Department of Biological Sciences, Clemson University, Clemson, SC, USA.
Department of Biology, University of Virginia, Charlottesville, VA, USA.
J Evol Biol. 2020 Apr;33(4):388-400. doi: 10.1111/jeb.13599. Epub 2020 Feb 18.
Colour phenotypes are often involved in communication and are thus under selection by species interactions. However, selection may also act on colour through correlated traits or alternative functions of biochemical pigments. Such forms of selection are instrumental in maintaining petal colour diversity in plants. Pollen colour also varies markedly, but the maintenance of this variation is little understood. In Campanula americana, pollen ranges from white to dark purple, with darker morphs garnering more pollinator visits and exhibiting elevated pollen performance under heat stress. Here, we generate an F2 population segregating for pollen colour and measure correlations with floral traits, pollen attributes and plant-level traits related to fitness. We determine the pigment biochemistry of colour variants and evaluate maternal and paternal fitness of light and dark morphs by crossing within and between morphs. Pollen colour was largely uncorrelated with floral traits (petal colour, size, nectar traits) suggesting it can evolve independently. Darker pollen grains were larger and had higher anthocyanin content (cyanidin and peonidin) which may explain why they outperform light pollen under heat stress. Overall, pollen-related fitness metrics were greater for dark pollen, and dark pollen sires generated seeds with higher germination potential. Conversely, light pollen plants produce 61% more flowers than dark, and 18% more seeds per fruit, suggesting a seed production advantage. Results indicate that light and dark morphs may achieve fitness through different means-dark morphs appear to have a pollen advantage whereas light morphs have an ovule advantage-helping to explain the maintenance of pollen colour variation.
颜色表型通常与物种间的相互作用有关，因此受到选择的影响。然而，选择也可能通过相关特征或生化色素的替代功能对颜色起作用。这种形式的选择对植物花瓣颜色多样性的维持起着重要作用。花粉颜色也有明显的变化，但对这种变化的维持却知之甚少。在风铃草属植物中，花粉颜色从白色到深紫色不等，较深的形态吸引了更多的传粉者访问，并在热应激下表现出更高的花粉性能。在这里，我们生成了一个分离花粉颜色的 F2 群体，并测量了其与花部特征、花粉属性以及与适合度相关的植物水平特征的相关性。我们确定了颜色变体的色素生物化学，并通过在形态内和形态间进行杂交来评估浅色和深色形态的母本和父本适合度。花粉颜色与花部特征（花瓣颜色、大小、花蜜特征）基本没有相关性，这表明它可以独立进化。较深的花粉粒更大，含有更高的花青素含量（矢车菊素和锦葵素），这可能解释了它们在热应激下为何比浅色花粉表现更好。总的来说，深色花粉的花粉相关适合度指标更高，深色花粉的父本产生的种子具有更高的萌发潜力。相反，浅色花粉植物产生的花比深色花粉多 61%，每果产生的种子比深色花粉多 18%，这表明它们具有种子生产优势。结果表明，浅色和深色形态可能通过不同的方式达到适合度——深色形态似乎具有花粉优势，而浅色形态具有胚珠优势——这有助于解释花粉颜色变异的维持。
