Floral biology of hybrid zones

Hybridization between closely related species is relatively common in angiosperms and can create a natural hybrid zone. We review recent experimental studies of floral biology in pairs of hybridizing species, emphasizing comparisons of the floral morphology and nectar rewards of hybrid plants with that of their progenitors, and quantifying the influence of these floral traits on pollinator behaviour and prezygotic and post-zygotic reproductive isolation. Floral traits of hybrids can be intermediate or transgressive. Floral differences between species, which in one case are attributable to particular chromosomal regions, can have differing impacts on interspecific flights by pollinators. A simulation model of mating in a hybrid zone between Ipomopsis aggregata and Ipomopsis tenuituba shows that behavioural responses by hummingbird and hawk-moth pollinators affect pre-zygotic ethological isolation more strongly than mechanical isolation. We apply this model to compare contact sites between these species which differ greatly in the frequency of natural hybrids. Striking differences in hawk-moth behaviour between the sites generated large differences in the rate of interspecific pollen movement, potentially explaining the dissimilar frequencies of hybrids. Although floral traits influence both the formation and fitness of hybrids primarily through effects on pollinators, impacts on plant enemies also need consideration. Recent research has revealed much about how floral traits influence pollinator visitation, but mainly for systems with hummingbird versus insect pollinators. Such studies should be extended to other pollinators. Further research is also needed on how floral traits influence pollen dispersal and other post-visitation events that impact reproductive isolation, and their genetic basis in natural populations.