expressing region is split into dorsal and ventral regions by the expression of the dorsal-specific gene apterous. (3) The D/V lineage restriction defined by marked dorsal and ventral clones lies in the middle of the cut-expressing region and is exactly congruent with the boundary of apterous expression. (4) No group of barrier cells is observed between dorsal and ventral clones. (5) Clones often run along the boundary for long distances, suggesting that they can grow along the D/V boundary without crossing it. These results thus do not support the existence of a groups of cells acting as a barrier between dorsal and ventral compartments. Nor do they support a critical role for division rates near the D/V boundary in establishing or maintaining the lineage restriction.

Shaggy (zeste-white 3) and the formation of supernumerary bristle precursors in the developing wing blade of Drosophila.

The development of supernumerary bristle precursors induced by the mutation shaggy (sgg; also known as zeste-white 3) was examined in the developing wing blade of imaginal and pupal Drosophila. sgg clones were induced by mitotic recombination; clones were marked using enhancer-trap flies which express beta-galactosidase ubiquitously in imaginal tissues, while bristle precursors were identified using sensillum and bristle-specific enhancer-trap lines. It was shown that the precursors of supernumerary sgg bristles in the wing blade mimicked the development of morphologically similar margin bristles, developing in a manner similar to that of anterior sensory bristles in anterior clones and posterior noninnervated bristles in posterior clones. Interestingly, supernumerary anterior sensory bristles appeared outside the normal regions of "proneural" gene activity as identified using anti-achaete. Moreover, sgg could induce the ectopic expression of achaete in anterior clones. Thus, in the anterior wing blade the sgg mutation leads to the formation of ectopic proneural regions.

The development of normal and ectopic sensilla in the wings of hairy and Hairy wing mutants of Drosophila.

We have utilized enhancer trap markers to follow the development of ectopic sensillar precursors in the wings of Drosophila induced by the mutations hairy and Hairy wing. Normal sensilla are still present in these mutations, and can be distinguished from ectopic sensilla based upon both the position and the timing of their development. This correlates well with the development of ectopic achaete expression in these mutations: such staining is detected only after the appearance of normal staining. Thus, neither mutation appears to alter the specification of proneural clusters in the wing, as identified with anti-achaete, or the specification of sensillar precursors within these clusters. Rather, both act to induce the formation of a temporally and spatially distinct phase of sensillar development.

Engrailed expression in the anterior lineage compartment of the developing wing blade of Drosophila.

The developing wing of Drosophila melanogaster was examined at larval and pupal stages of development to determine whether the anterior-posterior lineage boundary, as identified by lineage restrictions, was congruent with the boundaries defined by the expression of posterior-specific (engrailed, invected), and anterior-specific (cubitus interruptus-D) genes. The lineage boundary was identified by marking mitotic recombinant clones, using an enhancer trap line with ubiquitous beta-gal expression in imaginal tissues; clones of +/+ cells were identified by their lack of beta-gal expression. Domains of gene expression were localized using antibodies and gene specific lacZ constructs. Surprisingly, it was found that engrailed expression extended a small distance into the anterior lineage compartment of the wing blade, as identified with anti-en/inv mAb, anti-en polyclonal antiserum, or an en-promoter-lacZ insert, ryxho25. This anterior expression was not present in early third instar discs, but appeared during subsequent larval and pupal development. In contrast, the expression of cubitus interruptus-D, as identified using the ci-Dplac insert, appeared to be limited to the anterior lineage compartment. Thus, en expression is not limited to cells from the posterior lineage compartment, and en and ci-D activities can overlap in a region just anterior to the lineage compartment boundary in the developing wing. The lineage boundary could also be identified by a line of aligned cells in the prospective wing blade region of wandering third instar discs. A decapentaplegic-lacZ construct was expressed in a stripe several cells anterior to the lineage boundary, and did not define or overlap into the posterior lineage compartment.