Department of Evolution, Ecology, and Behavior, School of Integrative Biology, University of Illinois, Urbana, IL, USA.
Department of Biology, University of the South, Sewanee, TN, USA.
Oecologia. 2020 May;193(1):135-142. doi: 10.1007/s00442-020-04651-7. Epub 2020 Apr 19.
Populations of generalist grazers often contain genotypes with "powerful" and "efficient" strategies. Powerful genotypes grow rapidly on rich-quality resources, but slowly on poorer-quality ones, while efficient genotypes grow relatively better on poorer resources but cannot exploit richer resources as well. Via a "power-efficiency" trade-off, variation in resource quality could maintain genetic diversity. To evaluate this mechanism, we sampled six populations of the freshwater cladoceran Daphnia pulicaria. In persisting (year-round) populations, Daphnia consume resources that vary in quality, whereas in non-persisting (spring-only) populations, Daphnia primarily encounter rich-quality resources. We hypothesized that non-persisting populations harbor no efficient clones (hence should show lower growth on poor-quality resources). Although individuals from non-persisting populations remained smaller than individuals from persisting populations, no evidence arose for a trade-off between powerful and efficient strategies. In fact, growth rates on the two diets were positively correlated (instead of negatively, as predicted). Furthermore, in the persisting populations, we predicted that clonal selection from spring to summer should shift the distribution of genotypes from powerful (specialists on richer spring resources) to efficient (poorer, summer resources). Genetic composition of populations shifted from spring to summer, but not toward more efficient genotypes. Therefore, in these lakes, maintenance of variation among genotypes must stem from more complicated factors than population persistence patterns or seasonal shifts in resource quality alone.
杂食性草食动物群体通常包含具有“强大”和“高效”策略的基因型。强大的基因型在富营养资源上快速生长，但在贫营养资源上生长缓慢，而高效的基因型在贫营养资源上相对生长更好，但不能很好地利用富营养资源。通过“效能权衡”，资源质量的变化可以维持遗传多样性。为了评估这种机制，我们对淡水枝角类动物 Daphnia pulicaria 的六个种群进行了采样。在持续（全年）存在的种群中，Daphnia 消耗的资源质量存在差异，而在非持续（仅春季）存在的种群中，Daphnia 主要遇到的是富营养资源。我们假设非持续存在的种群不含有高效的克隆体（因此在贫营养资源上的生长速度应该较低）。尽管来自非持续存在种群的个体仍然比来自持续存在种群的个体小，但并没有证据表明强大和高效策略之间存在权衡。实际上，两种饮食的生长速度呈正相关（而不是预测的负相关）。此外，在持续存在的种群中，我们预测从春季到夏季的克隆选择应该会使基因型的分布从强大（富营养春季资源的专家）转变为高效（更贫瘠、夏季资源）。种群的遗传组成从春季到夏季发生了变化，但没有向更高效的基因型转变。因此，在这些湖泊中，基因型之间的变异维持必须源自比种群持续存在模式或资源质量季节性变化更复杂的因素。
