ECOLOGICAL AND GENETIC CONSEQUENCES OF POLLINATION BY SEXUAL DECEPTION IN THE ORCHID CALADENIA-TENTACTULATA

Only orchids affect pollination by the deceptive sexual attraction of male insects, a syndrome particularly well developed in Australia. We examined the ecological and genetic consequences of exclusive pollinat ion by sexually attracted male thynnine wasps in the orchid Caladenia tentaculata. Male wasps respond rapidly to flowers artificially presen ted in 1 x 1 m(2) experimental patches. Sixty of 287 wasps approached within centimeters of the flower but did not land. Of the remaining 79 % who made floral contact, only 7.5% attempted copulation, the step cr itical for pollination. Wasps only rarely moved among patches (19% of flights) and none attempted copulation a second time, resembling obser vations in natural populations. We confirmed outcrossing and long dist ance pollen flow by monitoring how colored pollen moved in natural pop ulations. Pollen movements approximated a linear rather than a leptoku rtic distribution (mean distance: 17 m; maximum: 58 m). Pollinator vis its varied independently of flower density in three of four population s with most solitary flowers being visited. Allozyme analysis revealed within-population fixation indices (F) close to zero and low levels o f differentiation (FST) among populations. Despite behavioral evidence for long distance pollen flow, significant local genetic structure ex ists, perhaps reflecting restricted seed dispersal. Long distance poll en flow in C. tentaculata may therefore promote outbreeding by minimiz ing pollen transfers among related neighbors. Although this species is self-compatible, outcrossed progeny develop significantly faster than selfed progeny. Effective pollination at low flower densities could a ccentuate this advantage. The data are consistent with the predictions that deceptive pollination will result in long distance pollen flow, which may be of selective advantage at low density. Comparative studie s of how food reward, food deceptive, and sexual deceptive pollination systems vary within a phylogenetic framework could further illuminate the evolution of sexual deception.