Many ways to be small: different environmental regulators of size generate distinct scaling relationships inDrosophila melanogaster

Abstract

Static allometries, the scaling relationship between body and trait size, describe the shape of animals in a population or species, and are generated in response to variation in genetic or environmental regulators of size. In principle, allometries may vary with the different size regulators that generate them, which can be problematic since allometric differences are also used to infer patterns of selection on morphology. We test this hypothesis by examining the patterns of scaling in Drosophila melanogaster subjected to variation in three environmental regulators of size: nutrition, temperature and rearing density. Our data indicate that different environmental regulators of size do indeed generate different patterns of scaling. Consequently, flies that are ostensibly the same size may have very different body proportions. These data indicate that trait size is not simply a read-out of body size, but that different environmental factors may regulate body and trait size, and the relationship between the two, through different developmental mechanisms. It may therefore be difficult to infer selective pressures that shape scaling relationships in a wild population without first elucidating the environmental and genetic factors that generate size variation among members of the population.

Comparison of the Allometry Coefficient of the Length-Weight and Length-Length Relationship between Selene brownii, S. vomer and S. setapinnis Caught in the Gulf of Mexico

“…When the value of b is <3, there can be said to be a negative allometric growth; when the value of b is >3, there is positive allometric growth (Shingleton et al, 2009). The value "b" was tested using a Student's t-test (Ho: b= 3 ∂= 5%) (Sokal & Rohlf).…”

Plastic flies

“…To test this we examined flies that were mutant for the developmental temperature (smaller at higher temperatures) than to changes in developmental nutrition (smaller at lower nutrition), when normalized against variation in overall body size. 12 Thus, there does not appear to be a single buffering mechanism for wing size. On the other hand, male genital size shows low levels of variability in response to both sources of environmental variation and also in response to genetic variation, supporting a common mechanism.…”

“…We have shown that environmental variability in trait size in Drosophila depends on the source of environmental variation. 12 For example, wing size is much more variable in response to changes in insulin-receptor (Inr). Mutations of the insulin receptor (Inr) genocopy starvation and, consistent with our hypothesis, caused a more substantial reduction in final wing size than final genital size.…”

“…Some organs, for example the human brain, are able to protect themselves from the effects of malnutrition and grow to more or less the same size in both well fed and poorly fed individuals. 12,16 Our hypothesis was that an organ's nutritional sensitivity is regulated by its individual response to changes in IIS. 25 We used the different adult organs of Drosophila to test this hypothesis, comparing size regulation of the nutritionally plastic wings with size regulation of the nutritionally implastic male genitalia.…”

“…25 We used the different adult organs of Drosophila to test this hypothesis, comparing size regulation of the nutritionally plastic wings with size regulation of the nutritionally implastic male genitalia. 12,25 Because adult insects have a hard exoskeleton, the size of the adult organs is controlled by their growth during the feeding larval stages. In holometabolous insects, adult organs grow as imaginal discs within the developing larva, which are sacks of cells sequestered from the epidermis during embryogenesis.…”