Abstract
BackgroundSelective 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.MethodsWe 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).ResultsGeneralized 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.ConclusionsWe 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.
Highlights
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
We provide a priori predictions for the direction of evolutionary and/or plastic trait divergence in G. roeselii by agents of natural and sexual selection based on a literature survey including other amphipods and freshwater invertebrates in general. ▲Predicted positive association; ▼predicted negative association dependence of local temperature and oxygen regimes [46]), reproductive life history traits known to be under natural selection from both abiotic and biotic selection factors, and traits that are primarily under sexual selection
Our present study is centred on two major questions: (1) Do we find small-scale phenotypic differentiation in non-native G. roeselii approximately 150 years upon their arrival in Central Europe [41] along repeated river gradients? To answer this question, we sampled amphipods from 16 sites along two streams in Germany (Fig. 1) and assessed an array of phenotypic traits (Table 1)
Summary
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. Streams and the associated limnic ecosystems are shaped by a widespread form of an environmental gradient in which an array of abiotic and biotic selection factors vary systematically in space and time, e.g., from source regions over smaller tributaries to slow-flowing lowland river sections [23,24,25] or between seasons [21, 23]. The stability of abiotic conditions rapidly decreases with increasing distance from the source(s), especially when the stream is no longer covered by forest canopy, such that water temperatures will attain greater variance due to solar input [25, 30] These river sections often experience pronounced fluctuations in abiotic conditions, as they undergo recurrent catastrophic flooding (e.g., after snow melt [31]), or temporal desiccation either during hot/dry seasons [32, 33] or as a result of climate change [34]. Abiotic conditions reattain a more stable state in downstream river portions, where multiple tributaries interconnect to form an extensive wetland system [23, 25]
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