Nectar characteristics of interspecific hybrids and their parents in Aesculus (Hippocastanaceae) and Iris (Iridaceae)

We studied nectar production characteristics (volume, sugar concentration, and sugar output) of parental and interspecific hybrid plants of the dicot Aesculus and the monocot Iris. in Aesculus, the bird-pollinated A. pal ia produced significantly larger volumes of significantly Less concentrated nectar than did the bee-pollinated A. sylvatica (7.81 +/- 0.46 mu l vs. 0.99 +/- 0.08 mu l, mean +/- SE, and 28.9 +/- 0.4% vs. 50.3 +/- 0.5%); sugar output for A. pavia was also significantly higher than in A. sylvatica (3.53 +/- 0.33 mg vs. 0.84 +/- 0.04 mg). Plants sampled from a natural hybrid swarm were intermediate between the parental species with respect to nectar volume (5.85 +/- 0.14 mu l) and sugar output (1.76 +/- 0.04 mg), but they produced nectar with significantly lower sugar concentration than both parental species (24.3 +/- 0.2%). In Iris. the bird-pollinated T. fulva produced significantly smaller volumes of significantly less concentrated nectar than did the bee-pollinated I. brevicaulis (20.4 +/- 2.5 mu l vs. 56.6 +/- 6.0 mu l, and 18.3 +/- 0.6% vs. 23.6 +/- 0.3%). The net result was significantly lower sugar output for I. fulva as compared to I. brevicaulis (4.3 =/- 0.6 mg vs. 14.7 +/- 1.7 mg). Regardless of which parental species served as the maternal parent, experimentally generated F-1 interspecific hybrids were almost perfectly intermediate between their parental species with respect to nectar volume (36.6 +/- 4.2 mu l and 36.2 +/- 2.1 mu l) and sugar output (9.8 +/- 0.7 mg and 9.8 +/- 1.4 mg), but their mean concentration was more similar to that of I. brevicaulis (23.3 +/- 0.3% and 24.0 +/- 0.4%). It is unclear whether or not pollinator-mediated selection has shaped and/or maintained the large differences in nectar characteristics between these species.