Data from: Pollinator-mediated selection on floral size and tube color in Linum pubescens: can differential behavior and preference in different times of the day maintain dimorphism?

Diversity of flower traits is often proposed as the outcome of selection exerted by pollinators. Positive directional pollinator-mediated selection on floral size has been widely shown to reduce phenotypic variance. However, the underlying mechanism of maintaining within-population floral color polymorphism is poorly understood. Divergent selection, mediated by different pollinators or by both mutualists and antagonists, may create and maintain such polymorphism, but it has rarely been shown to result from differential behavior of one pollinator.
We tested whether different behaviors of the same pollinators in morning and evening are associated with dimorphic floral trait in Linum pubescens, a Mediterranean annual plant that exhibits variable within-population frequencies of dark and light-colored flower tube. Usia bicolor bee-flies, the major pollinators of L. pubescens, are mostly feeding in the flower in the morning, while in the evening they are mostly visiting the flowers for mating. In two years of studying L. pubescens in a single large population in the Carmel, Israel, we found in one year that dark-centered flowers received significantly higher fraction of visits in the morning. Fitness was positively affected by number of visits, but no fitness differences were found between tube-color morphs, suggesting that both morphs have similar pollination success. Using mediation analysis, we found that flower size was under positive directional pollinator-mediated selection in both years, but pollinator behavior did not explain entirely this selection, which was possibly mediated also by other agents, such as florivores or a-biotic stresses.
While most pollinator-mediated selection studies show that flower size signals food reward, in L. pubescens it may also signals for mating place, which may drive positive selection. While flower size found to be under pollinator-mediated selection in L. pubescens, but differential behavior of the pollinators in morning and evening did not seem to explain flower color polymorphism.

花性状的多样性通常被认为是传粉媒介施加选择的结果。传粉媒介介导的花大小正向定向选择已被广泛证实会降低表型方差（phenotypic variance）。然而，维持种群内花色多态性（floral color polymorphism）的潜在机制仍不甚明晰。由不同传粉媒介介导的分化选择，或是由互利共生者与拮抗者共同介导的选择，可能会产生并维持这类多态性，但鲜有研究证明单一传粉类群的差异化行为可促成此类现象。
我们针对地中海一年生植物亚麻叶亚麻（Linum pubescens）展开研究，该物种种群内存在深色花管与浅色花管的频率变异。我们测试了同一传粉类群在早晨与傍晚的不同行为是否与其花管颜色二态性相关。双色蜂虻（Usia bicolor）是亚麻叶亚麻的主要传粉者，它们在早晨主要在花中取食，而在傍晚则主要到访花朵进行交配。我们在以色列卡梅尔地区的一个大型自然种群中对亚麻叶亚麻开展了为期两年的研究：其中一年的观测结果显示，早晨时段深色花管的花朵获得的访花比例显著更高。访花次数对植物适合度存在正向影响，但未发现不同花管颜色形态型（tube-color morphs）间存在适合度差异，这表明两类形态型的传粉成功水平相当。通过中介分析（mediation analysis），我们发现两年间花大小均处于传粉媒介介导的正向定向选择之下，但传粉媒介的行为并未完全解释该选择过程，其可能还受到其他因子的介导，例如花植食者或非生物胁迫。
尽管大多数传粉媒介介导的选择研究表明，花大小是食物奖励的信号，但在亚麻叶亚麻中，花大小同时也可作为交配场所的信号，这或许会驱动正向选择。虽然亚麻叶亚麻的花大小确实处于传粉媒介介导的选择之下，但传粉媒介在早晨与傍晚的差异化行为似乎并不能解释其种群内花色多态性的维持机制。