Evolution of reproductive strategies in the sexually deceptive orchid Ophrys sphegodes: How does flower-specific variation of odor signals influence reproductive success?

The orchid Ophrys sphegodes Miller is pollinated by sexually excited males of the solitary bee Andrena nigroaenea, which are lured to the flowers by v isual cues and volatile semiochemicals. In O. sphegodes, visits by pollinat ors are rare. Because of this low frequency of pollination, one would expec t the evolution of strategies that increase the chance that males will visi t 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. Using gas chromatography-mass spectrometry (GC-MS) analyses, we identified more than 100 compounds in the odor bouquets of labellum extracts from O. s phegodes; 24 compounds were found to be biologically active in male olfacto ry receptors based on gas chromatography with electroantennographic detecti on (GC-EAD). Gas chromatography (GC) analyses of odors from individual flow ers showed less intraspecific variation in the odor bouquets of the biologi cally active compounds as compared to nonactive compounds. This can be expl ained by a higher selective pressure on the pollinator-attracting communica tion signal. Furthermore, we found a characteristic variation in the GC-EAD active esters and aldehydes among flowers of different stem positions with in an inflorescence and in the n-alkanes and n-alkenes among plants from di fferent populations. In our behavioral field tests, we showed that male bee s learn the odor bouquets of individual flowers during mating attempts and recognize them in later encounters. Bees thereby avoid trying to mate with flowers they have visited previously, but do not avoid other flowers either of a different or the same plant. By varying the relative proportions of s aturated esters and aldehydes between flowers of different stem positions, we demonstrated that a plant may take advantage of the learning abilities o f the pollinators and influence flower visitation behavior. Sixty-seven per cent of the males that visited one flower in an inflorescence returned to v isit a second flower of the same inflorescence. However, geitonogamy is pre vented and the likelihood of cross-fertilization is enhanced by the time re quired for the pollinium deposited on the pollinator to complete its bendin g movement, which is necessary for pollination to occur. Cross-fertilizatio n is furthermore enhanced by the high degree of odor variation between plan ts. This variation minimizes learned avoidance of the flowers and increases the likelihood that a given pollinator would visit several to many differe nt plants within a population.