Print PageSpeciation involves the evolution of reproductive barriers between divergent lineages. Mechanical pollinator isolation, one of the prezygotic isolating barriers in plants, results from the inability of pollinators to successfully transfer pollen from the anther of one flower to the stigma of another. Mechanical pollinator isolation can be divided into two components: legitimacy of visitation and pollen transfer success.
The most ecologically similar of the Louisiana Iris species, I. brevicaulis and I. fulva display divergent floral morphologies and are primarily visited by different pollinators. In previous studies, bumblebees primarily visited I. brevicaulis whereas hummingbirds frequented I. fulva. However, visitation is not synonymous with pollination, and these pollinators may sometimes use illegitimately “rob” nectar or may be otherwise unsuccessful at transferring pollen between plants.
We observed the behavior of bumblebee and hummingbird pollinators that visited I. brevicaulis and I. fulva and classified the visit as “legitimate” or “illegitimate”. If the pollinators directly entered the pollination unit, the visit was recorded as legitimate. However, if the pollinators landed in the center of the flower and accessed overflowing nectar without entering the pollination unit, the visit was considered illegitimate.
To assay pollen transfer success, we dusted some fluorescent powder dyes on the anthers of I. brevicaulis and I. fulva. Once the pollinator legitimately visited the flowers which contained fluorescent dye and subsequently visited another flower that was not dusted, we collected the non-dusted flowers immediately after visitation and saved them in the sealed bags and used UV light to check whether the flowers were successful pollinated.
Results/Conclusions
The number of legitimate floral visits to each species significantly differed between pollinators. For I. fulva, legitimacy significantly differed between pollinator species, as 62 of 65 hummingbird visits were legitimate but only 52 of 158 of bumblebee visits were legitimate. For I. brevicaulis 6 of 58 hummingbird visits were legitimate and 52 or 70 bumblebee visits were legitimate. The number of successful and unsuccessful pollen analogue transfers also differed between pollinators for both Iris species.
For I. fulva flowers, only 1 of 13 bumblebee visits resulted in the successful transfer of the dye, while 26 of 37 hummingbird visits were classified as successful. For I. brevicaulis, 13 of 18 bumblebee visits were successful, while only 3 of 18 hummingbird visits were successful at transferring dye.
In conclusion, differences in legitimate visitation and pollen transfer result in mechanical pollinator isolation between Iris fulva and I. brevicaulis.
