Ernesto Sánchez-Herrero

The Drosophila segment polarity gene patched interacts with decapentaplegic in wing development.

The decapentaplegic (dpp) gene of Drosophila melanogaster encodes a polypeptide of the transforming growth factor‐beta family of secreted factors. It is required for the proper development of both embryonic and adult structures, and may act as a morphogen in the embryo. In wing imaginal discs, dpp is expressed and required in a stripe of cells near the anterior‐posterior compartment boundary. Here we show that viable mutations in the segment polarity genes patched (ptc) and costal‐2 (cos2) cause specific alterations in dpp expression within the anterior compartment of the wing imaginal disc. The interaction between ptc and dpp is particularly interesting; both genes are expressed with similar patterns at the anterior‐posterior compartment boundary of the disc, and mis‐expressed in a similar way in segment polarity mutant backgrounds like ptc and cos2. This mis‐expression of dpp could be correlated with some of the features of the adult mutant phenotypes. We propose that ptc controls dpp expression in the imaginal discs, and that the restricted expression of dpp near the anterior‐posterior compartment boundary is essential to maintain the wild‐type morphology of the wing disc.

The function of engrailed and the specification of Drosophila wing pattern.

The adult Drosophila wing (as the other appendages) is subdivided into anterior and posterior compartments that exhibit characteristic patterns. The engrailed (en) gene has been proposed to be paramount in the specification of the posterior compartment identity. Here, we explore the adult en function by targeting its expression in different regions of the wing disc. In the anterior compartment, ectopic en expression gives rise to the substitution of anterior structures by posterior ones, thus demonstrating its role in specification of posterior patterns. The en-expressing cells in the anterior compartment also induce high levels of the hedgehog (hh) and decapentaplegic (dpp) gene products, which results in local duplications of anterior patterns. Besides, hh is able to activate en and the engrailed-related gene invected (inv) in this compartment. In the posterior compartment we find that elevated levels of en product result in partial inactivation of the endogenous en and inv genes, indicating the existence of a negative autoregulatory mechanism. We propose that en has a dual role: a general one for patterning of the appendage, achieved through the activation of secreted proteins like hh and dpp, and a more specific one, determining posterior identity, in which the inv gene may be implicated.

Identification and characterization of a parasegment specific regulatory element of the abdominal-B gene of drosophila

We have characterized mutations of the Abdominal-B gene of the bithorax complex of Drosophila. We conclude that the gene contains two distinct genetic elements: one has a morphogenetic role and acts in parasegments 10, 11, 12, and 13, while the other acts on parasegment 14 and has primarily or exclusively a regulatory function. Evidence indicates that the latter suppresses the activity of the morphogenetic element of Abd-B and of other genes responsible for the development of sclerotic plates. The regulatory element also suppresses those BX-C genes and other homeotics that, in the absence of Polycomb or extra sex combs function, can become active in parasegment 14.

Sharp boundaries of Dpp signalling trigger local cell death required for Drosophila leg morphogenesis

Morphogens are secreted signalling molecules that govern many developmental processes. In the Drosophila wing disc, the transforming growth factor β (TGFβ) homologue Decapentaplegic (Dpp) forms a smooth gradient and specifies cell fate by conferring a defined value of morphogen activity. Thus, neighbouring cells have similar amounts of Dpp protein, and if a sharp discontinuity in Dpp activity is generated between these cells, Jun kinase (JNK)-dependent apoptosis is triggered to restore graded positional information. To date, it has been assumed that this apoptotic process is only activated when normal signalling is distorted. However, we now show that a similar process occurs during normal development: rupture in Dpp activity occurs during normal segmentation of the distal legs of Drosophila. This sharp boundary of Dpp signalling, independently of the absolute level of Dpp activity, induces a JNK—reaper-dependent apoptosis required for the morphogenesis of a particular structure of the leg, the joint. Our results show that Dpp could induce a developmental programme not only in a concentration dependent manner, but also by the creation of a sharp boundary of Dpp activity. Furthermore, the same process could be used either to restore a normal pattern in response to artificial disturbance or to direct a morphogenetic process.

The fu gene discriminates between pathways to control dpp expression in Drosophila imaginal discs

The genes decapentaplegic (dpp) and wingless (wg), which encode secreted factors of the TGF-beta and Wnt families, respectively, are required for the proper development of the imaginal discs. The expression of these genes must be finely regulated since their ectopic expression induces overgrowth and pattern alterations in wings and legs. Genes like patched (ptc) and costal-2 (cos-2), and the gene encoding the catalytic subunit of the protein kinase A gene (pkA) are required to restrict dpp and wg expression in their proper positions. We show here that some mutations in the cubitus interruptus (ci) gene also show ectopic dpp expression in the wing disc. We have also analyzed the functional hierarchy between these genes and the gene fused (fu), in the activation of dpp by the hedgehog (hh) signal. fu is required to transmit the hh signal in imaginal discs, since fu mutations rescue the phenotype due to the ectopic hh expression or to the lack of ptc activity. fu is also required for the activation of engrailed (en) caused when hh is ectopically activated in the wing disc. By contrast, fu mutations do not rescue the phenotypic consequences of the abnormal ci, cos-2 or pkA activity. Although fu, cos-2 and ci probably form part of the same pathway that controls dpp expression, pkA probably controls dpp transcription by a different pathway.

FUNCTIONAL SIMILARITY IN APPENDAGE SPECIFICATION BY THE ULTRABITHORAX AND ABDOMINAL-A DROSOPHILA HOX GENES

In Drosophila, the Ultrabithorax, abdominal‐A and Abdominal‐B HOX genes of the bithorax complex determine the identity of part of the thorax and the whole abdomen. Either the absence of these genes or their ectopic expression transform segments into the identity of different ones along the antero‐posterior axis. Here we show that misexpression of Ultrabithorax, abdominal‐A and, to some extent, Abdominal‐B genes cause similar transformations in some of the fruitfly appendages: antennal tissue into leg tissue and wing tissue into haltere tissue. abdominal‐A can fully, and Abdominal‐B partially, substitute for Ultrabithorax in haltere development. By contrast, when ectopically expressed, the three genes specify different segments in regions of the main body axis like notum or abdomen. Insects may have originally used the HOX genes primarily to specify this main body axis. By contrast, the homeotic requirement to form appendages is, in some cases, non‐specific.

The Abdominal-B gene of Drosophila melanogaster: overlapping transcripts exhibit two different spatial distributions.

The Abdominal‐B (Abd‐B) gene of Drosophila controls the specification of segment identities in the posterior abdomen. We describe here the spatial and temporal distribution of Abd‐B transcripts. At least five RNA species are detected on Northern blots with probes within the region of the Abd‐B homeobox. We have identified probes specific for subsets of these transcripts and have used these probes to study the distribution of each subset by in situ hybridization to embryonic tissue sections. The transcripts can be divided into two groups: one group is expressed maximally in parasegment (PS)13 and extends anteriorly to PS10; the other is expressed only in PS14 and 15. These two different patterns of expression correspond to the anatomical domains defined by two classes of mutations in Abd‐B. These results help explain the complex genetic interactions and phenotypes of mutations within this gene.

The genital disc of Drosophila melanogaster.

Abstract The genital disc of Drosophila, which gives rise to the genitalia and analia of adult flies, is formed by cells from different embryonic segments. To study the organization of this disc, the expressions of segment polarity and homeotic genes were investigated. The organization of the embryonic genital primordium and the requirement of the engrailed and invected genes in the adult terminalia were also analysed. The results show that the three primordia, the female and male genitalia plus the analia, are composed of an anterior and a posterior compartment. In some aspects, each of the three primordia resemble other discs: the expression of genes such as wingless and decapentaplegic in each anterior compartment is similar to that seen in leg discs, and the absence of engrailed and invected cause duplications of anterior regions, as occurs in wing discs. The absence of lineage restrictions in some regions of the terminalia and the expression of segment polarity genes in the embryonic genital disc suggest that this model of compartmental organization evolves, at least in part, as the disc grows. The expression of homeotic genes suggests a parasegmental organization of the genital disc, although these genes may also change their expression patterns during larval development.

The Ultrabithorax hox gene of Drosophila controls haltere size by regulating the Dpp pathway

The halteres and wings of Drosophila are homologous thoracic appendages, which share common positional information provided by signaling pathways. The activity in the haltere discs of the Ultrabithorax (Ubx) Hox gene establishes the differences between these structures, their different size being an obvious one. We show here that Ubx regulates the activity of the Decapentaplegic (Dpp) signaling pathway at different levels, and that this regulation is instrumental in establishing the size difference. Ubx downregulates dpp transcription and reduces Dpp diffusion by repressing the expression of master of thick veins and division abnormally delayed and by increasing the levels of thick veins, one of the Dpp receptors. Our results suggest that modulation in Dpp expression and spread accounts, in part, for the different size of halteres and wings.

A simple and efficient method to identify replacements of P-lacZ by P-Gal4 lines allows obtaining Gal4 insertions in the bithorax complex of Drosophila

The functional replacement of one gene product by another one is a powerful method to study specificity in development and evolution. In Drosophila, the Gal4/UAS method has been used to analyze in vivo such functional substitutions. To this aim, Gal4 lines that inactivate a gene and reproduce its expression pattern are required, and they can be frequently obtained by replacing pre-existing P-lacZ lines with such characteristics. We have devised a new method to quickly identify replacements of P-lacZ lines by P-Gal4 lines, and applied it successfully to obtain Gal4 insertions in the Ultrabithorax and Abdominal-B Hox genes. We have used these lines to study the functional replacement of a Hox gene by another one. Our experiments confirm that the abdominal-A gene can replace Ultrabithorax in haltere development but that it cannot substitute for Abdominal-B in the formation of the genitalia.