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Patterning the Drosophila embryo: A paradigm for RNA‐based developmental genetic regulation

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Abstract Embryonic anterior–posterior patterning is established in Drosophila melanogaster by maternally expressed genes. The mRNAs of several of these genes accumulate at either the anterior or posterior pole of the oocyte via a number of mechanisms. Many of these mRNAs are also under elaborate translational regulation. Asymmetric RNA localization coupled with spatially restricted translation ensures that their proteins are restricted to the position necessary for the developmental process that they drive. Bicoid (Bcd), the anterior determinant, and Oskar (Osk), the determinant for primordial germ cells and posterior patterning, have been studied particularly closely. In early embryos an anterior–posterior gradient of Bcd is established, activating transcription of different sets of zygotic genes depending on local Bcd concentration. At the posterior pole, Osk seeds formation of polar granules, ribonucleoprotein complexes that accumulate further mRNAs and proteins involved in posterior patterning and germ cell specification. After fertilization, polar granules associate with posterior nuclei and mature into nuclear germ granules. Osk accumulates in these granules, and either by itself or as part of the granules, stimulates germ cell division. This article is categorized under: RNA Export and Localization > RNA Localization Translation > Translation Regulation RNA in Disease and Development > RNA in Development
Cartoon illustrating various stages of Drosophila oogenesis, with sites of bcd RNA accumulation shown in blue. Germline stem cells (GSCs) at the anterior end of the germarium undergo four transit‐amplifying divisions and become enveloped by follicle cells (fc) to form egg chambers. The four mitotic divisions that germ cells undergo have incomplete cytokinesis, so the 16 germline cells remain interconnected by cytoplasmic bridges called ring canals. One of the germline cells that is connected to four other germline cells by ring canals becomes specified as the oocyte (oo) and is always located at the posterior end of the egg chamber. The other germline cells become nurse cells (nc). Nurse cells synthesize RNAs and proteins that can be transferred into the oocyte through ring canals. After stage 10, the cytoplasm of nurse cells is rapidly transferred into the oocyte. The nurse cells subsequently die and the mature stage 14 egg remains
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Cartoon illustrating the structured and heterogeneous nature of polar granules. Osk, Vas, Aub, and Tud proteins accumulate in the core region of polar granules along with cycB and nos mRNAs. pgc and gcl mRNAs accumulate in more peripheral regions (Trcek et al., 2015). Some polar granules contain only one species of mRNA while others accumulate several (Little, Sinsimer, Lee, Wieschaus, & Gavis, 2015)
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osk is under complex translational control. (a). Unlocalized osk is translationally repressed by oligomerization and by the translational repressor Bru1. osk is oligomerized by PTB, which can bind to multiple sites on the osk 3’ UTR. Bru1 binds to specific sites in the osk 3’ UTR and recruits Cup, a competitive inhibitor for eukaryotic initiation factor (eIF) 4E (4E) binding to eIF4G. (b). In the pole plasm osk translation is activated. Makorin1 (Mkrn1) displaces Bru1 and recruits poly(A) binding protein (pAbp), which stimulates translation
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osk mRNA localization proceeds through three distinct mechanisms. In early oogenesis (top panel), a microtubule organizing center (MTOC) forms in the oocyte, and microtubules (green lines) emanate from there through the ring canals into the nurse cells. The motor protein dynein transports osk (blue) along these microtubules into the oocyte. During stages 8–9 of oogenesis (middle panel), microtubules (shaded green) reorganize so that their minus‐ends accumulate all around the oocyte cortex except at the posterior pole. The plus‐end directed motor kinesin drives osk (blue) away from the cortex and it accumulates at the posterior thanks to a weak plus‐end microtubule bias. In late oogenesis (lower panel) rapid cytoplasmic streaming (red arrows) takes place, which transports macromolecules throughout the oocyte cytoplasm. osk mRNA is carried along by these movements and is anchored at the posterior, where there is a relative absence of microtubules, by myosin V
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Early Drosophila embryos are syncytial, containing many nuclei that share a common cytoplasm. From a tight anterior cap of bcd RNA (blue, top panel) a gradient of Bcd protein (green, lower panel) is produced, so that nuclei are exposed to different levels of Bcd depending upon their position along the anterior–posterior gradient. Somatic nuclei are depicted in yellow while primordial germ cells (pole cells) are in red
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RNA in Disease and Development > RNA in Development
Translation > Translation Regulation
RNA Export and Localization > RNA Localization

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