The Physiological Control of Body Size

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The sizes and shapes of bodies and appendages are the primary characteristics by which we recognize species of animals. The different shapes of different species are due to differences in the relative growth of body parts. Although much is known about how growth is controlled, little is known about the mechanisms that cause growth to stop when an animal reaches a species-specific size. How a growing animal assesses its body size is not known for any vertebrate, but is beginning to be understood in insects. The goal of the research is to expand upon recent discoveries about the mechanisms by which insects sense their body size, and to investigate how those lead to the cessation of growth. The project also investigates the mechanism by which insect wings grow in correct proportion to body size, using giant and dwarf genetic strains, as well as species that differ in body and wing size. The control of wing size involves signals emanating from the body wall muscles that control precisely for how long wings grow. This research will, for the first time, provide a detailed explanation of the difficult problem of how insects monitor and control their body size. The award supports training for college and high school students, as well as high school teachers in STEM fields, in physiological research, data analysis and presentation. The project tests hypotheses based on several recent discoveries about the physiological mechanisms by which insects sense their body size, and how those lead to the cessation of growth. Aim 1 investigates the mechanism that controls the size at which a growing larva molts to the next instar. This is called the critical weight, which is defined by the non-growing tracheal system that triggers the molt to the next larger instar when it is no longer able to supply sufficient oxygen to the ever-growing body. This trigger induces the secretion of the hormone ecdysone, which causes a molt to the next larger instar. Aim 2 investigates the mechanism that assesses the size of an instar and determines whether the next molt stops growth and initiates metamorphosis. This is called the threshold size, which is defined by the rise in myoglianin, a protein from muscles of the body wall, that causes the decline of juvenile hormone, that, in turn, triggers a pulse of ecdysone secretion, stopping growth altogether. Aim 3 investigates the mechanism by which wings grow to the correct proportion to body size. This is a special problem because wings do not begin to grow until after feeding and somatic growth have stopped. Body size after growth stops is also measured by myoglianin levels, which control the duration and level of ecdysone secretion, controlling the rate and duration of cell division in the wing so that the wing grows in correct proportion to body size. Graduate and undergraduate students and high school students and teachers will contribute to the research, contributing to STEM workforce development at multiple levels. The research team will also contribute to public education activities at local children’s museums.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.