Co-Gradient Variation in Growth Rate and Development Time of a Broadly Distributed Butterfly

Widespread species often show geographic variation in thermally-sensitive traits, providing insight into how species respond to shifts in temperature through time. Such patterns may arise from phenotypic plasticity, genetic adaptation, or their interaction. In some cases, the effects of genotype and temperature may act together to reduce, or to exacerbate, phenotypic variation in fitness-related traits across varying thermal environments. We find evidence for such interactions in life-history traits of Heteronympha merope, a butterfly distributed across a broad latitudinal gradient in south-eastern Australia. We show that body size in this butterfly is negatively related to developmental temperature in the laboratory, in accordance with the temperature-size rule, but not in the field, despite very strong temperature gradients. A common garden experiment on larval thermal responses, spanning the environmental extremes of H. merope's distribution, revealed that butterflies from low latitude (warmer climate) populations have relatively fast intrinsic growth and development rates compared to those from cooler climates. These synergistic effects of genotype and temperature across the landscape (co-gradient variation) are likely to accentuate phenotypic variation in these traits, and this interaction must be accounted for when predicting how H. merope will respond to temperature change through time. These results highlight the importance of understanding how variation in life-history traits may arise in response to environmental change. Without this knowledge, we may fail to detect whether organisms are tracking environmental change, and if they are, whether it is by plasticity, adaptation or both.

广布物种常表现出热敏感性状的地理变异，为理解物种随时间推移如何响应温度变化提供了关键视角。此类格局可由表型可塑性、遗传适应，或二者的交互作用所塑造。在部分场景中，基因型与温度的效应可共同作用，在不同热环境下削弱或加剧与适合度相关性状的表型变异。我们在澳大利亚东南部沿广阔纬度梯度分布的异纽灰蝶（Heteronympha merope）的生活史性状中，找到了这类交互作用的证据。研究发现，该蝴蝶的体型在实验室环境下与发育温度呈负相关，符合温度-体型法则（temperature-size rule），但在野外却未呈现这一规律，尽管野外存在极强的温度梯度。针对覆盖异纽灰蝶分布区环境极端范围的幼虫热响应开展的同质园（common garden）实验显示：相较于来自凉爽气候的种群，低纬度（暖气候）种群的蝴蝶具备相对更快的内在生长与发育速率。这类基因型与温度在景观尺度上的协同效应（即共梯度变异（co-gradient variation））或会加剧此类性状的表型变异，而在预测异纽灰蝶随时间如何响应温度变化时，必须将这一交互作用纳入考量。本研究结果凸显了阐明生活史性状变异如何响应环境变化的重要性。若缺乏此类认知，我们可能无法判断生物体是否在追踪环境变化；即便确实在追踪，也无法确定其背后的机制是可塑性、适应，还是二者兼具。