Temperature-size rule is mediated by thermal plasticity of critical size inDrosophila melanogaster

Abstract

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.

Metamorphosis is induced by food absence rather than a critical weight in the solitary bee, Osmia lignaria

“…1). This result is the complete opposite pattern from that observed in M. sexta and D. melanogaster, which delay metamorphosis if starved below the critical weight (20,22,23,30,35,51,52). While the effects of provision treatment (ANOVA, F 1,185 = 2,928.34, P < 0.0001), body mass (ANOVA, F 1,185 = 121.2, P < 0.0001), and their interaction (ANOVA, F 1,185 = 30.1, P < 0.0001) all weighed heavily on the duration of growth until the prepupal stage, two key problems made the determination of critical weight in O. lignaria untenable.…”

“…1). We observed no larval body mass at which the timing of growth cessation became invariant with respect to food removal, as expected with other critical weights for metamorphosis (20,22,23,30,35,51,52). Second, instead of starvation delaying development, excess feeding delayed development (Fig.…”

“…This suggests that there may be sex-specific differences in the minimum viable weight; however, this could not be confirmed or refuted because larvae, and pupae could not be reliably sexed. To determine the upper limit of body size, we used break-point analysis to calculate a size at which development time became invariant with respect to ad libitum feeding (7,22). We found a significant shift in the relationship between development time and body size with respect to ad libitum feeding at 75.2 mg ( Fig.…”

“…The "critical weight" refers to a mass threshold at which the growing larvae becomes committed to metamorphosis physiologically, "growth cessation" occurs when the larva stops feeding and gut purges before pupation, and the interval between the critical weight and growth cessation is the "interval to the cessation of growth," or "terminal growth period" (12,17,18,20). Collectively, variation in these factors explains adult body size variation in Manduca sexta and Drosophila melanogaster, including variation in response to different environmental conditions (21)(22)(23) and selection under laboratory conditions (24)(25)(26). Of the three factors, the critical weight is the most important, representing the decision to commit to metamorphosis (8,19,20), and is thus a central component of understanding size variation.…”

“…Research in M. sexta and D. melanogaster have demonstrated that oxygen sensing (23), growth of imaginal and endocrine tissues (30)(31)(32)(33)(34), and resource storage (35) contribute to intrinsic assessment of size. The critical weight can respond plastically to environmental factors including prior growth history (36), nutritional quality (21,22), oxygen availability during growth (23,37), and temperature (ref. 20, but see ref.…”