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WIREs Dev Biol
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The developmental control of size in insects

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The mechanisms that control the sizes of a body and its many parts remain among the great puzzles in developmental biology. Why do animals grow to a species‐specific body size, and how is the relative growth of their body parts controlled to so they grow to the right size, and in the correct proportion with body size, giving an animal its species‐characteristic shape? Control of size must involve mechanisms that somehow assess some aspect of size and are upstream of mechanisms that regulate growth. These mechanisms are now beginning to be understood in the insects, in particular in Manduca sexta and Drosophila melanogaster. The control of size requires control of the rate of growth and control of the cessation of growth. Growth is controlled by genetic and environmental factors. Insulin and ecdysone, their receptors, and intracellular signaling pathways are the principal genetic regulators of growth. The secretion of these growth hormones, in turn, is controlled by complex interactions of other endocrine and molecular mechanisms, by environmental factors such as nutrition, and by the physiological mechanisms that sense body size. Although the general mechanisms of growth regulation appear to be widely shared, the mechanisms that regulate final size can be quite diverse. WIREs Dev Biol 2014, 3:113–134. doi: 10.1002/wdev.124 This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms

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Insulin‐nutrition response pathways that regulate growth. This is the general pathway discussed in the text. Nutrients stimulate the insulin, prothoracicotropic hormone (PTTH)/ecdysone, and target of rapamycin (TOR) pathways. Although the entire pathway is potentially present in all cells, not all these reactions occur at the same time in development, and different tissues such as fat body and imaginal discs may express different subsets of this complex signaling system. Juvenile hormone (JH) can inhibit or direct ecdysone signaling depending on tissue and developmental stage.
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Broad mediates the effects of ecdysone and juvenile hormone (JH) on pupal gene expression during metamorphosis. During larval molts JH inhibits Broad so larval genes remain active and pupal gene expression is repressed. In the last larval stage of Manduca (but neither in earlier instars nor in Drosophila) JH also inhibits prothoracicotropic hormone (PTTH) secretion. When JH disappears in the last larval instar, ecdysone‐stimulated activation of Broad induces pupal commitment and represses expression of larval genes.
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Roles of genes, nutrition, and photoperiod in regulating body size in Manduca. Body size depends on the signal that causes growth to stop. In insects this signal is the secretion of ecdysone, whose secretion is regulated by prothoracicotropic hormone (PTTH). PTTH secretion is inhibited by juvenile hormone (JH) and gated by the photoperiod. The top panel illustrates a hypothetical scenario in which genetic differences in the rate of JH breakdown affects the duration of the period required for JH elimination after the larva passes the critical weight (red and blue bars). A slower rate of JH decay results in a larger body size. The bottom panel illustrates the effect of environmental variation on growth rate. Slower growth rate (without a change in the rate of JH decay) can cause larvae to miss a photoperiodic gate and stop growing at the next one, at a larger body size.
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Interorgan signaling pathways that regulate growth. Growth depends on nutrient input, but is mediated by target of rapamycin (TOR), insulin (insulin‐like peptide, ILP), and ecdysone signaling in each tissue. Details of the intracellular signaling pathways are shown in Figure . The prothoracicotropic hormone (PTTH)‐ecdysone axis appears to be universal in insects, as are the ILP pathways and the amino acid‐TOR axis. Other interactions illustrated have been studied in only one or two species and in relatively few tissues.
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Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing