Mating signals of bees and ophrys flowers: Mechanisms of chemical communication

The behavior releasing chemical substances in the sex pheromones of several species of bees and a scoliid wasp as well as in the flower scents of Ophrys orchids pollinated by them were identified and compared. These show a great agreement on all examined Ophrys- pollinator relationships, i.e. pollinator females and orchids attract the males with identical compounds. Furthermore we could prove reproductive strategies of Ophrys plants whose aim it is to increase the number of pollinated flowers and with that the number of seeds produced. Nearly all orchids of the genus Ophrys are pollinated by means of sexual deception, mostly by male bees and wasps. Lured by the odour and insect-like shape of the flower, the pollinator alights on the flower and attempts to copulate with it. During these "pseudocopulations" the pollinia become attached to the bees and are being transferred during further visits of the males on other flowers. In several Ophrys-pollinator-relationships, we identified and compared male attracting odour compounds of female bees (wasps) and Ophrys flowers. The aim was to get information about species- and individual-specific recognition signals and about basic mechanisms of chemical communication in the mating biology of bees. We performed behavioural tests, quantitative chemical analyses and electroantennogram assays. Ophrys flowers tend to produce complex bouquets of volatiles typically consisting of more than one hundred species-specific chemical compounds. Behavioral experiments with synthetic copies of the compounds produced by Ophrys flowers have shown that only certain volatiles are active in stimulating mating behavior in the males. In this study we could show for the first time that an Ophrys-orchid produces the same compounds that comprise the sex pheromone of its pollinator species. However, male attraction can be managed both by a bouquet of relatively simple compounds and by few, highly specific substances. In sexually deceptive orchids, visits by pollinators are rare. Because of this low frequency of pollination, one would expect the evolution of strategies that increase the chance that males will visit more than one flower on the same plant; this would increase the number of pollination events on a plant and therefore the number of seeds produced. Chemical analyses of individual flower odors showed that the floral volatile bouquets differed between flowers within an inflorescence. In behavioral field tests we could show that male bees learn the odor bouquets of individual flowers during mating attempts and recognize them in further encounters. They avoid trying to mate with flowers they have visited previously, but not with other flowers either on the same or on a different plant. Thereby, the chance of more than one flower being visited by the same male and the plant`s pollination success and individual fitness is increased. Another strategy to increase the number of pollinated flowers is to direct pollinators to unpollinated flowers of an inflorescence. In O. speculum, a species that is pollinated by males of the scoliid wasp Campsoscolia ciliata, the attractiveness of flowers is diminished after pollination due to smaller amounts of the male attracting compounds, mainly hydroxy acids. In Ophrys sphegodes, pollinated flowers produce farnesyl hexanoate, a substance which is usually produced by female bees to signal to males, that they are already mated, thereby directing pollinators to unpollinated flowers of an inflorescence. We conclude that pollinated flowers mimic the scent of mated females in order to guide pollinators to unpollinated flowers of the inflorescence. Thereby, the reproductive success of a plant is maximized.