Selective landscapes in rivers are made up by an array of selective forces that vary from source to downstream regions or between seasons, and local/temporal variation in fitness maxima can result in gradual spatio-temporal variation of phenotypic traits. This study aimed at establishing freshwater amphipods as future model organisms to study adaptive phenotypic diversification (evolutionary divergence and/or adaptive plasticity) along stream gradients. We collected Gammarus roeselii from 16 sampling sites in the Rhine catchment during two consecutive seasons (summer and winter). Altogether, we dissected n = 1648 individuals and quantified key parameters related to morphological and life-history diversification, including naturally selected (e.g., gill surface areas) as well as primarily sexually selected traits (e.g., male antennae). Acknowledging the complexity of selective regimes in streams and the interrelated nature of selection factors, we assessed several abiotic (e.g., temperature, flow velocity) and biotic ecological parameters (e.g., conspecific densities, sex ratios) and condensed them into four principal components (PCs). Generalized least squares models revealed pronounced phenotypic differentiation in most of the traits investigated herein, and components of the stream gradient (PCs) explained parts of the observed differences. Depending on the trait under investigation, phenotypic differentiation could be ascribed to variation in abiotic conditions, anthropogenic disturbance (influx of thermally polluted water), or population parameters. For example, female fecundity showed altitudinal variation and decreased with increasing conspecific densities, while sexual dimorphism in the length of male antennae—used for mate finding and assessment—increased with increasing population densities and towards female-biased sex ratios. We provide a comprehensive protocol for comparative analyses of intraspecific variation in life history traits in amphipods. Whether the observed phenotypic differentiation over small geographical distances reflects evolutionary divergence or plasticity (or both) remains to be investigated in future studies. Independent of the mechanisms involved, variation in several traits is likely to have consequences for ecosystem functions. For example, leaf-shredding in G. roeselii strongly depends on body size, which varied in dependence of several ecological parameters.

中文翻译：

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非原生片脚类动物沿复杂流梯度的小规模表型分化

河流中的选择性景观由一系列从源头到下游区域或季节之间变化的选择性力量组成，适应度最大值的局部/时间变化可导致表型性状的逐渐时空变化。本研究旨在建立淡水片足类动物作为未来的模式生物，以研究沿河流梯度的适应性表型多样化（进化分歧和/或适应性可塑性）。我们在连续两个季节（夏季和冬季）从莱茵河流域的 16 个采样点收集了 Gammarus roeselii。总之，我们解剖了 n = 1648 个人并量化了与形态和生活史多样化相关的关键参数，包括自然选择的（例如，鳃表面积）以及主要的性选择特征（例如，雄性触角）。承认溪流中选择机制的复杂性和选择因素的相互关联性，我们评估了几个非生物（例如，温度、流速）和生物生态参数（例如，同种密度、性别比）并将它们浓缩成四个主要成分（PCs ）。广义最小二乘模型揭示了本文研究的大多数性状的显着表型分化，并且流梯度 (PC) 的成分解释了观察到的部分差异。根据所研究的性状，表型分化可归因于非生物条件的变化、人为干扰（热污染水的流入）或种群参数。例如，雌性生殖力表现出海拔变化，并随着同种密度的增加而降低，而男性触角长度的性别二态性（用于寻找和评估配偶）随着人口密度的增加和偏向女性的性别比例而增加。我们提供了一个全面的协议，用于比较分析片脚类动物生活史特征的种内变异。在较小的地理距离上观察到的表型分化是否反映了进化分歧或可塑性（或两者）仍有待在未来的研究中进行调查。独立于所涉及的机制，几个性状的变化可能会对生态系统功能产生影响。例如，在 G. roeselii 中的叶子切碎很大程度上取决于体型，而体型取决于几个生态参数。