Evol Ecol Res 18: 429-441 (2017) Full PDF if your library subscribes.
Pollinator isolation in Louisiana iris: legitimacy and pollen transfer
Joshua P. Shaw, Sunni J. Taylor, Mary C. Dobson and Noland H. Martin
Department of Biology, Texas State University-San Marcos, San Marcos, Texas, USA
Correspondence: N.H. Martin, Department of Biology, Texas State University-San Marcos, San Marcos, TX 78666, USA.
Background: Iris brevicaulis and Iris fulva can be found in sympatric populations in southern Louisiana. Ethological pollinator isolation is an important reproductive barrier, as the pollination syndromes of Iris brevicaulis flowers result in primarily bumblebee visits, while the pollination syndromes of Iris fulva primarily result in hummingbird visits. Other potentially important components of pollinator isolation – namely, ‘legitimacy of visitation’ (i.e. whether or not a floral visitor physically interacts with a flower in such a way that it can pick up pollen from an anther or transfer pollen to a stigma) and ‘pollen transfer success’ (i.e. whether or not a ‘legitimate’ floral visitor successfully transfers pollen from one flower to the stigma of another) – have not been explored.
Hypotheses: Hummingbirds, the primary floral visitors of I. fulva, are more likely to visit I. fulva pollination units legitimately than they are the pollination units of I. brevicaulis. Furthermore, hummingbirds are more likely to successfully transfer pollen in I. fulva intraspecific visitation bouts compared with either interspecific or I. brevicaulis intraspecific floral visits. Bumblebees, the primary floral visitors of I. brevicaulis, are more likely to visit I. brevicaulis pollination units legitimately than they are those of I. fulva. In addition, bumblebees are more likely to successfully transfer pollen in I. brevicaulis intraspecific visitation bouts compared with interspecific or I. fulva intraspecific floral visits.
Methods: We recorded the proportion of legitimate floral visits made by bumblebees and hummingbirds to both I. fulva and I. brevicaulis flowers. We also assayed intraspecific and interspecific pollen transfer effectiveness among bumblebees and hummingbirds visiting I. fulva and I. brevicaulis flowers using powdered fluorescent dye as a pollen analogue.
Results: The proportion of legitimate hummingbird visits was higher in I. fulva flowers than in I. brevicaulis flowers, and hummingbirds were more effective at transferring pollen among I. fulva flowers than between I. fulva and I. brevicaulis flowers or among I. brevicaulis flowers. Conversely, the proportion of bumblebee visits that were legitimate was higher in I. brevicaulis than in I. fulva, and bumblebees were more effective at transferring pollen among I. brevicaulis flowers than between I. brevicaulis and I. fulva flowers or among I. fulva flowers. Calculations of reproductive isolation (RI ) were quite high such that reproductive isolation due to hummingbird visitation legitimacy was calculated as RI = 0.892, and that due to bumblebee visitation legitimacy as RI = 0.557. Isolation due to hummingbird pollen transfer efficiency was calculated as RI = 0.747, and that of bumblebees as RI = 0.711.
Keywords: bumblebee pollination, hummingbird pollination, hybridization, mechanical isolation, pollen transfer, pollinator isolation, pre-zygotic isolation, reproductive isolation.
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