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

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 fruit fly, Drosophila melanogaster, obeys the TSR using a novel mechanism: reduction of 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.

绝大多数变温动物（ectotherms）的发育温度与体型呈负相关关系，这一现象被称为温度体型规则（temperature size rule, TSR）。目前学界已提出多种相互竞争的假说，用以阐释该现象的成因。一派观点认为，温度体型规则源于温度对生长与分化速率不可避免的生物物理效应；而另一类观点则主张，温度体型规则是一种适应性特征，可通过不同类群（taxa）的多样机制实现。本研究数据显示，黑腹果蝇（Drosophila melanogaster）通过一种全新机制契合温度体型规则：在较高温度下降低其临界体型（critical size）。在全变态昆虫（holometabolous insects）中，临界体型的达成会启动终止生长的激素级联反应；据此，黑腹果蝇幼虫似乎会在较高温度下以更小的体型触发停止生长的信号。这与另一全变态昆虫——烟草天蛾（Manduca sexta）的研究发现相悖：烟草天蛾的温度体型规则源于温度对其生长速率与生长时长的影响。这一差异表明，温度体型规则并无统一的解释机制。相反，温度体型规则属于适应性特征，在近因层面上可通过不同类群的差异化机制得以实现。