Abstract
BackgroundSexual-size dimorphism (SSD) is replete among animals, but while the selective pressures that drive the evolution of SSD have been well studied, the developmental mechanisms upon which these pressures act are poorly understood. Ours and others’ research has shown that SSD in D. melanogaster reflects elevated levels of nutritional plasticity in females versus males, such that SSD increases with dietary intake and body size, a phenomenon called sex-specific plasticity (SSP). Additional data indicate that while body size in both sexes responds to variation in protein level, only female body size is sensitive to variation in carbohydrate level. Here, we explore whether these difference in sensitivity at the morphological level are reflected by differences in how the insulin/IGF-signaling (IIS) and TOR-signaling pathways respond to changes in carbohydrates and proteins in females versus males, using a nutritional geometry approach.ResultsThe IIS-regulated transcripts of 4E-BP and InR most strongly correlated with body size in females and males, respectively, but neither responded to carbohydrate level and so could not explain the sex-specific response to body size to dietary carbohydrate. Transcripts regulated by TOR-signaling did, however, respond to dietary carbohydrate in a sex-specific manner. In females, expression of dILP5 positively correlated with body size, while expression of dILP2,3 and 8, was elevated on diets with a low concentration of both carbohydrate and protein. In contrast, we detected lower levels of dILP2 and 5 protein in the brains of females fed on low concentration diets. We could not detect any effect of diet on dILP expression in males.ConclusionAlthough females and males show sex-specific transcriptional responses to changes in protein and carbohydrate, the patterns of expression do not support a simple model of the regulation of body-size SSP by either insulin- or TOR-signaling. The data also indicate a complex relationship between carbohydrate and protein level, dILP expression and dILP peptide levels in the brain. In general, diet quality and sex both affect the transcriptional response to changes in diet quantity, and so should be considered in future studies that explore the effect of nutrition on body size.
Highlights
Sexual-size dimorphism (SSD) is replete among animals, but while the selective pressures that drive the evolution of SSD have been well studied, the developmental mechanisms upon which these pressures act are poorly understood
While there is considerable evidence that males and females differ in the extent of their body size response to a variety of environmental variables [7]—a phenomenon called sex-specific plasticity (SSP)—the role that SSP plays in the developmental generation and evolution of sexual size dimorphism is largely unknown
Both male and female body size responded to changes in dietary protein as a negative quadratic, with body size increasing as protein concentration increased, but at a decreasing rate (Table 1)
Summary
Sexual-size dimorphism (SSD) is replete among animals, but while the selective pressures that drive the evolution of SSD have been well studied, the developmental mechanisms upon which these pressures act are poorly understood. Environmental variation can account for the vast majority of variation in body size within a population [4, 5], and if males and females differ in the extent to which the environment affects body size, this will generate changes in SSD across environments [6] This is a form of G × E interaction, where G is genetic sex and E is the environment. While there is considerable evidence that males and females differ in the extent of their body size response to a variety of environmental variables [7]—a phenomenon called sex-specific plasticity (SSP)—the role that SSP plays in the developmental generation and evolution of sexual size dimorphism is largely unknown. Despite an extensive literature on the role that growth rate and developmental time play in generating SSD [8,9,10,11,12,13] and on the genetic mechanisms that determine sex [14,15,16], the developmental mechanisms through which genetic sex affects growth parameters remains largely unknown
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