舌瓣鼠尾草退化杠杆雄蕊的相关花部特征及传粉机制

Floral morphology and pollination mechanism of Salvia liguliloba,a narrow endemic species with degraded lever-like stamens

杠杆状雄蕊是鼠尾草属(Salvia)物种形成的关键性状,背部杠杆传粉模式作为该属植物与传粉者精确互作的经典案例被广泛深入研究,但是在该属物种中还存在许多非典型的杠杆结构和传粉模式。雄蕊结构及其与传粉者互作的多样性,使得鼠尾草属成为研究植物传粉模式转变的模式材料,舌瓣鼠尾草(S.liguliloba)即是一种具非典型的退化杠杆状雄蕊结构和传粉特征的代表性物种。该文着重对舌瓣鼠尾草的花器官结构和传粉特征进行研究,并与具有短药隔杠杆的毛地黄鼠尾草(S.digitaloides)做比较分析,以期揭示退化杠杆可能的进化选择压力及其生态学意义。结果表明,舌瓣鼠尾草具有较短的花冠、更窄的冠筒和较短的雄、雌蕊(p<0.05)。退化萎缩的雄蕊下臂,冠筒内的狭小空间限制了唯一的有效传粉昆虫——三条熊蜂(Bombus trifasciatus)推动雄蕊做杠杆状运动,而是靠近花药直接利用头部完成授粉。相比经典的杠杆状雄蕊结构及其传粉过程,小型花冠和退化杠杆雄蕊是对专一性和活跃度较高传粉昆虫的适应,可能具有完全不同的进化途径和繁殖策略。

Aims Diverse stamen structures and interactions with pollinators make Salvia the model plants for studying evolutionary changes in plant pollination mechanisms. The dorsal pollination mechanism of lever-like stamens has been extensively investigated as a classic example for precise interactions between plants of Salvia genus and their pollinators. However, there are many atypical lever-like structures and pollination mechanisms among Salvia species. For example, Salvia liguliloba has floral organ structures and a pollination mechanism characterized by degenerated lever-like stamens. The aim of this work was to understand the selection pressure and ecological sig- nificance of Salvia plants that have the atypical staminal level mechanism. Methods In this study, we described the morphological features of S. liguliloba, a plant species endemic to the Tianmu Mountains, and investigated its pollination ecology in detail. Various components of the flower of S. liguliloba were measured, which included the corolla, corolla tube, corolla entrance, filament, connective and pis- til. Flower-visiting insects, pollinators, and the pollination process were observed and recorded by a digital video camera. Furthermore, we focused on comparing the floral organ structures and the pollination characteristics of S. liguliloba with those of S. digitaloides, which has a short-lever stamen. The relative frequency of insects, visiting time per flower, activity rate and visitation rate were measured and compared with the data from our previous study ofS. digitaloides, for which the flower structure and pollination features were well concluded. Important findings Salvia liguliloba has smaller corolla length, tube width, and shorter filament and pistil than S. digitaloides （p 〈 0.05）. The only effective pollinator was Bombus trifasciatus, and its average relative visiting frequency and the visiting time per flower were （0.959 ± 0.065） and （1.54 ± 0.60） s. The degenerated lower arm and limited moving space in the upper arm of the stamen restrict the pollinating insects from pushing the stamens in a lever-like motion. Thus, bumblebees completed pollination with the aid of their heads. Compared with the structure of the lever-like stamen and the pollination mechanism of S. digitaloides, the structural features of the floral organs of S. liguliloba make it adapt to a more specific pollinator with shorter visiting time and higher activ- ity rate. The results suggested that the species with degraded lever-like stamens might be different from other typical Salvia species in their evolution direction and reproductive strategy.