Genetic mosaics that place cells in competition within tissues may model features of tissue repair and tumor development and may reveal mechanisms of growth regulation. In one example, normal cells eliminate "Minute" cells that have reduced ribosomal protein gene dose and grow at their expense, replacing the Minute cells within developing compartments. We describe genes that are required by wild-type cells to kill Minute neighbors in Drosophila. The engulfment genes draper, wasp, the phosphatidylserine receptor, mbc/dock180, and rac1 are needed in wild-type cells for the death of Minute neighbors, whose corpses are engulfed by wild-type cells. Wild-type cells can themselves be killed by cells with elevated engulfing activity. Thus engulfment genes act downstream of growth differences between cells to eliminate cells with reduced ribosomal gene dose.

In Drosophila the process of segmentation depends on the function of coordinate, gap, pair-rule and segment-polarity genes. Mutations in segment-polarity genes cause defects in the pattern of every segment. Here the cloning of sequences from a segment-polarity gene, wingless, and the in situ localization of a transcript in embryos are described. The transcript is first detected in the anterior and posterior regions of the blastoderm embryo at celiularization, and accumulates in a series of stripes in the extended germ band, one stripe per metameric unit. Each stripe is localized to the most posterior cells of each parasegment. The signal is predominantly epidermal, and transcript accumulates only transiently in the mesoderm and nervous system. This pattern of expression is discussed with respect to models of pattern formation in segmental units.

Cell competition is a homeostatic mechanism that regulates the size attained by growing tissues. We performed an unbiased genetic screen for mutations that permit the survival of cells being competed due to haplo-insufficiency for RpL36. Mutations that protect RpL36 heterozygous clones include the tumor suppressors expanded, hippo, salvador, mats, and warts, which are members of the Warts pathway, the tumor suppressor fat, and a novel tumor-suppressor mutation. Other hyperplastic or neoplastic mutations did not rescue RpL36 heterozygous clones. Most mutations that rescue cell competition elevated Dppsignaling activity, and the Dsmurf mutation that elevates Dpp signaling was also hyperplastic and rescued. Two nonlethal, nonhyperplastic mutations prevent the apoptosis of Minute heterozygous cells and suggest an apoptosis pathway for cell competition . In addition to rescuing RpL36 heterozygous cells, mutations in Warts pathway genes were supercompetitors that could eliminate wild-type cells nearby. The findings show that differences in Warts pathway activity can lead to competition and implicate the Warts pathway, certain other tumor suppressors, and novel cell death components in cell competition, in addition to the Dpp pathway implicated by previous studies. We suggest that cell competition might occur during tumor development in mammals.

Ribosomes perform protein synthesis but are also involved in signaling processes, the full extent of which are still being uncovered. We report that phenotypes of mutating ribosomal proteins (Rps) are largely due to signaling. Using Drosophila, we discovered that a bZip-domain protein, Xrp1, becomes elevated in Rp mutant cells. Xrp1 reduces translation and growth, delays development, is responsible for gene expression changes, and causes the cell competition of Rp heterozygous cells from genetic mosaics. Without Xrp1, even cells homozygously deleted for Rp genes persist and grow. Xrp1 induction in Rp mutant cells depends on a particular Rp with regulatory effects, RpS12, and precedes overall changes in translation. Thus, effects of Rp mutations, even the reductions in translation and growth, depend on signaling through the Xrp1 pathway and are not simply consequences of reduced ribosome production limiting protein synthesis. One benefit of this system may be to eliminate Rp-mutant cells by cell competition.

The basic Helix-Loop-Helix (bHLH) proteins represent a well-known class of transcriptional regulators. Many bHLH proteins act as heterodimers with members of a class of ubiquitous partners, the E-proteins. A widely-expressed class of inhibitory heterodimer partners- the Inhibitor of DNA-binding (ID) proteins- also exists. Genetic and molecular analyses in humans and in knockout mice implicate E-proteins and ID-proteins in a wide variety of diseases, belying the notion that they are non-specific partner proteins. Here, we explore relationships of E-proteins and ID-proteins to a variety of disease processes and highlight gaps in knowledge of disease mechanisms.

In eye development, inhibition by Notch activity is restricted to specific phases of proneural gene expression, beginning when prepattern decays and is replaced by autoregulation. We suggest that Notch signalling inhibits atonal autoregulation, but not expression by other mechanisms, and that a transition from prepattern to autoregulation is necessary for patterning neural cell determination. Distinct neural tissues might differ in their proneural prepatterns, but use Notch in a similar mechanism.

Summary Spatial and temporal expression of specific basic Helix-Loop-Helix (bHLH) transcription factors defines many types of differentiation. We find that the much broader expression of the heterodimer partners of these specific factors is also highly regulated, through a distinct mechanism. A cross-interacting regulatory network links expression of the Drosophila E-protein Daughterless, required to heterodimerize with bHLH proteins, with expression of the Id protein Extramacrochaetae, an antagonist of bHLH proteins. Coupled transcriptional feedback loops maintain the widespread Extramacrochaetae expression that restrains Daughterless expression, opposing bHLH-dependent differentiation while enhancing growth and cell survival. Where extracellular signals repress extramacrochaetae, Daughterless expression is then able to increase. This defines regions of proneural ectoderm, independently from the proneural bHLH genes. Similar regulation is found in multiple Drosophila tissues and in mammalian cells, and may be a conserved general feature of developmental regulation by HLH proteins.

Mutations in the Drosophila scabrous {sea) gene affect eye and bristle development, leading to irregular spacing of ommatidia and bristle duplications in the adult fly. We have cloned the sea gene by P-element tagging. The sea transcription unit is 12 kb and consists of four exons that are joined in a 3.2-kb mRNA. In an enhancer trap screen we have isolated several P[/flcZ] insertions close to the sea transcription start site. We have examined the expression pattern of sea by in situ hybridization to sea transcripts, by p-galactosidase localization in the P[iacZ] lines, and by immunocytochemistry with an anti-sea antiserum. During embryogenesis, sea is expressed in a dynamic pattern associated with neural development. During imaginal development, sea is mainly expressed in the R8 photoreceptor precursor cells in the eye imaginal disc and in sensory organ precursor cells in other discs. In the wing disc, sea expression is coextensive with the anlagen for bristles and is controlled by genes of the aebaete-seute complex. Based on its loss-of-function phenotype, expression pattern, and the predicted structure of its product, a secreted peptide with homology to the fibrinogen gene family, we propose that sea encodes a signal involved in lateral inhibition within individual domains of the developing nervous system.