The origin of islet-like cells in Drosophila identifies parallels to the vertebrate endocrine axis

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   Fig. 1.

   IPC lineage development in the Drosophila brain. The diagram, at top left, shows the positions of the IPCs, within in the pars intercerebralis, and the CC cells within the ring gland, relative to the embryonic and larval protocerebrum. All brains labeled by antibodies are as indicated with the text color corresponding to color channels in merged images. The position of the midline is indicated by dotted lines with anterior to the left when midline is horizontal, or anterior to the top when midline is vertical. Embryonic stages of labeled brains are as indicated. For an indication of scale, note that the individual cells of the IPC lineage are typically 3–8 μm at all stages. (a) Dorsal view of IPCs in third-instar larval brain shows the specificity of the tio enhancer for IPCs (arrows). (b) IPCs in the pharate larval brain express tio and Dac (arrow). (c) Before onset of Dilp2 expression, tio ⁺ cells comprise the anterior part of a 10- to 12-cell cluster of Dac⁺ Ey⁺ cells (arrow). (d) The expansion of the Dac⁺ IPC lineage from one cell at stage 12 to 10–12 cells by stage 17 (arrows). (Upper) Series shows Dac expression only. (Lower) Series shows the Dac⁺ cluster expands on the posterior-lateral side of the anterior PI (6), marked by dChx1. (e) The Dac⁺ IPC lineage is marked by a single marked clone at early stage 17 (red arrow). The two focal planes show the entire Dac⁺ group. Ey expression appears in the older anterior cells of the Dac⁺ group at this stage. (f and g) The IPC lineage is produced from a dividing Dpn⁺ Dac⁺ neuroblast with membrane-localized Pros (red arrow). At stage 12 (f), neuroblast has divided once and the GMC daughter cell shows nuclear localized Pros (white arrow). At stage 16 (g), the neuroblast is still present (red arrow), as are the GMCs (white arrow). Cells to the anterior of the Dac⁺ group are early IPCs, lacking both Dpn and Pros expression (yellow arrow). (h) IPCs arise from the first-born GMCs. tio expression (white arrow), which labels the IPCs, is in the anterior of the Dac⁺ group, whereas the IPC neuroblast is at the posterior (red arrow). (i) Lineage model for IPCs. (j) The dChx1⁺ Cas⁺ L'Sc⁺ proneural group, which gives rise to the IPC neuroblast, in the anterior neuroectoderm (area in dotted outline, arrow). (k) The dChx1⁺ Cas⁺ cells delaminate as a chain of neuroblasts (bracket). The first to delaminate is the Dac⁺ IPC neuroblast (arrow). (Scale bar: 5 μm.)

   
   
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   Fig. 2.

   CC cell lineage development in the Drosophila brain. All brains labeled by antibodies are as indicated with the text color corresponding to color channels in merged images. The position of the midline is indicated by dotted lines with anterior to the left when midline is horizontal, or anterior to the top when midline is vertical. Embryonic stages of labeled brain are as indicated. For an indication of scale, note that the individual cells of the CC lineage are typically 3–8 μm at all stages. (a) Gl expression labels the CC cell precursor group. The CC cell cluster (red arrows) expands from a single cell (stage 11) to a group of six to eight (stage 16). The cell cluster migrates out of the developing brain (stage 13), along the developing foregut (stage 14), to coalesce in the presumptive ring gland (stage 16). (b) The first Gl⁺ cells to appear (white arrow) are gt1⁺ and are within a cell diameter of the dChx1⁺ Cas⁺ neuroectoderm (red arrow). (c–e) In stage 10–11 embryos, the CC cell precursor group is associated with a Kr-GFP ⁺ neuroblast. (c) The Gl⁺ cells are labeled by the Kr-GFP reporter (arrow), as is an adjacent neuroblast that is Dpn⁺ (d, arrow) and that has membrane localized Pros (e, arrow). (f) The Gl⁺ CC cell lineage is marked by a single marked clone at early stage 17 (red arrow). The CC cell group from the contralateral brain hemisphere is not labeled by the clone. (g–l) Early gene expression within the CC cell group in stage 10–11 embryos. (g) The first Gl⁺ cell to appear expresses So and gt1 (arrow). (h) The Kr-GFP ⁺ CC cell group expresses So and Eya. (i) So and gt1 coexpression in the surface neuroectoderm epithelium (arrow). (j) 1–2 Dpn⁺ neuroblasts arise from the So⁺ gt1 neuroectoderm (arrow). (k and l) Gl⁺ CC cell precursors are a component of the gt head stripe 1 (k, arrows), but not the gt head stripes 2 and 3 (l, arrows). (Scale bar: 5 μm.)

   
   
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   Fig. 3.

   The gt head stripe 1 in stage 10–11 embryos has molecular similarity to the vertebrate hypohyseal placode. (a) A summary diagram of expression labeling data from b–f, which shows the subdivision of neuroectoderm and underlying neuroblast lineages into 2–12 cell groups with common gene expression. The color-coding for gene expression is given in the diagram. The CC cell and IPC neuroblast progenitors delaminate from the surface neuroectoderm at stages 10 and 12, respectively. At stage 13, regions B (anterior PI) and D (Pars lateralis) invaginate to form neurogenic placodes underneath the surface neuroectoderm and continue to generate neuroblasts (6). The CC cell and IPC neuroblasts are not included in the brain neuroblast map of Urbach and Technau (35) (data not shown). (b–f) All brains labeled by antibodies are as indicated with the text color corresponding to color channels in merged images. Letter labels correspond to those in the diagram (a). Anterior is to the left. (b) Embryo head with dotted outline showing region of placode gene expression (So, D-six4, Optix, and Eya). The position of the midline is indicated by dotted line. Expression patterns of Eya, Cas, and dChx1. (c) Expression patterns of Eya, Optix, and dChx1. Region B shows low-level Eya expression. (d) Expression patterns of Eya, Cas, and gt1. Region F shows high-level gt1 expression. (e) Expression patterns of So, Cas, and dChx1. The Mz-VUM enhancer of the Dchx1 gene (shown) labels the dChx1⁺ cells of region A at this stage. (f) Expression patterns of Fas2, gt1, and Cas. Fas2 labeling of region D corresponds to the developing Pars lateralis (6). (g) Comparison of hypophyseal gene expression (Optix/Six6 and Eya) in the fate maps of mouse and fly during the early development of the brain endocrine axis and pharynx. The neurohypohyseal diverticulum of the infundibuar region (ir) will give rise to neurosecretory hypothalamic cells (neuhypophysis). Rathke's pouch (rp) is an invagination of the oral ectoderm that gives rise to the anterior pituitary (adenohypohysis). In both mammals and flies, Optix/Six6⁺ Eya cells are fated to become either pharynx (ph) or endocrine progenitors. (Scale bar: 5 μm.)