Segment boundary formation inDrosophilaembryos

Abstract

In Drosophila embryos, segment boundaries form at the posterior edge of each stripe of engrailed expression. We have used an HRP-CD2 transgene to follow by transmission electron microscopy the cell shape changes that accompany boundary formation. The first change is a loosening of cell contact at the apical side of cells on either side of the incipient boundary. Then, the engrailed-expressing cells flanking the boundary undergo apical constriction, move inwards and adopt a bottle morphology. Eventually,grooves regress, first on the ventral side, then laterally. We noted that groove formation and regression are contemporaneous with germ band retraction and shortening, respectively, suggesting that these rearrangements could also contribute to groove morphology. The cellular changes accompanying groove formation require that Hedgehog signalling be activated, and, as a result, a target of Ci expressed, at the posterior of each boundary (obvious targets like stripe and rhomboid appear not to be involved). In addition, Engrailed must be expressed at the anterior side of each boundary,even if Hedgehog signalling is artificially maintained. Thus, there are distinct genetic requirements on either side of the boundary. In addition,Wingless signalling at the anterior of the domains of engrailed (and hedgehog) expression represses groove formation and thus ensures that segment boundaries form only at the posterior.

Mapping chromatic pathways in the Drosophila visual system

“…Our GRASP results suggest that LT11 receives synaptic inputs from all four classes of chromatic Tm neurons but that the number of these contacts is much lower than the inputs these form to Li4. To examine the synaptic contacts between Tm and LT neurons at the electron microscopic (EM) level, we have developed a "two-tag" double labeling system that can highlight both pre-and postsynaptic neurons in the same preparation, by combining two orthogonal expression systems and two different peroxidases, HRP (horseradish peroxidase; Larsen et al, 2003;Edwards and Meinertzhagen, 2009) and APX (ascorbate peroxidase; Martell et al, 2012). In this system, Gal4-driven HRP::DsRed::GPI (Han et al, 2012) labels the cell membrane of postsynaptic dendrites (LT11), whereas LexA-driven mito::APX::HA, APX 41F fused to a mitochondrion matrix targeting peptide highlights mitochondria and identifies presynaptic terminals (Tm5c).…”

Genetic control of epidermis differentiation in Drosophila.

“…This PS frontier separates two groups of cells along the AP axis, the anterior and posterior compartments. Each segment corresponds to the juxtaposition of an anterior and a posterior compartment (Martinez Arias, 1993), that will be highlighted by the segmental grooves (Larsen et al, 2003), first observable at mid-embryogenesis and that persist through the larval periods. In early stages of embryogenesis, Wg and hh expressions are interdependent: reception of the Wg signal in posterior cells activates the expression of the transcription factor Engrailed, which triggers the expression of hh (Martinez Arias, 1993).…”

An Unconventional Nuclear Localization Motif Is Crucial for Function of the Drosophila Wnt/Wingless Antagonist Naked Cuticle

“…First, during early to middle germband extension (stages 8-11, $3.5-6 hr AEL), Wg maintains the transcription of the target genes hh and engrailed (en) at close range in the two to three rows of cells posterior to Wg-producing cells (reviewed by DiNardo et al 1994;Hatini and DiNardo 2001). Thus the width of the hh/en-expressing stripe-the posterior border of which marks the segment boundary, a guidepost for axonal pathfinding and muscle attachment-is a readout of the Wg-signaling gradient ( Figure 1A) (Larsen et al 2003). Second, ventral cells exposed to Wg after $6 hr AEL, upon differentiation, suppress the synthesis of cell protrusions termed denticles and appear ''naked,'' whereas cells out of range of Wg produce denticle bands that facilitate larval locomotion Dougan and DiNardo 1992).…”