BUZZ-POLLINATION AND PATTERNS IN SEXUAL TRAITS IN NORTH EUROPEAN PYROLACEAE

Flower biology and pollination of Moneses uniflora, Orthilia secunda, Pyrola minor, P. rotundifolia, P. chlorantha, and Chimaphila umbellata are described and discussed in relation to patterns in sexual traits and possible evolution of buzz-pollination within the group. The large number of pollen grains are packed into units of monads in Orthilia, tetrads in Moneses and Pyrola, or polyads in Chimaphila. Pollen is the sole reward to visiting insects except in the nectar-producing O. secunda and C. umbellata. Correlations are present between several sexual traits. Pollen: ovule ratios are very low, indicating efficient pollen transfer. Use of polyads may reduce the number of siring males. This trend may be counteracted by larger stigmas as in C umbellata. Increasing pollen unit size may enhance relatedness of seeds, reduce abortion, and improve seed set. This may explain the evolution, within the Pyrolaceae and other families, of higher ovule numbers with larger pollen units. The plants are mainly pollinated by bumblebees. Chimaphila umbellata is visited by nectar-collectors; Moneses uniflora, Pyrola minor, P. rotundifolia, and P. chlorantha are visited exclusively by pollen-buzzers; and Orthilia secunda is visited by both nectar-collectors and pollen-buzzers. Nectar secretion is suggested to be an ancient attractant in the group with C. umbellata having the least specialized pollination system, and buzz-pollination in M. uniflora and in the studied Pyrola species regarded as an advanced system.