Department of Biology, East Carolina University, Greenville, NC, USA.
Plant Biol (Stuttg). 2018 Mar;20(2):199-204. doi: 10.1111/plb.12677. Epub 2018 Jan 7.
Self-fertilisation that is delayed until after opportunities for outcrossing have ceased has been argued to provide both the reproductive assurance benefits of selfing and the genetic advantages of outcrossing. In the Campanulaceae, presentation of pollen on stylar hairs and progressive stigma curvature have been hypothesised to facilitate delayed selfing, but experimental tests are lacking. Stigma curvature is common in Campanula, a genus largely characterised by self-incompatibility, and therefore is unlikely to have initially evolved to promote self-fertilisation. In derived self-compatible species, however, stigma curvature might serve the secondary function of delayed selfing. We investigated delayed selfing in Triodanis perfoliata, a self-compatible relative of Campanula. Using floral manipulation experiments and pollen tube observations, we quantified the extent and timing of self-pollination. Further, we hypothesised that, if stigma curvature provides the benefit of delayed selfing in Triodanis, selection should have favoured retention of self-pollen through the loss of a stylar hair retraction mechanism. Results of a stigma removal experiment indicated that autonomous selfing produces partial seed set, but only some selfing was delayed. Pollen tube observations and a flower senescence assay also supported the finding of partial delayed selfing. Scanning electron microscopy revealed that pollen-collecting hairs retract during anthesis, which may limit the extent of delayed selfing. Delayed selfing appeared to be only partially effective in T. perfoliata. The stylar hair retraction in this species would seem to contradict selection for selfing. We suggest that caution and rigour are needed in interpreting floral traits as adaptive mechanisms for delayed selfing.
自花受精（即在异交机会结束后发生的自花受精）被认为同时提供了自交的生殖保障益处和异交的遗传优势。在桔梗科中，花粉在柱头上的呈现和柱头逐渐弯曲被假设为促进延迟自交，但缺乏实验测试。在以自交不亲和性为主要特征的风铃草属中，柱头弯曲很常见，因此不太可能最初是为了促进自交而进化的。然而，在衍生的自交亲和物种中，柱头弯曲可能具有延迟自交的次要功能。我们研究了三叶堇（Triodanis perfoliata）中的延迟自交现象，三叶堇是风铃草属的一个自交亲和的近缘种。通过花朵操作实验和花粉管观察，我们量化了自花授粉的程度和时间。此外，我们假设如果柱头弯曲在三叶堇中提供了延迟自交的益处，那么选择应该通过失去柱头毛发回缩机制来保留自花粉。柱头去除实验的结果表明，自主自交可以产生部分种子，但只有部分自交是延迟的。花粉管观察和花朵衰老测定也支持部分延迟自交的发现。扫描电子显微镜显示，花粉收集毛发在开花期间回缩，这可能限制了延迟自交的程度。延迟自交在 T. perfoliata 中似乎只是部分有效。该物种中的柱头毛发回缩似乎与自交的选择相悖。我们建议在将花朵特征解释为延迟自交的适应性机制时需要谨慎和严格。
