Brinker possesses multiple mechanisms for repression because its primary co-repressor, Groucho, may be unavailable in some cell types.

Transcriptional repressors function primarily by recruiting co-repressors, which are accessory proteins that antagonize transcription by modifying chromatin structure. Although a repressor could function by recruiting just a single co-repressor, many can recruit more than one, with Drosophila Brinker (Brk) recruiting the co-repressors CtBP and Groucho (Gro), in addition to possessing a third repression domain, 3R. Previous studies indicated that Gro is sufficient for Brk to repress targets in the wing, questioning why it should need to recruit CtBP, a short-range co-repressor, when Gro is known to be able to function over longer distances.

Current topics in organogenesis and gametogenesis.

At the 49(th) Annual Drosophila Research Conference from April 3-8, 2008 in San Diego there were eight talks and over ninety posters in the section on Organogenesis and Gametogenesis. These covered a wide range of topics within the two broad categories of organ-specific stem cells (including germ cells) and organ-specific developmental programs. Here we discuss eleven of these presentations describing current research into the formation of the gonad, intestine, trachea, muscle and leg joint.

Genetic interactions among scribbler, Atrophin and groucho in Drosophila uncover links in transcriptional repression.

In eukaryotes, the ability of DNA-binding proteins to act as transcriptional repressors often requires that they recruit accessory proteins, known as corepressors, which provide the activity responsible for silencing transcription. Several of these factors have been identified, including the Groucho (Gro) and Atrophin (Atro) proteins in Drosophila. Here we demonstrate strong genetic interactions between gro and Atro and also with mutations in a third gene, scribbler (sbb), which encodes a nuclear protein of unknown function.

Generating and interpreting the Brinker gradient in the Drosophila wing.

The transcription factor Brinker (Brk) represses gene expression in the Drosophila wing imaginal disc, where it is expressed in symmetrical lateral-to-medial gradients, a pattern that is established by inverse gradients of the TGF-beta, Dpp, which is in turn transduced into graded phosphorylated Mad (pMad, an R-Smad). pMad is part of a complex which directly represses brk. sal and omb are targets of Brk and are, thus, only expressed medially with their domains extending mediolaterally into the region where Brk is graded.

Regulation of gene expression in the distal region of the Drosophila leg by the Hox11 homolog, C15.

The distal region of the Drosophila leg, the tarsus, is divided into five segments (ta I-V) and terminates in the pretarsus, which is characterized by a pair of claws. Several homeobox genes are expressed in distinct regions of the tarsus, including aristaless (al) and lim1 in the pretarsus, Bar (B) in ta IV and V, and apterous (ap) in ta IV. This pattern is governed by regulatory interactions between these genes; for example, Al and B are mutually antagonistic resulting in exclusion of B expression from the pretarsus.

Repression of Dpp targets in the Drosophila wing by Brinker.

Patterning along developing body axes is regulated by gradients of transcription factors, which activate or repress different genes above distinct thresholds. Understanding differential threshold responses requires knowledge of how these factors regulate transcription. In the Drosophila wing, expression of genes such as omb and sal along the anteroposterior axis is restricted by lateral-to-medial gradients of the transcriptional repressor Brinker (Brk).

Distalization of the Drosophila leg by graded EGF-receptor activity.

Arthropods and higher vertebrates both possess appendages, but these are morphologically distinct and the molecular mechanisms regulating patterning along their proximodistal axis (base to tip) are thought to be quite different. In Drosophila, gene expression along this axis is thought to be controlled primarily by a combination of transforming growth factor-beta (TGF-beta) and Wnt signalling from sources of ligands, Decapentaplegic (Dpp) and Wingless (Wg), in dorsal and ventral stripes, respectively. In vertebrates, however, proximodistal patterning is regulated by receptor tyrosine kinase (RTK) activity from a source of ligands, fibroblast growth factors (FGFs), at the tip of the limb bud.

Initiation of the proximodistal axis in insect legs.

Much of the cell-cell communication that controls assignment of cell fates during animal development appears to be mediated by extracellular signaling molecules. The formation of the proximodistal (P/D) axis of the legs of flies is controlled by at least two such molecules, a Wnt and a TGFbeta, encoded by the wingless (wg) and decapentaplegic (dpp) genes, respectively. The P/D axis appears to be initiated from the site where cells expressing wg are in close association with those expressing dpp.

Cell behaviour during postembryonic pattern regulation in the insect abdomen (Oncopeltus fasciatus). II. Intrasegmental regulation.

Cells from different levels in the anteroposterior axis of an abdominal segment of Oncopeltus were confronted by scraping away the strip of epidermis that separated these levels. The cells migrate over the wound and meet in the centre. The subsequent behaviour of the epidermal cells was followed by preparing whole mounts of integument at various times after confrontation.

Cell behaviour during postembryonic pattern regulation in the insect abdomen (Oncopeltus fasciatus). I. Regeneration of segment borders.

The confrontation of cells from the anterior region of an abdominal segment of Oncopeltus with those from the posterior region of the same or the adjacent segment results in the generation of a segment border. The behaviour of epidermal cells during this regulation is described. It consists primarily of cell division and transverse elongation of cells at the site of confrontation.