Pollen flow and post-pollination barriers in two varieties of Dactylorhiza incarnata s.l. (Orchidaceae)

Dactylorhiza orchids are known for their high variation in morphology and distinct varieties have been named in D. incarnata s.l. However, it is not known how these varieties interact in mixed populations and why they remain stable. We conducted three field experiments in West-Estonian populations of D. incarnata to examine if the two most common varieties co-occurring these are separated from each other either by pre-pollination or post-pollination reproductive barriers. We found that pollinia were far more frequently transferred between the purple-flowered var. incarnata and the pale-flowered var. ochroleuca than between plants of the same variety. Furthermore, in hand-pollination and germination experiments concerning the same two varieties, we found that pollen source (self-pollination, within- or between-varieties pollination) did not affect seed production or probability of fungus infection of the germinating seeds. These two varieties of D. incarnata thus had no pre-pollination or early functioning post-pollination reproductive barriers. Post-pollination barriers may, however, act later in seedling or adult stage.