Peter A. Lawrence

Morphogens, Compartments, and Pattern: Lessons from Drosophila?

We thank Javier Sampedro for planning Figure 4Figure 4 and Dan Barbash, Jose Casal, Jim Smith, and Jean-Paul Vincent for help with the manuscript. The advice and support of Hugh Pelham and Jim Smith is much appreciated.

Distribution of the wingless gene product in drosophila embryos: A protein involved in cell-cell communication

wingless, a segment polarity gene required in every segment for the normal development of the Drosophila embryo, encodes a cysteine-rich protein with a signal peptide. A polyclonal antiserum localizes the wingless protein in approximately the same region of the embryo as the wingless mRNA. The pattern of antigen localization changes rapidly during development. In the extended germband stage, stripes of wingless staining are present in the trunk region just anterior to the parasegment boundary; wingless-expressing cells abut engrailed-expressing cells across that boundary. wingless antigen is seen both inside and outside the cell by electron microscopy: inside the cell, in small membrane-bound vesicles and in multivesicular bodies; outside the cell, close to or on the plasma membrane and associated with material in the intercellular space. The multivesicular bodies containing the wingless protein are occasionally found in engrailed-positive cells, suggesting that the wingless protein behaves as a paracrine signal.

Control of Drosophila body pattern by the hunchback morphogen gradient.

Most of the thoracic and abdominal segments of Drosophila are specified early in embryogenesis by the overlapping activities of the hunchback (hb), Krüppel, knirps, and giant gap genes. The orderly expression of these genes depends on two maternal determinants: bicoid, which activates hb transcription anteriorly, and nanos, which blocks translation of hb transcripts posteriorly. Here we provide evidence that the resulting gradient of hb protein dictates where the Krüppel, knirps, and giant genes are expressed by providing a series of concentration thresholds that regulate each gene independently. Thus, hb protein functions as a classical morphogen, triggering several distinct responses as a function of its graded distribution.

Induction across germ layers in Drosophila mediated by a genetic cascade

We report an induction process occurring between two germ layers in the Drosophila embryo that involves a cascade of five interacting genes. Two of these, Ultrabithorax and abdominal-A, encode nuclear homeobox proteins; each of them is expressed in one of two adjacent parasegments in the visceral mesoderm and directs expression in its parasegment of a separate target gene, decapentaplegic in parasegment 7 and wingless in parasegment 8. The activity of both target genes is required for normal expression of another homeotic gene, labial, in cells of the adhering midgut epithelium. Their products are putative extracellular proteins, which presumably act as signals between the two germ layers. Positional instruction of this kind may be needed since the endoderm, unlike the mesoderm, appears unsegmented at first as it originates from two primordia near the embryonic poles, outside the realm of segmentation genes.

Compartments in the wing of Drosophila: a study of the engrailed gene

Abstract It has recently been suggested that the wildtype alleles of homeotic genes are responsible for controlling the development of compartments. Because the mutation engrailed gives the posterior wing compartment anterior characteristics, it can be regarded as such a homeotic gene. Our experiments confirm the role of the engrailed gene in development of the posterior wing compartment, results which strongly support and extend the compartment hypothesis. Clonal analysis reveals that the state of the engrailed gene is immaterial to the entire anterior compartment, and crucial to the normal development of the posterior compartment, where it controls the pattern of veins and bristles. The presence of a straight and precisely positioned compartment border is dependent on the activity of the engrailed gene until late in development. We suggest that this is due to the genes effects on cell affinities of the posterior compartment. The engrailed mutation increases the size and changes the shape of the posterior compartment. engrailed clones cause local wing enlargement only if they are dorsal and include the posterior margin of the wing. Wildtype cells outside the clone contribute to this change of shape. This result suggests that the postero-dorsal margin is primarily responsible for the control of shape, and that the ventral compartment is, to some extent, modeled on the dorsal.

Planar cell polarity: one or two pathways?

In multicellular organisms, cells are polarized in the plane of the epithelial sheet, revealed in some cell types by oriented hairs or cilia. Many of the underlying genes have been identified in Drosophila melanogaster and are conserved in vertebrates. Here we dissect the logic of planar cell polarity (PCP). We review studies of genetic mosaics in adult flies — marked cells of different genotypes help us to understand how polarizing information is generated and how it passes from one cell to another. We argue that the prevailing opinion that planar polarity depends on a single genetic pathway is wrong and conclude that there are (at least) two independently acting processes. This conclusion has major consequences for the PCP field.

Phenocopies induced with antisense RNA identify the wingless gene

Earlier work suggested that the wingless gene of Drosophila is required for cooperation within discrete groups of cells during development. We show that antisense RNA made from a 3 kb transcript produces wingless mutant phenocopies when injected into wild-type eggs, proving that this transcript executes the wingless function. In the accompanying paper, Rijsewijk et al. report cloning of a Drosophila homolog (Dint-1) of the mouse int-1 gene and show that this gene is identical to wingless+. The partial sequence of the wingless cDNA that we have isolated is identical to part of the complete Dint-1 sequence described by Rijsewijk et al.

Two separate molecular systems, Dachsous/Fat and Starry night/Frizzled,act independently to confer planar cell polarity

Planar polarity is a fundamental property of epithelia in animals and plants. In Drosophila it depends on at least two sets of genes: one set, the Ds system, encodes the cadherins Dachsous (Ds) and Fat (Ft), as well as the Golgi protein Four-jointed. The other set, the Stan system, encodes Starry night (Stan or Flamingo) and Frizzled. The prevailing view is that the Ds system acts via the Stan system to orient cells. However, using the Drosophila abdomen, we find instead that the two systems operate independently: each confers and propagates polarity, and can do so in the absence of the other. We ask how the Ds system acts; we find that either Ds or Ft is required in cells that send information and we show that both Ds and Ft are required in the responding cells. We consider how polarity may be propagated by Ds-Ft heterodimers acting as bridges between cells.

The development of wingless, a homeotic mutation of Drosophila.

Abstract The mutation wingless produces a homeotic transformation in which the distal structures (appendages) of both the wing and haltere discs are replaced by a duplication of the proximal structures (thorax). However, not all of the mutant discs show mutant phenotype; some of them differentiate normal appendages. Gynandromorph and clonal analyses suggest that the phenotype does not result from massive cell death followed by regeneration and/or duplication. We conjecture that the mutant phenotype is caused by a specific failure in the process of compartmentalization. In contrast to other homeotic mutants, wingless is not cell autonomous; that is, mutant clones show wildtype phenotype when produced in wildtype wings.

Cell lineage in the developing retina of Drosophila

Analysis of the cell lineage of the Drosophila retina is reported. Mitotic recombination within the white locus results in the formation of small red spots in white eyes; these are found under the dissecting microscope. The spot frequency is low (never more than 130 eyes) so that there can be no doubt that each spot is a single clone. Eyes bearing a clone are serially sectioned and all retinula and all pigment cells scored as white or white+. We describe the constitution of 101 clones and examine the disposition of the marked cells in the retinal lattice. The clones are apparently random combinations of the marked cell types—for example, two-celled clones containing one pigment and one retinula cell are frequently found. Our results appear to rule out fixed cell lineage as a determinative mechanism in ommatidial development.