Dl protein is present on myogenic cell membranes.

Expression of Dl is widespread in the mesoderm at embryonic stage 12. By the end of the period, it becomes restricted to cardioblasts. Based on cell size and shape and the expression levels of Dl, dorsal and ventral subdomains within the cardiogenic mesoderm of stage 12 embryos can be distinguished. Cells of the ventral domain are small and express moderate levels of Dl and tin. Dorsally, larger cells expressing higher levels of tin are observed. Dl expression is highly dynamic in the dorsal domain. At most timepoints during stage 12, anterior and posterior clusters of strongly Dl-positive cells are seen, flanking a central cluster with lower Dl levels. The central cluster expresses eve. eve expression defines a dorso-central cluster from which the pair of eve-positive pericardial cells is specified. The later eve-positive/low-Dl dorso-central cluster gives rise to a dorsal muscle. The anterior and posterior, high-Dl clusters form the definitive cardiogenic mesoderm. They will give rise to the cardioblasts of the dorsal vessel and the odd-positive blood progenitors and pericardial nephrocytes. The ventral domain showing low levels of tin and Dl contribute to the dorsal musculature. Individual lineages derived from the cardiogenic mesoderm segregate from each other and begin to differentiate during the second half of stage 12. The anterior and posterior high-Dl cluster of each segment move closer together, while the central, low-Dl cluster moves out of the way, migrating laterally and ventrally. Dl expression becomes restricted to the cardioblasts. In the thoracic segments, each of the high-Dl clusters forms two cardioblasts while in the abdominal segments, three cardioblasts arise per cluster. However, only two cells of each abdominal cluster maintain a high level of Dl and tin. The third cell down-regulates these genes and expresses svp. A pattern of alternating sets of four tin/Dl-positive and two svp-positive cardioblasts characteristic of the abdominal dorsal vessel is generated.

The expression of ey, ap, and Dll were compared in outer optic lobes (OPC) starting in late third instar larvae. At this stage they were expressed as three distinct cell populations. In anterior sections, the three genes are expressed a three parallele stripes of cells that represent rows of neurons that emerge from the OPC. They correspond to progeny from the youngest to oldest neuroblasts. In middle sections, Dll-positive cells are generated in the progeny of the oldest neuroblasts, with ey-positive and ap-positive cells often placed below Dll-positive (in cells that had emerged earlier from the these neuroblasts). By the beginning of pupation, the number of cells origination from the OPC increased. A major reorganization of optic lobe structure occurs around P20 such that the three stripes are no longer distinguishable and the three cell populations are extensively interspersed within the adult medulla cortex.

Dl protein is strongly expressed in neuroepithelial cells of the inner and outer optic anlagen (IPC, OPC) from late second to late third instar larval stages. In the OPC, Dl immunoreactivity is stronger in the medial neuroepithelial cells that border the medulla neuroblasts. Dl protein expression is detected at a lower level in medulla neuroblasts, but is higher in newly generated medulla neurons; however it is not detected in mature medulla neurons or their axons. Dl protein is weakly expressed in the lamina anlage, and in anterior cells in the lamina; it is more strongly expressed as punctate dots in posterior lamina cells.

Dl protein is observed primarily in intracellular vesicles in eye discs though some cytoplasmic staining is seen in cells within the morphogenetic furrow.

Dl protein is localized to the segment boundaries of all leg segments in leg discs.

Dl protein is used as a marker for the embryonic ventral large intestine.

Dl protein is expressed in presumptive wing veins in the wing disc.

Dl protein is expressed in the invaginating ectodermal cells of the keyhole structure of the developing embryonic proventriculus. Dl protein is downregulated in the anterior- and posterior-most cell rows of the keyhole structure after embryonic stage 15.

Dl is expressed along the ventral side of the DV boundary and along the longitudinal veins in the ventral compartment. Dl is not expressed in dorsal cells.

At embryonic stage 11, Dl protein expression is observed in cells surrounding the cells of visceral mesoderm, in particular in the cells surrounding the fusion-competent myoblasts.

Dl protein is rapidly internalized and is detected intracellularly in developing eye discs of third instar larvae.

Well defined staining of crossveins is observed by 23-26 hr APF.

Dl protein is expressed in all microchaeta proneural cells and microchaeta sensory organ precursors (SOPs) and is expressed dynamically in SOP progeny. Dl expression in microchaeta proneural cells is detected before ac expression.

Dl protein is first detected in cells in the morphogenetic furrow. It is primarily accumulated in vesicles located apically within each cell though some cytoplasmic staining is seen. After cells emerge from the furrow, Dl protein is localized exclusively to vesicles that are primarilly localized apically. In earlier rows, Dl protein accumulates in vesicles in R8, R2 and R5. Subsequently, Dl protein disappears from those cells and is seen only in R3 and R4 by row 5. Between rows 6 and 8, vesicles are also observed in R1 and R6 and three rows later, vesicles are apparent in R3, R4, R1, R6, and R7. Dl protein ceases to accumulate in R3, R1, and R6, but continues to be found in R4 and R7 until at least row 14.

Dl protein can be detected throughout oogenesis. It is first detected in the germarium where diffuse cytoplasmic staining and small bright vesicular staining is observed. In stages 1-3, diffuse cytoplasmic staining is again seen. Dl protein accumulates in vesicular features associated with the membranes of nurse cells and oocytes starting in stages 4-5. In stages 5-6, intense staining is observed at the junction between the follicle cells and the nurse cells and oocyte. Dl protein is also apparent in the membranes surrounding the ring canals. By stages 7-8, Dl protein levels fall to background at the membranes at the junction of the oocyte and follicle cells but remain high at the junctions of nurse cells and follicle cells. uring stages 9 and 10A, Dl protein accumulation becomes reduced in the follicle cell, nurse cell, and oocyte membranes and becomes more pronounced in vesicles. Starting in stage 10B-11, Dl protein appears to be transferred to the oocyte from the nurse cells. It is also expressed in a subset of centripetally migrating follicle cells. From stage 11 on, it is expressed at background levels throughout the follicle except at the nurse cell-oocyte border. N protein and Dl protein localization were compared during oogenesis. In the germarium, cytoplasmic N and Dl protein staining are observed. In contrast to Dl protein, more intense N staining is seen in the membranes of follicle cells in regions 2 and 3 of the germarium. Diffuse cytoplasmic staining of N and Dl proteins is bserved in stages 1-6. In contrast to Dl protein, follicle cell membrane staining of N protein is observed during this whole period. In stages 4-5, N and Dl protein accumulation is apically polarized within the membranes of all follicle cells but some N protein is also present in the basal membranes. N and Dl protein staining is also observed in nurse cell membranes and cytoplasm but the membrane staining is stronger for Dl protein than N protein. By stages 7-8, in contrast to Dl protein, N protein is still present in the membranes between oocytes and follicle cells. N protein is expressed in the membranes of all follicle cells that surround the egg chamber in stages 7-9. From stage 9, N protein accumulation decreases in follicle cell membranes but persists in urse cell membranes. N protein also accumulates in two specialized groups of follicle cells situated dorsolaterally at the nurse cell chamber-oocyte junction which eventually form the chorionic appendages. No Dl accumulation is seen in these cells. While Dl protein appears to be transferred from nurse cells to the oocyte during stage 11, N protein is not transferred.

Dl protein is first detected in the cortical membrane of precellular blastoderm embryos. Just before gastrulation, the level of Dl protein decreases in the presumptive mesoderm region of the embryo and profuse vesicular subcellular staining is observed. These vesicles are associated with endocytosis from the membrane. Dl protein is expressed in the ectodermal cells within the neurogenic region and in mesectodermal cells through the waves of neuroblast segregation. Dl protein is not apparent in the segregated neuroblasts but continues to be expressed in the developing epidermis. It is expressed transiently in the mesoderm at the end of neuroblast segregation and is also detected in the procephalic neurogenic region and withinndodermal derivatives including the anterior and posterior midgut invaginations and part of the hindgut. By stage 11, expression is mainly restricted to the developing epidermis and the posterior midgut. During germ band retraction, Dl protein is expressed in a number of tissues including what appears to be the primordia of the optic lobes, the stomatogastric nervous system or antennomaxillary complex, and the epiphysis. It is also expressed in the tracheal trunks, proventriculus, hindgut, pharynx, proesophageal ganglion, and anterior and posterior midguts. It's expression in the ventral nerve cord appears to be restricted to dividing cells both in the midline and in the CNS. In larvae, Dl expression is observed in a number of tissues. Dl protein is expressed in CNS neuroblasts andtheir progeny in all three larval instars and within the developing proliferation centers. It is also expressed in cells along the ventral midline that may be glial. In eye discs, Dl protein expression is first seen on the surfaces of unpatterned cells ahead of the morphogenetic furrow. It is then observed in clusters of cells in the morphogenetic furrow and extending behind the furrow. Expression appears to be restricted to apical vescicles (thought to be multivesicular bodies) near the center of each developing ommatidium. The Dl-expressing cells appear to include the photoreceptor cells. Later Dl protein is expressed in cone cells and in the peripodial membrane. Dl expression is also observed in the antennal portion of the eye-antennal disc. A complex pattern of Dl protein epression is described in the wing disc. Dl expression is observed in nearly all cells of the wing disc but at an elevated level in some areas. These include two bands of cells flanking the anterior wing margin that give rise to sensory organ precursors. Two bands of cells flanking the posterior wing margin also express elevated levels of dl protein and may give rise to non-innervated epidermal hairs. In the notum regions, cells that express elevated Dl protein levels appear to correspond to macrochaeta proneural groups. Six hours after puparium formation, Dl protein is expressed in regions where developing bristles are forming along the anterior wing margin, where epidermal hairs are forming along the posterior wing margin, and within the presumptive wing veins. Two groups of intnsely staining cells in the third longitudinal vein correspond to the developing campaniform sensilla. Finally Dl and N expression are compared in the larval CNS, wing discs, and eye-antennal discs.