Effects of local density and flower color polymorphism on pollination and reproduction in the rewardless orchid Dactylorhiza sambucina (L.) Soò

The rewardless orchid Dactylorhizasambucina shows a stable flower colour polymorphism,with both yellow- and red-flowered morphsgrowing sympatrically. Pollination biology andbreeding system were investigated to examine theeffects of density of plants, colour polymorphism,inflorescence dimension, and flower position withininflorescence on male and female reproductivesuccess in three natural populations of D. sambucina.There were significant differences among sitesin the number of pollinia removed and in fruit setper inflorescence. Number of removed pollinia andcapsule production in D. sambucina were independentfrom flower and inflorescence size or flowerposition. As a whole, the red morphs showedthe highest number of capsules produced, while theyellow morphs had the greatest male success. Therelative male and female reproductive success wereindependent from plant density but were significantlycorrelated with the yellow morph frequencyat the population level. Overall, our findings showthat the contribution to the total reproductivesuccess deriving from the two colour morphs doesnot conform with the predictions of negativefrequency-dependent selection.