Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON M5S 3B2, Canada;
Department of Biology, Lund University, SE-223 62 Lund, Sweden.
Proc Natl Acad Sci U S A. 2021 Feb 16;118(7). doi: 10.1073/pnas.2017185118.
Developing organisms typically mature earlier and at larger sizes in favorable growth conditions, while in rarer cases, maturity is delayed. The rarer reaction norm is easily accommodated by general life history models, whereas the common pattern is not. Theory suggests that a solution to this paradox lies in the existence of critical size thresholds at which maturation or metamorphosis can commence, and in the evolution of these threshold sizes in response to environmental variation. For example, ephemeral environments might favor the evolution of smaller thresholds, enabling earlier maturation. The threshold model makes two unique and untested predictions. First, reaction norms for age and size should steepen, and even change sign, with decreases in threshold size; second, food reductions at sizes below the threshold should delay maturation, while those occurring after the threshold should accelerate maturation. We test these predictions through food manipulations in five damselfly species that theory suggests should differ in threshold size. The results provide strong support for the threshold model's predictions. In all species, early food reductions delayed maturation, while late reductions accelerated maturation. Reaction norms were steeper, and the effect of food reductions changed from decelerating to accelerating at a much smaller size in species from ephemeral habitats. These results support the view that developmental thresholds can account for the widespread observation of negative correlations between age and size at maturity. Moreover, evolution of the threshold appears to be both predictable and central to the observed diversity of reaction norms for age and size at maturity.
在有利的生长条件下，发育中的生物通常会更早、更大地成熟，而在更罕见的情况下，成熟会被延迟。更罕见的反应规范很容易被一般的生活史模型所适应，而常见的模式则不然。理论表明，解决这个悖论的方法在于存在临界大小阈值，在这些阈值上，成熟或变态可以开始，并且这些阈值大小可以响应环境变化而进化。例如，短暂的环境可能有利于较小阈值的进化，从而使成熟更早发生。阈值模型做出了两个独特且未经测试的预测。首先，随着阈值大小的减小，年龄和大小的反应规范应该变陡，甚至改变符号；其次，在阈值以下的大小减少食物应该延迟成熟，而在阈值之后的大小减少食物应该加速成熟。我们通过对五种蜻蜓物种进行食物处理来检验这些预测，理论上这些物种的阈值大小应该有所不同。结果强烈支持阈值模型的预测。在所有物种中，早期减少食物会延迟成熟，而晚期减少食物会加速成熟。在来自短暂栖息地的物种中，反应规范变得更陡峭，并且食物减少的效果从减速变为加速的尺寸要小得多。这些结果支持这样一种观点，即发育阈值可以解释在成熟时年龄和大小之间普遍存在的负相关关系。此外，阈值的进化似乎是可预测的，并且是观察到的成熟时年龄和大小的反应规范多样性的核心。
