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 p roduced significantly larger volumes of significantly Less concentrated nec tar 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 outp ut 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 volu me (5.85 +/- 0.14 mu l) and sugar output (1.76 +/- 0.04 mg), but they produ ced nectar with significantly lower sugar concentration than both parental species (24.3 +/- 0.2%). In Iris. the bird-pollinated T. fulva produced sig nificantly smaller volumes of significantly less concentrated nectar than d id 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 a lmost 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 s imilar to that of I. brevicaulis (23.3 +/- 0.3% and 24.0 +/- 0.4%). It is u nclear whether or not pollinator-mediated selection has shaped and/or maint ained the large differences in nectar characteristics between these species .