Petal ontogeny, structure, and pollination system of four Aquilegia species in Midwest China

Abstract

The diversification of spur length in Aquilegia is often thought to be driven by pollinators. In this study, the morphology and pollination biology of four sympatric Aquilegia species distributed in China with different petal spur lengths were investigated through scanning electron microscopy, light microscopy, and field observations and measurements. The morphology and spur lengths of the four Aquilegia species differed: Aquilegia kansuensis has long incurved spurs (14.82 ± 0.18 mm), A. rockii has long straight spurs (16.34 ± 0.18 mm), A. semicalcarata has short slender spurs (5.16 ± 0.17 mm), and A. ecalcarata is effectively spurless (1.20 ± 0.03 mm). Early petal development was similar among the species; petals undergo initiation and delayed development, resulting in mature petals with different spur lengths and morphologies via developmental remodeling. The long spurs of A. kansuensis and A. rockii were derived from an increased cell number and anisotropic cell expansion, with well-developed secretory tissue forming at the spur bottom. No secretory tissue formed in the spurs of A. semicalcarata and A. ecalcarata, even though their cell number increased. Although their spur morphology and nectary structure show considerable differences, the pollination systems of the four species are similar. Rhingia campestris is the dominant pollinator of A. kansuensis, A. semicalcarata, and A. ecalcarata, whereas Pipiza familiaris is the dominant pollinator of A. rockii. Both R. campestris and P. familiaris forage for pollen when visiting flowers. Given that the four species studied differ in spur length but are all pollinated by short proboscid pollinators, there is no obvious one-to-one correspondence between spur length and pollinator mouthpart length in A. rockii, A. kansuensis, A. semicalcarata and A. ecalcarata, which is similar to the pattern in the European Aquilegia alpina, and abiotic factors may be an important explanation for spur lengths in our study.