Fruit syndromes in Viburnum: correlated evolution of color, nutritional content, and morphology in bird-dispersed fleshy fruits

Premise

A key question in plant dispersal via animal vectors is where and why fruit colors vary between species and how color relates to other fruit traits. To better understand the factors shaping the evolution of fruit color diversity, we tested for the existence of syndromes of traits (color, morphology, and nutrition) in the fruits of Viburnum. We placed these results in a larger phylogenetic context and reconstructed ancestral states to assess how Viburnum fruit traits have evolved across the clade.

Results

We find that blue Viburnum fruits are not very juicy, have high lipid content, and large, round endocarps surrounded by a small quantity of pulp. Red fruits display the opposite suite of traits: they are very juicy with low lipid content and smaller, flatter endocarps. The ancestral Viburnum fruit may have gone through a sequence of color changes before maturation (green to yellow to red to black), though our reconstructions are equivocal. In one major clade of Viburnum (Nectarotinus), fruits mature synchronously with reduced intermediate color stages. Most transitions between fruit colors occurred in this synchronously fruiting clade.

Conclusions

It is widely accepted that fruit trait diversity has primarily been driven by the differing perceptual abilities of bird versus mammal frugivores. Yet within a clade of largely bird-dispersed fruits, we find clear correlations between color, morphology, and nutrition. These correlations are likely driven by a shift from sequential to synchronous development, followed by diversification in color, nutrition, and morphology. A deeper understanding of fruit evolution within clades will elucidate the degree to which such syndromes structure extant fruit diversity.

研究背景
借助动物媒介实现传播的植物中，不同物种间果实颜色存在差异的成因与分布规律，以及果实颜色与其他果实性状间的关联，是植物传播生态学领域的核心问题之一。为阐明驱动果实颜色多样性演化的核心因素，本研究针对荚蒾属（Viburnum）植物果实的性状（颜色、形态与营养组成）综合征是否存在展开验证。本研究将所得结果置于更大尺度的系统发育框架下进行分析，并通过祖先状态重建，探究荚蒾属整个演化支上果实性状的演化历程。

研究结果
本研究发现，蓝色荚蒾属果实含水量较低，脂质含量较高，且内果皮（endocarps）大而圆润，包裹的果肉量较少。红色荚蒾属果实则呈现出完全相反的性状组合：含水量高，脂质含量低，内果皮更小且更扁平。尽管本研究的重建结果尚存不确定性，但荚蒾属祖先果实可能在成熟前经历了一系列颜色变化（绿色→黄色→红色→黑色）。在荚蒾属的一个主要演化支——蜜腺荚蒾支系（Nectarotinus）中，果实同步成熟，且中间色阶段减少。果实颜色的多数转变均发生在该同步成熟的演化支中。

研究结论
学界普遍认为，果实性状多样性主要由鸟类与哺乳类食果动物不同的视觉感知能力所驱动。但本研究在一个以鸟类传播为主的演化支中，却发现果实颜色、形态与营养组成间存在显著关联。这种关联可能源于果实发育从渐次成熟转向同步成熟，随后伴随颜色、营养组成与形态的多样化。对演化支内果实演化的更深入研究，将有助于阐明这类性状综合征对现存果实多样性的塑造程度。