Effective pollinators of Asian sacred lotus (Nelumbo nucifera): contemporary pollinators may not reflect the historical pollination syndrome

Abstract

Background and aims: If stabilizing selection by pollinators is a prerequisite for pollinator-mediated floral evolution, spatiotemporal variation in the pollinator assemblage may confuse the plant-pollinator interaction in a given species. Here, effective pollinators in a living fossil plant Nelumbo nucifera (Nelumbonaceae) were examined to test whether beetles are major pollinators as predicted by its pollination syndrome.

Methods: Pollinators of N. nucifera were investigated in 11 wild populations and one cultivated population, and pollination experiments were conducted to examine the pollinating role of two major pollinators (bees and beetles) in three populations.

Key results: Lotus flowers are protogynous, bowl shaped and without nectar. The fragrant flowers can be self-heating during anthesis and produce around 1 million pollen grains per flower. It was found that bees and flies were the most frequent flower visitors in wild populations, contributing on average 87.9 and 49.4 % of seed set in Mishan and Lantian, respectively. Beetles were only found in one wild population and in the cultivated population, but the pollinator exclusion experiments showed that beetles were effective pollinators of Asian sacred lotus.

Conclusions: This study indicated that in their pollinating role, beetles, probable pollinators for this thermoregulating plant, had been replaced by some generalist insects in the wild. This finding implies that contemporary pollinators may not reflect the pollination syndrome.

Fig. 1.

The four types of flower visitors of N. nucifera. (A) A bee (Apis mellifera) collecting pollen from one Day-2 flower. (B) Bees (left bottom Lasioglossum species and the other Apis mellifera) collecting pollen in a Day-2 flower. (C) Numerous pollen beetles (Haptoncus luteolus) collecting pollen among anthers. (D) Three beetles crawling on stigmas of a Day-1 flower. (E) A fly (Eristalis cerealis L.) feeding on the stigmatic secretion. (F) Numerous thrips (arrow) collecting pollen.

Fig. 2.

Visitation frequency (mean ± s.e.) of bees and flies over the three days of anthesis in two wild populations during 2005 and 2006. Note different vertical scales between graphs.

Fig. 3.

A comparison of seed sets (mean ± s.e.) of flowers bagged using small-mesh nets and open-pollinated in Mishan and Lantian populations.

Fig. 4.

A comparison of seed sets (mean ± s.e.) of lotus flowers enclosed in different types of bag to examine the pollination role of beetles in the Wuhan population. The same letter indicates no significant difference among treatments.