Posterior-to-anterior transformation in engrailed wing imaginal disks of Drosophila

Segments in the Drosophila adult are divided into clonally distinct anterior and posterior compartments. Mutations at the engrailed locus can affect the pattern of cuticular structures in the posterior compartments of segments, but have no obvious effect on anterior structures; for example, bristles that are normally seen only on the anterior wing margin in wild-type flies can be found on the posterior margin of engrailed wings. These and clonal analysis data led to the hypothesis that engrailed causes a transformation of posterior to anterior identity in the wing cells. Despite some striking examples of this transformation, a common engrailed phenotype is the disruption or elimination of posterior pattern elements, without a clear replacement by anterior structures; this, together with indications that localized cell death can mimic some of the observed posterior-to-anterior transformations, has led some investigators to question the original engrailed hypothesis. Recently, monoclonal antibodies displaying region-specific binding patterns on the wing imaginal disk have been described, and one of these antibodies in particular provides a novel probe for the engrailed phenotype in the larval precursors of the adult wing. Here I compare the antibody binding patterns on engrailed and wild-type wing disks. The results strongly support the notion that engrailed mutations cause a posterior-to-anterior transformation in these cells.     

Patterns of dye coupling in the imaginal wing disk of Drosophila melanogaster

Responses of developing tissues to experimental disruption demonstrate that cell interaction is important both in generating positional information and in controlling growth. However, the mechanism by which cells interact and the range over which the interactions are effective are not known. In the imaginal disks of Drosophila melanogaster, experiments on pattern regulation following surgical ablation suggest that the cell interactions are very local in nature; in fact, most of the data can be explained by assuming that cells interact only with their immediate neighbours. In contrast, studies of cell division patterns in the same tissue indicate that the "local' proliferative response to an ablation extends over a distance of up to about eight cell diameters. Still longer-range interactions have been proposed on the basis of theoretical considerations. It is possible that the interactions are mediated by the transfer of small molecules through gap junctions, as gap junctions are abundant in imaginal disks at the appropriate developmental stages. We have explored the range, timing and directionality of dye coupling between the cells of the wing disk as a test of the possible role of gap junctions in imaginal disk patterning. Our results indicate that interactions over different ranges are possible depending on the nature of the molecule being transferred.     

Suppression of Polycomb group proteins by JNK signalling induces transdetermination in Drosophila imaginal discs

During the regeneration of Drosophila imaginal discs, cellular identities can switch fate in a process known as transdetermination. For leg-to-wing transdetermination, the underlying mechanism involves morphogens such as Wingless that, when activated outside their normal context, induce ectopic expression of the wing-specific selector gene vestigial. Polycomb group (PcG) proteins maintain cellular fates by controlling the expression patterns of homeotic genes and other developmental regulators. Here we report that transdetermination events are coupled to PcG regulation. We show that the frequency of transdetermination is enhanced in PcG mutant flies. Downregulation of PcG function, as monitored by the reactivation of a silent PcG-regulated reporter gene, is observed in transdetermined cells. This downregulation is directly controlled by the Jun amino-terminal kinase (JNK) signalling pathway, which is activated in cells undergoing regeneration. Accordingly, transdetermination frequency is reduced in a JNK mutant background. This regulatory interaction also occurs in mammalian cells, indicating that the role of this signalling cascade in remodelling cellular fates may be conserved.     

A zone of non-proliferating cells at a lineage restriction boundary in Drosophila

The use of X-ray-induced mitotic recombination to genetically mark individual cells and their descendants during development has led to the discovery of lineage restriction boundaries in Drosophila imaginal disks, dividing the disks into areas called compartments. Clones of cells initiated after a given developmental stage are unable to grow across these boundaries, even if provided with a growth rate advantage over the remaining cells. It has been suggested that cells within compartments are distinguished by the differential activation of selector genes and that the lineage restrictions are maintained by adhesivity differences between the cells in different compartments, but other mechanisms have not been ruled out. Recently a discrete population of cells with unusual permeability properties has been described along an intersegmental lineage restriction boundary in Oncopeltus, suggesting that a lineage restriction could be maintained by a zone of cells which present a barrier to clone growth. Here we demonstrate by autoradiography the presence of a narrow zone of non-proliferating cells (ZNC) coincident with the presumptive wing margin in the Drosophila wing disk, and suggest that this could account for the observed lineage restriction between presumptive dorsal and ventral surfaces of the wing. As the anterior/posterior compartment boundary does not coincide with a ZNC, the results indicate that different lineage boundaries may be maintained by different mechanisms.     

Distortion of proximodistal information causes JNK-dependent apoptosis in Drosophila wing.

Distinct and evolutionarily conserved signal-transduction cascades mediate the survival or death of cells during development. The c-Jun amino-terminal kinases (JNKs) of the mitogen-activated protein kinase superfamily are involved in apoptotic signalling in various cultured cells. However, the role of the JNK pathway in development is less well understood. In Drosophila, Decapentaplegic (Dpp; a homologue of transforming growth factor-beta) and Wingless (Wg; a Wnt homologue) proteins are secretory morphogens that act cooperatively to induce formation of the proximodistal axis of appendages. Here we show that either decreased Dpp signalling in the distal wing cells or increased Dpp signalling in the proximal wing cells causes apoptosis. Inappropriate levels of Dpp signalling lead to aberrant morphogenesis in the respective wing zones, and these apoptotic zones are also determined by the strength of the Wg signal. Our results indicate that distortion of the positional information determined by Dpp and Wg signalling gradients leads to activation of the JNK apoptotic pathway, and the consequent induction of cell death thereby maintains normal morphogenesis.     

C. elegans odour discrimination requires asymmetric diversity in olfactory neurons

Caenorhabditis elegans senses at least five attractive odours with a single pair of olfactory neurons, AWC, but can distinguish among these odours in behavioural assays. The two AWC neurons are structurally and functionally similar, but the G-protein-coupled receptor STR-2 is randomly expressed in either the left or the right AWC neuron, never in both. Here we describe the isolation of a mutant, ky542, with specific defects in odour discrimination and odour chemotaxis. ky542 is an allele of nsy-1, a neuronal symmetry, or Nsy, mutant in which STR-2 is expressed in both AWC neurons. Other Nsy mutants exhibit discrimination and olfactory defects like those of nsy-1 mutants. Laser ablation of the AWC neuron that does not express STR-2 (AWCOFF) recapitulates the behavioural phenotype of Nsy mutants, whereas laser ablation of the STR-2-expressing AWC neuron (AWCON) causes different chemotaxis defects. We propose that odour discrimination can be achieved by segregating the detection of different odours into distinct olfactory neurons or into unique combinations of olfactory neurons.     

Context generalization in Drosophila visual learning requires the mushroom bodies.

The world is permanently changing. Laboratory experiments on learning and memory normally minimize this feature of reality, keeping all conditions except the conditioned and unconditioned stimuli as constant as possible. In the real world, however, animals need to extract from the universe of sensory signals the actual predictors of salient events by separating them from non-predictive stimuli (context). In principle, this can be achieved if only those sensory inputs that resemble the reinforcer in their temporal structure are taken as predictors. Here we study visual learning in the fly Drosophila melanogaster, using a flight simulator, and show that memory retrieval is, indeed, partially context-independent. Moreover, we show that the mushroom bodies, which are required for olfactory but not visual or tactile learning, effectively support context generalization. In visual learning in Drosophila, it appears that a facilitating effect of context cues for memory retrieval is the default state, whereas making recall context-independent requires additional processing.     

Genetic divergence, speciation and morphological stasis in a lineage of African cichlid fishes.

Since their discovery at the turn of the century, the species assemblages of cichlid fishes in the East African Lakes Victoria, Malawi and Tanganyika have fascinated evolutionary biologists. Many models have attempted to account for the 'explosive' evolution of several hundred species within these lakes. Here we report a case of surprisingly large genetic divergence among populations of the endemic Tropheus lineage of Lake Tanganyika. This lineage of six species contains twice as much genetic variation as the entire morphologically highly diverse cichlid assemblage of Lake Malawi and six times more variation than the Lake Victoria species flock. Although it is highly variable in coloration, this group of species and its closest relatives have not undergone appreciable morphological change. The observed geographic pattern of genetic variation suggests that major lake level fluctuations affected the distribution and speciation of this lineage of cichlid fishes.