Plasticity and scaling
The ability ability of an organism to respond to its environment is a defining quality of life. However, why are some characteristics or individuals sensitive to environmental change whilst others are not? We identified the mechanism that controls the response of growing organs to a particularly important environmental factor – developmental nutrition. In all animals, a decrease in developmental nutrition reduces final body and organ size. However, the size of some organs is less responsive to changes in nutrition than others. In a male fruit fly it is the size of the genitals that is resistant to dietary restriction. This is achieved by the male fruit fly reducing expression of a key gene in their genitalia. This gene, FOXO, forms part of the insulin signaling system, which signals food levels to tissues in all animals. By lowering the production of FOXO, the genitalia are able to ‘ignore’ hormonal signals that tell the rest of the body to grow slowly due to limited food. The ability of tissues to become insensitive to nutritional information is a characteristic of many tumors and also underlies type 2 diabetes. Our data may therefore provide insight into the origin and treatment of both conditions.
Excitingly, our data also reveal the developmental mechanisms that regulate morphological scaling. By reducing their nutritional plasticity, the genitals of Drosophila are the same size in large and small individual, a condition called hypoallometry. Morphological diversity is dominated by variation in scaling, and yet the developmental mechanisms that regulate the relationship between organ size and body size have remained elusive - until now. Our data is the first example of a mechanism that controls how organs scale with each other and with body size.
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