Distylous plants have two floral morphs: pin flowers with long styles and short stamens, and thrum flowers with short styles and long stamens. The height of stigmas in pin flowers corresponds to the height of anthers in thrum flowers, and vice versa. Most distylous plants are self- and intra-morph incompatible and distyly has traditionally been viewed as an adaptive trait to promote inter-morph pollen transfer.
Nectar robbers are birds, insects, or other flower visitors, that remove nectar from flowers through a hole pierced or bitten in the corolla. Nectar robbers usually obtain reward without providing a pollination service, so they have frequently been described as cheaters in the plant–pollinator mutualism system since Darwin. However, some studies have shown that robbers can directly pollinate flowers. In all cases of robber-like pollination, the stamens and pistils protruded from corollas, and pollination occurred when nectar robbers (usually bumble-bees or carpenter bees) unintentionally came into contact with these sexual organs. It remains unclear whether nectar robbers could pollinate flowers with sexual organs deeply hidden within corollas.
Under the guidance of Prof. LI Qingjun, XTBG Master candidates ZHU Xingfu and WAN Jinpeng conducted a study at Yunnan, China to examine the possibility of robber-like pollination in a distylous plant Primula secundiflora. Flowers of P. secundiflora were frequently robbed by Bombus Richardsi and robbing holes were always positioned between high and low sexual organs for both floral morphs. Using detailed observations and measuring seed production of manipulated flowers that were visited only by robbers, they proved their hypothesis that nectar robbers touch low sexual organs and thus transfer pollen grains from pin flowers to thrum flowers. Such asymmetrical pollen flow caused by nectar robbers may act as an important selective agent in floral fitness and evolution of distyly.The research result, entitled “Nectar robbers pollinate flowers with sexual organs hidden within corollas in distylous Primula secundiflora (Primulaceae)”, was published on the recently issued journal of Biology Letters(Volume 6, 2010, 785-787).