Department of Plant Sciences, University of Cambridge, Cambridge, UK.
Real Jardín Botánico (RJB-CSIC), Madrid, Spain.
Ann Bot. 2018 Nov 3;122(5):801-809. doi: 10.1093/aob/mcx213.
Nectar spurs (tubular outgrowths of a floral organ which contain, or give the appearance of containing, nectar) are hypothesized to be a 'key innovation' which can lead to rapid speciation within a lineage, because they are involved in pollinator specificity. Despite the ecological importance of nectar spurs, relatively little is known about their development. We used a comparative approach to investigate variation in nectar spur length in a clade of eight Iberian toadflaxes.
Spur growth was measured at the macroscopic level over time in all eight species, and growth rate and growth duration compared. Evolution of growth rate was reconstructed across the phylogeny. Within the clade we then focused on Linaria becerrae and Linaria clementei, a pair of sister species which have extremely long and short spurs, respectively. Characterization at a micromorphological level was performed across a range of key developmental stages to determine whether the difference in spur length is due to differential cell expansion or cell division.
We detected a significant difference in the evolved growth rates, while developmental timing of both the initiation and the end of spur growth remained similar. Cell number is three times higher in the long spurred L. becerrae compared with L. clementei, whereas cell length is only 1.3 times greater. In addition, overall anisotropy of mature cells is not significantly different between the two species.
We found that changes in cell number and therefore in cell division largely explain evolution of spur length. This contrasts with previous studies in Aquilegia which have found that variation in nectar spur length is due to directed cell expansion (anisotropy) over variable time frames. Our study adds to knowledge about nectar spur development in a comparative context and indicates that different systems may have evolved nectar spurs using disparate mechanisms.
花蜜spur（花器官的管状突起，包含或表现出包含花蜜）被假设为一种“关键创新”，可以导致一个谱系内的快速物种形成，因为它们与传粉者的特异性有关。尽管花蜜spur 在生态上很重要，但对它们的发育却知之甚少。我们使用比较的方法来研究 8 种伊比利亚黄花九轮草中花蜜spur 长度的变化。
在所有 8 个物种中，我们在宏观水平上随时间测量 spur 的生长，比较生长速度和生长持续时间。在系统发育上重建了生长速率的进化。在这个分支内，我们专注于 Linaria becerrae 和 Linaria clementei，这一对姐妹种，它们的 spur 长度极长和极短。在一系列关键发育阶段进行微观形态学特征分析，以确定 spur 长度的差异是由于细胞扩展或细胞分裂的差异。
我们检测到进化的生长速率有显著差异，而 spur 生长的开始和结束的发育时间仍然相似。长spur 的 L. becerrae 中的细胞数量是短spur 的 L. clementei 的三倍，而细胞长度仅长 1.3 倍。此外，两种物种之间成熟细胞的总体各向异性没有显著差异。
我们发现细胞数量的变化，因此细胞分裂在很大程度上解释了 spur 长度的进化。这与之前在 Aquilegia 中发现的花蜜spur 长度的变化是由于在不同时间框架内的定向细胞扩展（各向异性）的研究结果形成对比。我们的研究在比较的背景下增加了对花蜜spur 发育的认识，并表明不同的系统可能使用不同的机制来进化出花蜜spur。
