Pollination ecology of Monarda didyma L., M. clinopodia L. and hybrids in the southern Appalachian Mountains

Monarda didyma L. and M. clinopodia L. (Lamiaceae) occasionally form introgressive populations in the mountains of Tennessee and North Carolina. Natural hybridization, pollination ecology, breeding systems and crossing relationships of the two species were studied to evaluate the role of pollinator specificity and constancy in the isolating mechanisms of the two. Hybrid indices and scatter diagrams were used to analyze morphological relationships in six populations. The following characteristics were examined to determine whether any differences existed among species and hybrids: : flower color, size and odor; breeding systems; pollen stainability; nectar composition, volume and concentration; natural seed set; and crossing relationships. Flower visitors were observed in natural populations and in evenly-spaced mixed plots to determine the pollinators of each species, their relative frequencies, and the constancy of the major pollinators.

Hummingbirds are the major pollinator of M. ( didyma, while M. clinopodia is pollinated by bumblebees. Pollinator specificity is not absolute; bumblebees occasionally visit and pollinate both species. The bees displayed a low but significant constancy in the mixed plots; the constancy of hummingbirds was not statistically significant. Pollinator specificity and constancy do not constitute a major barrier to hybridization, and temporal and genetic barriers are also weak. Adaptation to different edaphic conditions is probably the major barrier to gene flow between the two species.