Replicated latitudinal clines in reproductive traits of European and North American yellow dung flies

Geographic variation in phenotypic traits is commonly correlated with spatial variation in the environment, e.g., seasonality and mean temperature, providing evidence that natural selection generates such patterns. In particular, both body size and egg size of ectothermic animals are commonly larger in northern climates, and temperature induces plastic responses in both traits. Size-independent egg quality can also vary with latitude, though this is rarely investigated. For the widespread yellow dung fly (Scathophaga stercoraria; Diptera: Scathophagidae), we investigated whether there are latitudinal clines in reproductive traits (clutch size, egg size and egg composition), whether these clines are due to variation in body and/or egg size, and whether such clines replicate across independent experiments performed on different continents (North America and Europe). Egg size generally increased with latitude (especially in Europe), an effect largely explained by body size of the mother, while clutch size did not; overall reproductive effort thus increased with latitude. Both the absolute and relative (correcting for egg size) amount of egg protein increased with latitude, egg glycogen decreased with latitude, while latitudinal trends for egg lipids and total egg energy content were complex and non-linear. Altitude sometimes showed relationships analogous to those of latitude (egg proteins and glycogen) but occasionally opposite (egg size), possibly because latitude and altitude are negatively related among populations of this cold-adapted species. There was no evidence of a trade-off between egg size and number across latitudinal populations; if anything, the relationship was positive. All traits, including body and egg size, varied with rearing temperature (12°C, 18°C, 24°C), generally following the temperature-size-rule. Clines based on common garden rearing, thus reflecting genetic differentiation, were qualitatively but not always quantitatively consistent between continents, and were similar across rearing temperatures, suggesting they evolved due to natural selection, although the concrete selective mechanisms involved require further study.

表型性状的地理变异通常与环境的空间变异（如季节节律与平均气温）呈显著相关，这为自然选择塑造此类演化格局提供了科学佐证。具体而言，变温动物的成体体型与卵大小通常在北方气候环境中更大，且温度会对这两类性状诱导产生可塑性响应。不依赖于体型的卵质量也可能随纬度发生变化，但此类研究迄今尚不多见。针对广布性物种黄粪蝇（*Scathophaga stercoraria*；双翅目：粪蝇科），本研究探讨了三个核心科学问题：其一，繁殖相关性状（产卵量、卵大小与卵成分）是否存在纬度渐变群；其二，此类渐变群是否由成体体型和/或卵大小的变异所驱动；其三，这类渐变群在北美与欧洲两个大陆开展的独立实验中是否具有可重复性。研究结果显示，卵大小总体随纬度升高而增大（欧洲种群尤为显著），该效应在很大程度上可由母代成体体型解释，而产卵量则未呈现此类纬度梯度；因此，整体繁殖投入随纬度升高而增加。卵内蛋白质的绝对含量与校正卵大小后的相对含量均随纬度升高而增加，卵糖原含量则随纬度升高而降低；而卵脂质含量与总卵能量含量的纬度变化趋势则较为复杂，呈非线性特征。海拔高度与卵性状的关联有时与纬度趋势一致（如卵蛋白与糖原），有时则恰好相反（如卵大小），这可能是因为该冷适应物种的不同种群间，纬度与海拔呈负相关关系。在不同纬度的种群间，未发现卵大小与卵数量存在进化权衡关系；实际上二者的关联甚至呈正向。所有性状（包括成体体型与卵大小）均随饲养温度（12℃、18℃、24℃）发生变化，总体符合温度-体型法则（temperature-size rule）。基于共同花园饲养实验得到的渐变群（该实验设计可反映遗传分化），在大陆间仅在定性层面保持一致，而非总是定量吻合；且不同饲养温度下的渐变群模式相似，这表明此类渐变群是自然选择驱动演化的结果，不过其中具体的选择机制仍有待进一步深入研究。