Systematics and floral evolution of Nicotiana

In the first part of this project, we reconstructed a phylogenetic hypothesis of Nicotiana using sequence data of the nuclear ribosomal internal transcribed spacer region. Most sections agree with Goodspeed's phonetic hypothesis for the genus, however, our analysis suggests a rearrangement at the sub-familial level. ITS sequences in Nicotiana appear to be evolving rapidly and did not give resolution at the species level.; We then investigated pre- and post-mating barriers to reproduction between two interfertile species of section Alatae, Nicotiana alata and N. forgetiana. Hawkmoths pollinate N. alata while primarily hummingbirds visit N. forgetiana. Two experiments were conducted near an area of parapatry in Rio Grande do Sul, Brazil to examine reproductive isolation and hybrid fitness: (1) experimental sympatric plots containing both species and (2) plots of both species plus F 1 hybrids. In 1998 and 1999 in the mixed species plots, hawkmoths showed strong preference for N. alata but visited N. forgetiana on average 16.5% of the total visits. Hummingbirds were only observed in 1999 and only visited N. forgetiana. Significantly less hybrid offspring were produced than the frequency of interspecific pollinator movements and N. forgetiana was the seed parent of ninety-seven percent of the F1 hybrids. In plots with F1 hybrids and both parental species, hawkmoths preferred N. alata in two of three plots while hummingbirds preferred N. forgetiana in all plots. F1 hybrids received as many or more pollinator visits overall as the parental species in two of three plots. Hybrid plants were equally fit as the parent species in several life history traits. Pollinator discrimination among species is strong but is an imperfect premating isolation mechanism and is lower when F1 hybrids are present.; In a third field experiment, we used two plots of F2 hybrids from Nicotiana alata and N. forgetiana to simulate initial stages of sympatric speciation via pollinator-mediated selection under natural conditions in Southern Brasil. Since hawkmoths pollinate N. alata while hummingbirds primarily pollinate N. forgetiana, we hypothesized that hawkmoths and hummingbirds would overvisit plants with floral traits similar to N. alata and N. forgetiana , respectively. Hummingbirds were not seen in either plot. In plot 1, hawkmoths significantly preferred flowers with wider floral limbs and longer floral tubes. F3 plants descended from these hawkmoth-pollinated fruits had significantly wider limbs and longer tubes relative to random hand-pollinated control plants descended from the same F2 plants. In plot 2, hawkmoths only preferred flowers with longer floral tubes and larger annulus diameters and these preferences were weaker than in plot 1. F3 plants descended from these F3 hawkmoth-pollinated fruits were not significantly different for any floral trait means relative to F3 plants descended from random hand-pollinated control crosses. The selection differential applied by hawkmoths was smaller in plot 2 because the mean flower size in F2 plot 2 was larger but the average size flowers visited by hawkmoths was similar for both plots.