Data from: 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、平均气温）显著相关，这为自然选择塑造此类格局提供了实证依据。具体而言，变温动物的体型与卵粒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表型可塑性响应。不依赖体型的卵质也可随纬度发生变化，但此类研究尚较为匮乏。针对广布性物种黄粪蝇（*Scathophaga stercoraria*；双翅目：粪蝇科），本研究探讨了其繁殖性状（窝卵数、卵粒大小及卵成分）是否存在纬度梯度变异，这些梯度变异是否由体型或卵粒大小的变化所驱动，以及此类梯度变异在不同大陆（北美与欧洲）开展的独立实验中是否具有可重复性。卵粒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体体型解释；但窝卵数并无此类纬度梯度变化，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相对含量（校正卵粒大小后的数值）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。海拔维度有时呈现与纬度相似的关联（如卵蛋白与糖原），但偶尔也会出现相反的关联（如卵粒大小），这可能是因为该冷适应物种的种群间纬度与海拔呈负相关关系。在纬度梯度的种群间，并未发现卵粒大小与卵数量之间存在权衡关系；反而二者间呈现正相关趋势。包括体型与卵粒大小在内的所有性状，均随饲养温度（12℃、18℃、24℃）发生变化，且总体符合温度-体型规则（temperature-size rule）。基于同质园饲养实验得到的纬度梯度变异（可反映遗传分化），在大陆间的定性结果基本一致，但定量结果并非始终统一；且不同饲养温度下的梯度变异模式相似，这表明此类梯度变异是由自然选择演化而来，不过其中具体的选择机制仍有待进一步研究。