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The cost of males should give asexual females an advantage when in competition with sexual females. In addition, high-fecundity asexual genotypes should have an advantage over low-fecundity clones, leading to reduction in clonal diversity over time. To evaluate fitness components in a natural population, we measured the annual reproductive rate of individual sexual and asexual female Potamopyrgus antipodarum , a New Zealand freshwater snail, in field enclosures that excluded competitors and predators. We used allozyme genotyping to assign the asexual females to particular clonal genotypes. We found that the most fecund asexual clones had similar or higher fecundity as the top 10 % of sexual families, suggesting that fecundity selection, even without the cost of males, would lead to replacement of the sexual population by clones. Consequently, we expected that the clones with the highest fecundity would dominate the natural population. Counter to this prediction, we found that high annual reproductive rates did not correlate with the frequency of clones in the natural population. When we exposed the same clones to parasites in the laboratory, we found that resistance to infection was positively correlated with the frequency of clones in the population. The correlation between fecundity and parasite resistance was negative, suggesting a trade-off between these two traits. Our results thus suggest that parasite resistance is an important short-term predictor of the success of asexual P. antipodarum in this population. Our study aimed to understand the relative contribution of fitness components to the success of asexual clones in a natural population of coexisting sexual and asexual snails. In conclusion, our results demonstrated that parasite resistance is a better predictor of the frequency clones than high performance in basic life history traits (survival, reproduction). In our enclosure experiment we measured reproductive rate of P. antipodarum clones in conditions that excluded the majority of ecological interactions (parasitism, intra-specific competition). We found that the mixed population of Potamopyrgus snails in Lake Alexandrina consists of many clones that have the reproductive potential to be able to outcompete the sexual lineages. Indeed, our results suggest that the conditions in which the sexuals are favoured in Lake Alexandrina require a large advantage in ecological interactions. This is in accordance with laboratory competition experiments where asexual lineages have been shown to increase in frequency in the absence of parasite pressure Jokela While we were able to explain the frequencies of clones in the population with infection experiments that revealed the genotype specificity of parasite resistance, such an experiment is not sufficient to address longer term advantage of sexual lineages under variable parasite pressure. Indeed, parasite-mediated selection for sex has been shown to fluctuate in Lake Alexandrina What our study suggests is that strong ecological interactions are required in order to explain why sexual lineages are successful in this population. In Lake Alexandrina the sexual population is most abundant in the shallow regions of the lake, where parasite risk is highest and Lively Studies including a larger geographic scale support important to the success of sex in many of these snail populations

在与有性生殖的雌性竞争时，雄性的成本应该会给无性生殖的雌性带来优势。此外，高繁殖力的无性基因型应该比低繁殖力的克隆体更具优势，随着时间的推移会导致克隆多样性的降低。为了评估自然种群中的适合度组成部分，我们测量了新西兰淡水螺Potamopyrgus antipodarum有性和无性生殖的雌性个体在排除竞争者和捕食者的野外围栏中的年繁殖率。我们使用同工酶基因分型将无性生殖的雌性归为特定的克隆基因型。我们发现繁殖力最强的无性克隆体与繁殖力最高的前10%的有性家族具有相似或更高的繁殖力，这表明即使没有雄性的成本，繁殖力选择也会导致克隆体取代有性生殖种群。因此，我们预期繁殖力最高的克隆体将在自然种群中占主导地位。与这一预测相反，我们发现高年繁殖率与自然种群中克隆体的频率并无关联。当我们在实验室中将相同的克隆体暴露于寄生虫时，我们发现对感染的抵抗力与种群中克隆体的频率呈正相关。繁殖力和寄生虫抵抗力之间呈负相关，这表明这两种特性之间存在权衡。我们的研究结果因此表明，寄生虫抵抗力是该种群中无性生殖的Potamopyrgus antipodarum成功的一个重要短期预测指标。我们的研究旨在了解适合度组成部分对有性和无性生殖的蜗牛共存的自然种群中无性克隆体成功的相对贡献。总之，我们的研究结果表明，寄生虫抵抗力比基本生活史特征（生存、繁殖）的高性能更能预测克隆体的频率。在我们的围栏实验中，我们在排除了大多数生态相互作用（寄生、种内竞争）的条件下测量了Potamopyrgus antipodarum克隆体的繁殖率。我们发现亚历山德里娜湖中的Potamopyrgus蜗牛混合种群由许多克隆体组成，这些克隆体具有能够胜过有性谱系的繁殖潜力。事实上，我们的研究结果表明，亚历山德里娜湖中有利于有性生殖的条件需要在生态相互作用方面具有很大优势。这与实验室竞争实验一致，在没有寄生虫压力的情况下，无性谱系的频率已被证明会增加（约凯拉）。虽然我们能够通过揭示寄生虫抵抗力基因型特异性的感染实验来解释种群中克隆体的频率，但这样的实验不足以解决在可变寄生虫压力下有性谱系的长期优势。事实上，寄生虫介导的性别选择已被证明在亚历山德里娜湖中会波动。我们的研究表明，需要强烈的生态相互作用来解释为什么有性谱系在该种群中是成功的。在亚历山德里娜湖中，有性生殖种群在湖的浅水区最为丰富，那里寄生虫风险最高（莱弗利）。包括更大地理范围的研究支持在许多这些蜗牛种群中性别成功的重要性。