Abstract:

For many plants, nectar is hidden in the depth of nectar spurs. The diverse morphology and length of nectar spurs can attract pollinators with various proboscis lengths and promote the differentiation of pollination niche, which plays an important role in the formation of species diversity. Therefore, nectar spur is considered as a key innovation trait in promoting speciation and an ideal trait for studying floral evolution and plant-pollinator coevolution in angiosperm. This review systematically summarizes the worldwide plant taxonomic databases and published articles, classifies the nectar spur plants, and reviews the diversity and evolutionary mechanisms of nectar spur plants. We investigated 3 427 nectar spur species belonging to 13 orders, 23 families and 271 genera of angiosperm. Among them, Orchidaceae has the largest number of species, with 1 536 species, following by Papaveraceae with 487 species, Ranunculaceae with 351 species, Balsaminaceae with 284 species and Violaceae with 197 species, accounting for 83.3% of all the species. According to the sources of the development of nectar spurs, the nectar spurs can be divided into six types: petal spur, corolla spur, sepal spur, calyx spur, tepal spur and hypanthium spur. The petal spurs are the predominant type, detected in 212 genera. The average length of nectar spurs in Balsaminaceae is the longest (23.8 mm) among the 2 546 species whose nectar spurs have been measured. The shortest nectar spurs are reported in Lycelandaceae, which are only 1.6 mm on average. Most nectar spurs have different degrees of curvature, likely helping to improve the pollination efficiency. The number of nectar spur(s) in a flower ranges from 1 to 6, and most species have only one nectar spur in a flower. Multiple nectar spurs may increase the visiting time of pollinators. Some plants in Orchidaceae and Violaceae have appendages inside the nectar spurs, which may have the function of nectar secretion. The pollinators of plants with nectar spur mainly comprise insects from Hymenoptera, Lepidoptera, and some long-proboscis species from Diptera and birds. Nectar robbers are mainly bees and birds. There is a general association of geographic mosaic theory of coevolution between the length of nectar spur and the length of pollinator proboscis. Evolutionary ecologists have proposed the hypothesis of “Darwin’s Mechanistic Model” and “The Pollinator Shift Model” to explain this phenomenon. In future studies of the mechanisms of nectar spur evolution, we should not only focus on the morphological characteristics of spur length and proboscis length and the pollination ecology, but also explore the driving forces of nectar spurs evolution from cell division/elongation, functional genes, abiotic factors, with the help of novel technological approaches including developmental anatomy and genomics.

Key words: nectar spur, spur length, key evolutionary innovation, floral structure, nectar robbing