Hybrid intermediacy between pollination syndromes in Penstemon, and the role of nectar in affecting hummingbird visitation

Flower size dimensions, colour, and nectar quantities potentially account for differences in pollinator species between Penstemon centranthifolius (Benth.) Benth. (tubular, red, nectar-rich, hummingbird pollinated) and Penstemon spectabilis Thurber (wide, purple, nectar-poor, hymenopteran pollinated). For this study, floral characters and pollinator attraction were measured for the two species, F-1 hybrids, and backcrosses. Floral dimensions, nectar characters, and color spectra combined in the hybrids in a largely linear fashion, consistent with an interpretation of net additive gene expression. In other words, the changes from one pollination syndrome to another appeared to be quantitative. There were some deviations from linearity, but not so much as to obscure the linear effect. Generally, backcrosses were not more variable than F-1 hybrids or pure parents. Hummingbirds preferred P. centranthifolius over P. spectabilis. The strength of this preference varied by year, and the birds did not respond to hybrids in a strictly linear fashion. When there was a preference, adding nectar hourly to under-visited plants quickly changed the foraging of hummingbirds. Bees in the genus Ceratina preferred the bee-pollination syndrome of P. spectabilis.