The difference in body size between females and males, or sexual size dimorphism (SSD), is ubiquitous, and yet we have a poor understanding of the developmental-genetic mechanisms that generate it, and how these mechanisms may vary within and among species. Such an understanding of the genetic architecture of SSD is important if we are to evaluate alternative models of SSD evolution, but is difficult to describe because SSD is a characteristic of populations, not individuals. Here, we overcome this challenge by using isogenic lineages of Drosophila to measure SSD for 196 genotypes. We demonstrate extensive genetic variation for SSD, primarily driven by higher levels of genetic variation for body size among females than males. While we observe a general increase in SSD with sex-averaged body size (pooling for sex) among lineages, the vast majority of variation in SSD is independent of sex-averaged body size, and shows a strong genetic correlation with sex-specific plasticity, such that increased female-biased SSD is associated with increased body-size plasticity in females. Our data are consistent with the condition-dependence hypothesis of sexual dimorphism, and suggest that SSD in Drosophila is a consequence of selection on the developmental-genetic mechanisms that regulate the plasticity of body size.  

We collected data from 196 DGRP lineages. For each lineage, flies were reared following previously publish protocols. Eggs were collected over 12-20 h, transferred in lots of 50 into 7ml fly food, and the larvae were reared at 22˚C, in standard cornmeal-molasses medium until the starvation treatment was applied. Larvae were removed from food at precisely timed developmental stages and starved starting at either 0–24 h or 24-48h before pupation to generate variation in body size. Because larvae stop feeding approximately 24h before pupation, larvae removed from the food 0–24 h before pupation were essentially allowed to feed ad libitum and are referred to as fed flies. In contrast, larvae removed from the food 24-48h before pupation were starved during the period when adult body size is affected by nutrition, and are referred to as starved flies. After being removed from the food, larvae were transferred to an empty vial containing a wet cotton plug and left until pupariation. To allow us to associate each adult fly with its pupal case post-eclosion, pupae were transferred to individual 2.5 mL Eppendorf tubes with a small puncture in the top for gas exchange. After flies emerged from their pupal cases, they were sexed and the pupal case was imaged and size quantified using semi-automated software developed in the Shingleton lab. We used area of the pupal silhouette (dorsal view) as a proxy for adult body size. Flies were collected in nine temporal blocks, with five lineages repeated across blocks to serve as a control.