Plastic background colour matching in the springbok mantis

Within-species variation in colour phenotypes is widespread in animals. One mechanism by which such variation can be maintained is plastic background matching, where individuals plastically develop a similar colour to that of their surroundings. A few examples are known from insects that exhibit green-brown colour polyphenisms. But the extent to which plastic colour responses are shaped by other factors, such as genetic variation in plasticity or the interaction of other environmental cues, is poorly understood. Here, we investigate the plasticity of body colouration in the springbok mantis, Miomantis caffra—a species where hatchlings emerge brown in colour and typically change to green but sometimes remain entirely or partly brown through successive moults. We reared 350 mantises from 10 full-sib families on a green or brown background under a high or low temperature and a high or low humidity using a fully-factorial, split-brood design, and recorded colour phenotypes (all green, all brown, or mixed colouration) after 14 weeks of development. We found very strong evidence of developmental plasticity for background matching: the green background induced a higher incidence of the all-green phenotype, whereas the brown background produced more of the all-brown and mixed phenotypes. The all-green phenotype was also universally more common under higher humidity, and under higher temperature when the background was green. However, not all body parts showed the same level of environmental sensitivity: the steepest reaction norms were observed in the mid-legs and hindlegs, potentially reflecting the selection for disruptive colouration of the body outline in browner environments. Using model comparison techniques, we found little evidence of genotype-level variation in colour plasticity—a pattern likely the result of strong viability selection for camouflage. Our study shows how developmental plasticity in colouration can be triggered directly by the colour of the environment and indirectly by climatic cues associated with habitat colouration. We argue that this high level of developmental plasticity has likely evolved due to the diversity of habitats but the sedentary lifestyle of this sit-and-wait predator.

Methods
In a laboratory experiment with a split brood design that used full sib offspring from 10 families, we manipulated temperature (18 versus 28 degrees Celsius), humidity (50% versus 100%), and background colouration (green versus brown) to assess the extent to which colour development in Miomantis caffra is environmentally induced. Fourteen weeks after hatchlings entered the experiment, we recorded the colour of each of 6 major body parts (head, thorax, abdomen, foreleg femur, mid-leg femur, and hindleg femur) as well as the colour of the overall phenotype (all brown, all green, mixed colouration).

动物种内的颜色表型变异广泛存在于动物类群中。维持这类变异的一种机制是可塑性背景匹配（plastic background matching），即个体可通过塑性发育形成与周围环境相似的体色。目前已知少数具备绿-棕颜色多型现象的昆虫类群存在此类案例。然而，关于体色塑性响应在多大程度上受其他因素（如可塑性的遗传变异或其他环境信号的交互作用）调控，目前仍知之甚少。
本研究以南非螳螂（Miomantis caffra）为对象，探究其体色的发育可塑性：该物种的幼体孵化时为棕色，通常会在多次蜕皮后转变为绿色，但有时会完全或部分保持棕色。我们采用全因子分窝实验设计，将来自10个全同胞家系的350只螳螂分别置于绿色或棕色背景下，同时设置高温/低温、高湿/低湿四种环境组合，在饲养14周后记录其颜色表型（全绿、全棕或混合体色）。
研究结果为背景匹配的发育可塑性提供了强有力的证据：绿色背景下全绿表型的发生率更高，而棕色背景下全棕和混合体色的个体更多。全绿表型在高湿环境下普遍更为常见，且在背景为绿色的高温环境中占比更高。不过，不同身体部位的环境敏感性存在差异：中足和后足的反应规范最为显著，这可能反映了在偏棕色的环境中，对身体轮廓干扰色（伪装色）存在选择压力。通过模型比较技术，我们未发现体色可塑性存在显著的基因型水平变异，这一模式可能源于伪装所需的强存活选择作用。
本研究揭示，体色的发育可塑性既可直接由环境颜色触发，也可通过与栖息地颜色相关的气候信号间接调控。我们认为，这种高度的发育可塑性很可能是由于该物种作为伏击型捕食者的静栖生活方式，同时其栖息环境具有多样性，由此演化而来。
方法
本实验采用分窝设计，使用来自10个家系的全同胞后代，通过操控温度（18℃与28℃）、湿度（50%与100%）以及背景颜色（绿色与棕色），评估南非螳螂（Miomantis caffra）的体色发育在多大程度上受环境诱导。在幼体进入实验14周后，我们记录了其6个主要身体部位（头部、胸部、腹部、前足股节、中足股节及后足股节）的颜色，以及整体的颜色表型（全棕色、全绿色或混合体色）。