Centre for Functional Ecology and Department of Life Sciences, University of Coimbra, Coimbra, Portugal.
Frontiers in Evolutionary Zoology research group, Natural History Museum, University of Oslo, Oslo, Norway.
Am J Bot. 2019 Feb;106(2):231-243. doi: 10.1002/ajb2.1238.
Ray flowers commonly observed in daisies' flowering heads are a well-known example of advertising structures for enhancing pollinator attraction. Despite this, ray loss has occurred in multiple lineages, which still rely on pollinators, suggesting that rayless phenotypes could also be adaptive for animal-pollination. Here, we investigate the benefits and costs of these specialized floral advertising structures by comparing rayed and rayless phenotypes in two hybridizing closely related species.
We assessed the advantages and costs of ray production in terms of floral visitor's attraction, pollen limitation, and female reproductive success using the broad natural variation on ray size and number at the contact zone of A. clavatus (rayed) and A. valentinus (rayless). In addition, we experimentally explored the effect of rays under controlled neighborhoods and the effect of ray removal on fruit production.
In sympatry, rayed phenotypes attracted significantly more visitors than rayless plants, in which seed production was pollen limited. However, rayed phenotypes did not show higher fruit set or seed production than rayless phenotypes. Fruit set and seed production benefited from denser neighborhood displays and larger individual floral displays, respectively. The removal of ray florets did not appear to enable resource reallocation to fruit production.
Rayless heads compensated their lower visitation rate by means of a higher number of flowers per head achieving similar fecundity levels to rayed plants. The larger size of rayless heads might thus indicate an inflorescence-level trade-off between attraction and fertility.
雏菊头状花序中常见的射线花是增强传粉者吸引力的广告结构的一个著名例子。尽管如此，多个谱系已经失去了射线，这些谱系仍然依赖传粉者，这表明无射线表型也可能对动物授粉具有适应性。在这里，我们通过比较两种杂交近缘种的有射线和无射线表型，研究这些专门的花广告结构的好处和成本。
我们使用 A. clavatus（有射线）和 A. valentinus（无射线）接触区的射线大小和数量的广泛自然变异，从花访客的吸引力、花粉限制和雌性生殖成功等方面评估了射线产生的优势和成本。此外，我们在受控环境中实验性地探索了射线的作用以及去除射线对果实产量的影响。
在同域中，有射线表型比无射线植物吸引了更多的访客，而无射线植物的种子生产受到花粉限制。然而，有射线表型的果实结实率或种子产量并不比无射线表型高。果实结实率和种子产量分别受益于更密集的邻域展示和更大的个体花展示。去除射线花瓣似乎并没有使资源重新分配到果实生产中。
无射线头状花序通过增加每个头状花序的花朵数量来弥补其较低的访问率，从而达到与有射线植物相似的生殖水平。因此，无射线头状花序的较大尺寸可能表明在吸引力和生殖力之间存在花序级别的权衡。
