PREMISE Spiny pollen has evolved independently in multiple entomophilous lineages. Sexual selection may act on exine traits that facilitate male mating success by influencing the transfer of pollen from the anther to the body of the pollinator, while natural selection acts to increase pollen survival. We postulated that relative to sexual congeners, apomictic dandelions undergo relaxed selection on traits associated with male mating success. METHODS We explored sexual selection on exine traits by measuring the propensity for Taraxacum spp. pollen to attach to hairs of flower-visiting bumblebees (Bombus spp.) or flies (Diptera: Syrphidae and Muscoidea) and assessed natural selection by testing whether pollen traits defend against consumption. RESULTS Pollen picked up by bumblebees exhibited a narrower subset of spine-spacing phenotypes, consistent with stabilizing selection. Flies picked up larger pollen from flowers than expected at random. Surveys of corbiculae (pollen basket) contents from foraging bumblebees and feces of flies showed that pollen grains consumed by both kinds of visitors are similar in spine characteristics and size to those produced by the donor. When bees visit inflorescences of apomictic T. officinale, they pick up pollen with spine-spacing phenotypes above the mean and shifted toward those of sexual T. ceratophorum. CONCLUSIONS We demonstrate that traits under sexual selection during pollen pickup vary among pollinators, while natural selection for pollen defense is nil in T. ceratophorum. In hybrid zones between apomictic and sexual dandelions, pollen traits place apomictic donors at a dispersal disadvantage, potentially reinforcing reproductive isolation.

中文翻译：

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蒲公英花粉形态的性和自然选择

前提 多刺花粉在多个虫媒谱系中独立进化。性选择可以通过影响花粉从花药到传粉者身体的转移来影响雄性交配成功的外壁性状，而自然选择的作用是增加花粉的存活率。我们假设相对于性同源物，无融合生殖的蒲公英在与雄性交配成功相关的特征上经历了宽松的选择。方法我们通过测量蒲公英属的倾向来探索外壁性状的性选择。花粉附着在访花大黄蜂（Bombus spp.）或苍蝇（双翅目：Syrphidae 和 Muscoidea）的毛发上，并通过测试花粉特性是否能抵御消费来评估自然选择。结果 大黄蜂采集的花粉表现出更窄的脊柱间距表型子集，与稳定选择相一致。苍蝇从花中随机拾取比预期更大的花粉。对觅食的大黄蜂和苍蝇粪便的球体（花粉篮）含量的调查表明，两种游客食用的花粉粒在脊柱特征和大小上与捐赠者产生的花粉粒相似。当蜜蜂访问无融合生殖 T. officinale 的花序时，它们会采集棘间距表型高于平均值的花粉，并转向有性 T. ceratophorum 的花粉。结论我们证明了在花粉采集过程中性选择的特征在传粉者之间有所不同，而花粉防御的自然选择在 T. ceratophorum 中为零。在无融合生殖和有性蒲公英之间的杂交区，花粉特性使无融合生殖供体处于传播劣势，