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WIREs Dev Biol
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Specification of the somatic musculature in Drosophila

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The somatic muscle system formed during Drosophila embryogenesis is required for larvae to hatch, feed, and crawl. This system is replaced in the pupa by a new adult muscle set, responsible for activities such as feeding, walking, and flight. Both the larval and adult muscle systems are comprised of distinct muscle fibers to serve these specific motor functions. In this way, the Drosophila musculature is a valuable model for patterning within a single tissue: while all muscle cells share properties such as the contractile apparatus, properties such as size, position, and number of nuclei are unique for a particular muscle. In the embryo, diversification of muscle fibers relies first on signaling cascades that pattern the mesoderm. Subsequently, the combinatorial expression of specific transcription factors leads muscle fibers to adopt particular sizes, shapes, and orientations. Adult muscle precursors (AMPs), set aside during embryonic development, proliferate during the larval phases and seed the formation of the abdominal, leg, and flight muscles in the adult fly. Adult muscle fibers may either be formed de novo from the fusion of the AMPs, or are created by the binding of AMPs to an existing larval muscle. While less is known about adult muscle specification compared to the larva, expression of specific transcription factors is also important for its diversification. Increasingly, the mechanisms required for the diversification of fly muscle have found parallels in vertebrate systems and mark Drosophila as a robust model system to examine questions about how diverse cell types are generated within an organism. WIREs Dev Biol 2015, 4:357–375. doi: 10.1002/wdev.182 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Signaling Pathways > Cell Fate Signaling
The embryonic and larval muscle pattern. (a) Lateral view of a stage 16 embryo stained with an antibody against myosin heavy chain to reveal the segmentally repeated muscle pattern. Scale bar, 50 µm. For this and all subsequent images, dorsal is up and anterior is left. (b) Close‐up of a single abdominal hemisegment from (a), showing the 30 unique muscles in the hemisegment. Scale bar, 10 µm. (c) External (left) and internal (right) schematics of the abdominal muscle pattern. Muscles can be grouped into four regions: dorsal (DA1, DA2, DO1, DO2), dorsolateral (DO3, DO4, DO5, DA3, DT1, LL1), lateral (LT1‐4, LO1, SBM), and ventral (VA1‐3, VL1‐4, VO1‐6, VT1). The alternate muscle numbering system is shown in parentheses after the muscle name. (Reprinted with permission from Ref . Copyright 2010 Elsevier)
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Adult muscle pattern. Schematic depicting the arrangement of muscles in the adult thorax as seen in a lateral view (a) and a cross‐section through the thorax (b). Anterior is to the left in both panels; dorsal is up in panel (a) and (b) represents a dorsal view. Both groups of indirect flight muscles (IFMs) are shown: 6 pairs of dorsal longitudinal muscles (DLMs, magenta) and 7 pairs of dorsoventral muscles (DVMs, green). Direct flight muscles (DFMs) are shown in light blue, and the tergal depressor of the trochanter (TDT, also known as the jump muscle) is shown in dark blue. (Reprinted with permission from Ref . Copyright 2001 Cell Press. Reprinted with permission from Ref . Copyright 2010 Frontiers)
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Identity genes are expressed in incompletely overlapping subsets of muscles. Schematic of the muscle pattern in a single abdominal hemisegment at stage 16. Muscles are color‐coded to show identity gene expression patterns. A colored outline marks identity gene expression in progenitors that is lost from FCs. Note that some identity genes, such as ap, are expressed in FCs at stage 13 but lost in the final muscle at stage 16. (Reprinted with permission from Ref . Copyright 1998 Cell Press)
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Progenitor and founder cell specification. (a) Cartoon showing the locations of the L'sc‐expressing equivalence groups within a single abdominal hemisegment. (Reprinted with permission from Ref . Copyright 1995 Cold Spring Harbor Laboratory Press) (b) Progenitors (P) divide to give rise to FCs for particular muscles as well as pericardial cells (PCs) and dorsal, lateral, and ventral adult muscle precursors (DAMPs, LAMPs, and VAMPs, respectively). Sibling FCs are depicted, and the number of the equivalence group (subscript) from which the progenitor arose is indicated if known. (c) Stereotypical arrangement of FCs in a single abdominal hemisegment at stage 12. Sibling FCs share the same color. (Reprinted with permission from Ref . Copyright 2007 Elsevier)
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Mesodermal subdivision and equivalence group formation. (a) Cartoon of a stage 10 embryo showing Twi expression modulated into high (dark blue) and low (light blue) domains. Cells of the high Twi domain give rise to all somatic muscles. (b) Cartoon of a single hemisegment, showing mesodermal subdivision and the tissues that arise from those regions. CM, cardiac muscle (pink); VM, visceral muscle (orange); FB, fat body (purple); MG, mesodermal glia cells (gray). The somatic muscle is colored green with yellow nuclei. (c) Expression of L'sc marks groups of mesodermal cells that are competent to become muscle progenitors. Lateral inhibition restricts l'sc expression to a single progenitor. Cells that lose L'sc expression become FCMs. Asymmetric division of progenitors generate two FCs (blue and purple) or an FC and an AMP (blue and red, respectively). (Reprinted with permission from Ref . Copyright 1998 Cell Press)
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