Evidence of post-pollination barriers among three colour morphs of the deceptive orchid Dactylorhiza sambucina (L.) Soó

Floral-colour polymorphism in rewardlessorchids has been hypothesized to be maintained bymeans of naı¨ve insects, which after visiting a flowerwithout reward will tend to fly elsewhere, looking for aflower of a different colour. In this study, levels of maleand female reproductive success were monitored inSouthern Italy populations of the deceptive orchidDactylorhiza sambucina, through field observations over3 years. These populations were characterized by thepresence of a rare pink morph which is sympatric withthe more frequent yellow and red morphs. In addition,final plant fertility was evaluated through percentages ofembryo-containing seeds produced in both naturalconditions and hand-pollination experiments. Resultsshowed that pollinator preferences were independent ofthe morph frequencies and thus do not promote thepredicted negative frequency-dependent selection. Althoughyellow and pink morphs showed significantlyhigher male reproductive success (RS), we found comparablelevels of female RS, which suggest that pollinatorbehaviour cannot be the main mechanism whichmaintains this polymorphism. Interestingly, we founddifferent percentages of embryo-containing seeds infruits set under natural conditions as well as in thoseobtained from experimental crosses. In particular, pinkmorph showed a very low intrinsic fertility. Moreover,fertility in intra- was higher than in inter-morph crosses.To our knowledge, this is the first study pointing out theoccurrence in the orchid family of post-pollinationreproductive barriers. Findings are discussed in light ofpresent hypothesis on the evolution and maintenance ofcolour polymorphism in deceptive orchids and otherangiosperms.