Pollinator Preferences for Nicotiana Alata, N. Forgetiana, and Their F1 Hybrids

Abstract

The role of pollinators in plant speciation and maintenance of species boundaries is dubious because most plant species are visited by several types of pollinators, and most pollinator species visit several species of plants. We investigated pollinator preferences and their efficacy as ethological isolation mechanisms between two interfertile species, Nicotiana alata and N. forgetiana and their F1 hybrids. Hawkmoths pollinate N. alata, while primarily hummingbirds and occasionally small hawkmoths visit N. forgetiana. F1 hybrids are easily produced in the greenhouse and although the species grow in similar habitats, hybrids have not been found in nature. In Rio Grande do Sul, Brazil, near where both species are found, experimental plots were studied containing both species, and both species plus F1 hybrids. In the mixed‐species plots, hawkmoths showed a strong preference for N. alata. Hummingbirds were less common and only visited N. forgetiana. Hybrid seed was produced but plants made significantly fewer hybrid offspring than predicted by the frequency of interspecific pollinator movements. Nicotiana forgetiana was the seed parent of 97% of the F1 offspring, suggesting an asymmetry in pollen delivery or postpollination processes. In plots containing F1 hybrids plus both parental species, hawkmoths preferred N. alata and undervisited the other two phenotypes, except that in the third plot they visited hybrids in proportion to the hybrid frequency. Hummingbirds strongly preferred N. forgetiana in all plots but also visited F1 hybrids in proportion to their frequency in the third plot. Overall, F1 hybrids were well pollinated and were frequently visited immediately before or after one of the parental species. Thus hybrids could facilitate gene flow between the parental species. We conclude that pollinator discrimination among species is strong but is an imperfect isolation mechanism, especially if hybrids are present.

Post-pollination hybridization barriers in Nicotiana section Alatae

“…PTGR and NSS data are consistent with field experiments Ippolito et al (2004) studied experimental sympatry plots of N. alata and N. forgetiana, using meta-populations that extended to within 300 m of each other. Hawkmoths preferred N. longiflora overall, but visited N. forgetiana in all four plots.…”

“…N. forgetiana, N. langsdorffii, N. mutabilis, and a putative new species, 'Rastroensis' are all visited by hummingbirds, although the latter two are also visited by Bombus and Halictid bees (Kaczorowski et al 2005). Because hawkmoth probosci are long enough to extract nectar from these bird/bee species, there is the possibility that hawkmoths may also visit them, as we have seen in artificial sympatry plots of N. alata and N. forgetiana (Ippolito et al 2004). Unusually small flowers of N. alata occasionally received hummingbird visits in that experiment so gene flow is possible in both directions where those species' ranges overlap in the Serra Geral in Santa Catarina (SC) and Rio Grande do Sul (RS), Brazil.…”

“…The SI n = 9 clade of section Alatae is comprised of six species, but the phylogenies have not been resolved for N. mutabilis and putative species 'Rastroensis' (dashed lines). Rastroensis and N. mutabilis are placed within section Alatae with nrITS sequence data (adapted from Ippolito 2000). The SC n = 10 clade consists of N. longiflora and N. plumbaginifolia amounts of inter-and intraspecific pollen deposited on receptive species stigmas led to at least 60% conspecific seed set (SS).…”

On ‘various contrivances’: pollination, phylogeny and flower form in the Solanaceae

“…Species of section Alatae with bird and moth-type flowers asymmetrically shared pollinators, with hawkmoths visiting both floral types (Ippolito et al 2004); a similar pattern has been shown in Petunia (Lorenz- Lemke et al 2006). Kaczorowski et al (2005) have suggested that nectar traits, rather than floral form per se, have shaped pollinator preferences in the Alatae.…”

Increased relative abundance of an invasive competitor for pollination, Lythrum salicaria, reduces seed number in Mimulus ringens

“…In this analysis, we used blocks as a replicated measure, and expected values for plant-to-plant transitions were calculated based on the frequency of visits pollinators made to each species (Ippolito et al 2004). Therefore, deviation from expected values should be independent from patterns of pollinator preference (Husband and Barrett 1992;Dafni et al 2005).…”