The selective pressures leading to the evolution of Sexual Size Dimorphism (SSD) have been well studied in many organisms, yet, the underlying developmental mechanisms are poorly understood. By generating a complete growth profile by sex in Drosophila melanogaster, we describe the sex-specific pattern of growth responsible for SSD. Growth rate and critical size for pupariation significantly contributed to adult SSD, whereas duration of growth did not. Surprisingly, SSD at peak larval mass was twice that of the uneclosed adult SSD with weight loss between peak larval mass and pupariation playing an important role in generating the final SSD. Our finding that weight loss is an important regulator of SSD adds additional complexity to our understanding of how body size is regulated in different sexes. Collectively, these data allow for the elucidation of the molecular-genetic mechanisms that generate SSD, an important component of understanding how SSD evolves.

Most ectotherms show an inverse relationship between developmental temperature and body size, a phenomenon known as the temperature-size rule (TSR). Several competing hypotheses have been proposed to explain its occurrence. According to one set of views, the TSR results from inevitable biophysical effects of temperature on the rates of growth and differentiation, whereas other views suggest the TSR is an adaptation that can be achieved by a diversity of mechanisms in different taxa. Our data reveal that the fruitfly, Drosophila melanogaster, obeys the TSR using a novel mechanism: reduction in critical size at higher temperatures. In holometabolous insects, attainment of critical size initiates the hormonal cascade that terminates growth, and hence, Drosophila larvae appear to instigate the signal to stop growth at a smaller size at higher temperatures. This is in contrast to findings from another holometabolous insect, Manduca sexta, in which the TSR results from the effect of temperature on the rate and duration of growth. This contrast suggests that there is no single mechanism that accounts for the TSR. Instead, the TSR appears to be an adaptation that is achieved at a proximate level through different mechanisms in different taxa.

SUMMARYRearing oxygen level is known to affect final body size in a variety of insects, but the physiological mechanisms by which oxygen affects size are incompletely understood. In Manduca sexta and Drosophila melanogaster, the larval size at which metamorphosis is initiated largely determines adult size, and metamorphosis is initiated when larvae attain a critical mass. We hypothesized that oxygen effects on final size might be mediated by oxygen effects on the critical weight and the ecdysone titers, which regulate growth rate and the timing of developmental transitions. Our results showed that oxygen affected critical weight, the basal ecdysone titers and the timing of the ecdysone peak, providing clear evidence that oxygen affected growth rate and developmental rate. Hypoxic third instar larvae (10% oxygen) exhibited a reduced critical weight, slower growth rate, delayed pupariation, elevated baseline ecdysone levels and a delayed ecdysone peak that occurred at a lower larval mass. Hyperoxic larvae exhibited increased basal ecdysone levels, but no change in critical weight compared with normoxic larvae and no significant change in timing of pupariation. Previous studies have shown that nutrition is crucial for regulating growth rate and the timing of developmental transitions. Here we show that oxygen level is one of multiple cues that together regulate adult size and the timing and dynamics of growth, developmental rate and ecdysone signaling. Supplementary material available online at

We show that two complementary asymmetric isolating mechanisms, likely mediated by divergence in body size, underlie the evolution of incipient reproductive isolation between a set of Drosophila melanogaster populations selected for rapid development and their ancestral controls. Selection has led to great reduction in body size in the fast developing lines. Small males belonging to fast developing lines obtain few matings with large control females, both in presence and absence of large control line males, giving rise to unidirectional, premating isolation caused by sexual selection. Conversely, small selected line females suffer greatly increased mortality following mating with large control males, causing unidirectional postcopulatory prezygotic isolation. We discuss preliminary evidence for evolution of reduced male harm caused to females upon mating in the fast developing lines, and speculate that the females from these lines have coevolved reduced resistance to male harm such that they can no longer resist the harm caused by males from control lines. This potentially implicates differing levels of sexual conflict in creating reproductive barrier between the selected line females and the control males. We also show that a large difference in development time is not sufficient to cause postzygotic incompatibilities in the two sets of populations reaffirming the belief that prezygotic isolation can evolve much earlier than postzygotic isolation.

In most ectotherms, a reduction in developmental temperature leads to an increase in body size, a phenomenon known as the temperature size rule (TSR). In Drosophila melanogaster, temperature affects body size primarily by affecting critical size, the point in development when larvae initiate the hormonal cascade that stops growth and starts metamorphosis. However, while the thermal plasticity of critical size can explain the effect of temperature on overall body size, it cannot entirely account for the effect of temperature on the size of individual traits, which vary in their thermal sensitivity. Specifically, the legs and male genitalia show reduced thermal plasticity for size, while the wings show elevated thermal plasticity, relative to overall body size. Here, we show that these differences in thermal plasticity among traits reflect, in part, differences in the effect of temperature on the rates of cell proliferation during trait growth. Counterintuitively, the elevated thermal plasticity of the wings is due to canalization in the rate of cell proliferation across temperatures. The opposite is true for the legs. These data reveal that environmental canalization at one level of organization may explain plasticity at another, and vice versa.

Canalization denotes the robustness of a trait against genetic or environmental perturbation. Plasticity, in contrast, indicates the environmental sensitivity of a trait. Stabilizing selection is often thought to increase the canalization of a trait, whereas directional selection is often thought to lead to decanalization. However, the relationship between selection, canalization, and plasticity remains largely unclear. Using experimental evolution, here, we ask whether long-term directional selection for reduced pre-adult development time in Drosophila melanogaster results in the evolution of increased canalization for development time, the trait under primary selection. We additionally investigate whether pre-adult survivorship, a trait only secondarily under selection in this experimental regime, also evolves to become canalized. We examine canalization both in terms of stability of population means and of within population variability across two environmental axes, reflecting macro-and microenvironmental canalization, respectively. We used four large outbred populations of D. melanogaster selected for rapid pre-adult development and early reproduction for 295 generations, and four corresponding ancestral control populations that were not under conscious selection for development time or early reproduction. The selected populations had evolved ∼25% reduction in both development time and pre-adult survivorship at the time of this study. We studied development time and pre-adult survivorship of the selected populations and controls across various combinations of rearing temperature and larval density. Development time in the selected populations had become more canalized than controls against both macro-and microenvironmental variation, but only with regard to density, and not temperature. Canalization of development time across density appears to have evolved due to evolutionary changes in the life-history and physiology of the selected populations. Pre-adult survivorship, only a secondary correlate of fitness in the selected populations, did not show any clear trend in terms of canalization with regard to either density or temperature, and overall variation in the trait was greater compared to development time within and across environments. Whether long-term directional Ghosh et al. Canalization of Drosophila Development Time selection canalizes or not, therefore, appears to be dependent in a complex way on specific interactions of trait, selection regime, and environmental factor in the context of the ecology and physiology of the populations under study.

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to either density or temperature, and, overall variation in the trait was greater compared to development time within and across environments. Whether long-term directional selection canalizes or not, therefore, appears to be dependent in a complex way on specific interactions of trait, selection regime and environmental factor in the context of the ecology and physiology of the popualtions under study.