A Quantitative Analysis of Growth and Size Regulation in Manduca sexta: The Physiological Basis of Variation in Size and Age at Metamorphosis.

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Body size and development time are important life history traits because they are often highly correlated with fitness. Although the developmental mechanisms that control growth have been well studied, the mechanisms that control how a species-characteristic body size is achieved remain poorly understood. In insects adult body size is determined by the number of larval molts, the size increment at each molt, and the mechanism that determines during which instar larval growth will stop. Adult insects do not grow, so the size at which a larva stops growing determines adult body size. Here we develop a quantitative understanding of the kinetics of growth throughout larval life of Manduca sexta, under different conditions of nutrition and temperature, and for genetic strains with different adult body sizes. We show that the generally accepted view that the size increment at each molt is constant (Dyar's Rule) is systematically violated: there is actually a progressive increase in the size increment from instar to instar that is independent of temperature. In addition, the mass-specific growth rate declines throughout the growth phase in a temperature-dependent manner. We show that growth within an instar follows a truncated Gompertz trajectory. The critical weight, which determines when in an instar a molt will occur, and the threshold size, which determines which instar is the last, are different in genetic strains with different adult body sizes. Under nutrient and temperature stress Manduca has a variable number of larval instars and we show that this is due to the fact that more molts at smaller increments are taken before threshold size is reached. We test whether the new insight into the kinetics of growth and size determination are sufficient to explain body size and development time through a mathematical model that incorporates our quantitative findings.





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Grunert, Laura W, Jameson W Clarke, Chaarushi Ahuja, Harish Eswaran and H Frederik Nijhout (2015). A Quantitative Analysis of Growth and Size Regulation in Manduca sexta: The Physiological Basis of Variation in Size and Age at Metamorphosis. PLoS One, 10(5). p. e0127988. 10.1371/journal.pone.0127988 Retrieved from https://hdl.handle.net/10161/10745.

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H. Frederik Nijhout

John Franklin Crowell Distinguished Professor of Biology

Fred Nijhout is broadly interested in developmental physiology and in the interactions between development and evolution. He has several lines of research ongoing in his laboratory that on the surface may look independent from one another, but all share a conceptual interest in understanding how complex traits arise through, and are affected by, the interaction of genetic and environmental factors. 1) The control of polyphenic development in insects. This work attempts to understand how the insect developmental hormones, ecdysone and juvenile hormone, act to control alternative developmental pathways within a single individual. His studies and those of his students have dealt with the control of sequential polyphenism in metamorphosis, of alternate polyphenisms in caste determination of social insects and the many seasonal forms of insects. 2) The regulation of organ and body size in insects. Ongoing research deals with the mechanism by which insects asses their body size and stop growing when they have achieved a characteristic size. Other studies deal with the control of growth and size of imaginal disks. This work is revealing that the control of body and organ size does not reside in any specific cellular or molecular mechanism but that it is a systems property in which cellular, physiological and environmental signals all contribute in inextricable ways to produce the final phenotype. 3) The development and evolution of color patterns in Lepidoptera. Ongoing research attempts to elucidate the evolution of mimicry using genetic and genomic approaches. 4) The development, genetics and evolution of complex traits. Complex traits are those whose variation is affected by many genes and environmental factors and whose inheritance does not follow Mendel’s laws. In practice this involves understanding how genetic and developmental networks operate when there is allelic variation in their genes. This work attempts to reconstruct complex traits through mathematical models of the genetic and developmental processes by which they originate, and uses these models to study the effects of mutation and selection. Currently metabolic networks are being used to develop a deeper understanding of the functional relationships between genetic variation and trait variation, and of the mechanisms by which genetic and environmental variables interact to produce phenotypes. More on web page: http://www.biology.duke.edu/nijhout/

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