At species' range edges, individuals often face novel environmental conditions that may limit range expansion until populations adapt. The potential to adapt depends on genetic variation upon which selection can act. However, populations at species' range edges are often genetically depauperate. One mechanism increasing genetic variation is reshuffling existing variation through sex. Sex, however, can potentially limit adaptation by breaking up existing beneficial allele combinations (recombination load). The gene swamping hypothesis predicts this is specifically the case when populations expand along an abiotic gradient and asymmetric dispersal leads to numerous maladapted dispersers from the range core swamping the range edge. We used the ciliate Tetrahymena thermophila as a model for testing the gene swamping hypothesis. We performed replicated range expansions in landscapes with or without a pH-gradient, while simultaneously manipulating the occurrence of gene flow and sexual versus asexual reproduction. We show that sex accelerated evolution of local adaptation in the absence of gene flow, but hindered it in the presence of gene flow. However, sex affected adaptation independently of the pH-gradient, indicating that both abiotic gradients and the biotic gradient in population density lead to gene swamping. Overall, our results show that gene swamping alters adaptation in life-history strategies.

中文翻译：

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基因淹没改变了原生生物嗜热四膜虫范围扩张过程中的进化。

在物种的活动范围边缘，个体经常面临新的环境条件，这些条件可能会限制活动范围的扩展，直到种群适应为止。适应的潜力取决于选择所发挥作用的遗传变异。然而，处于物种分布范围边缘的种群在遗传上往往是贫乏的。增加遗传变异的一种机制是通过性别重新调整现有的变异。然而，性别可能会通过破坏现有的有益等位基因组合（重组负载）来限制适应。基因淹没假说预测，当种群沿着非生物梯度扩张并且不对称扩散导致大量适应不良的扩散者从范围核心淹没范围边缘时，就会出现这种情况。我们使用纤毛虫嗜热四膜虫作为模型来检验基因淹没假说。我们在有或没有 pH 梯度的景观中进行了重复的范围扩张，同时操纵基因流和有性繁殖与无性繁殖的发生。我们发现，在没有基因流的情况下，性别加速了局部适应的进化，但在存在基因流的情况下却阻碍了局部适应的进化。然而，性别对适应的影响与pH梯度无关，表明种群密度中的非生物梯度和生物梯度都会导致基因淹没。总的来说，我们的结果表明基因淹没改变了生活史策略的适应。