With few exceptions, the flowers of Gesneriaceae are cross-pollinated and animal-pollinated (zoophilous), with insects (mainly bees), birds and bats acting as pollen vectors. The evolutionary diversification of the family and speciation is closely linked with the mode of pollination. Field studies on pollination have been performed to a limited extent, and much work remains to be done. Nonetheless, some major trends have been explored or can be concluded from the flower characters and pollination syndromes.
Avoidance of self-pollination
In order to present self-pollination, three principal mechanisms have been developed in gesneriad flowers: (1) protandry (the pollen of the anthers ripens and is shed before the stigma of the same flower is receptive), (2) protogyny (the stigma is receptive before the pollen of the same flower is shed and no longer receptive when the pollen is shed), and (3) enantiostyly (left-right-position of the style combined with a reciprocal position of the anthers).
Protandry dominates by far in the Gesneriaceae, while protogyny seems to be restricted to some bat-pollinated species of Gesneria. Enantiostylous flowers occur in Didymocarpus, Henckelia and Saintpaulia (where, however, the stamens remain in a stable position).
As far as can be inferred from the floral characters (pollination syndromes), animals involved in pollination include insects, birds and bats. Regarding the insects, bees (Apidae, including the Euglossini in the neotropics, Anthophoridae, probably Xylocopinae and others) play the dominant role. Pollination by butterflies seems to be rare and the same holds true for moths and dipterans. Wasps, gnats, hover flies and other unusual flower visiting insects are possibly involved in pollination as well, but certainly play an insignificant role.
Floral rewards, dummies and respective flower categories
Regarding the floral rewards (or imitation of rewards), five types can be distinguished:
Attraction of pollinators is primarily by visual cues. The majority of Gesneriaceae has flowers that are devoid of any floral scent. This particularly holds true for the ornithophilous flowers. However, for a number of species (from c. 15 genera, essentially neotropical) fragrance has been reported. The range of odours is surprisingly wide: sweet honey-like, lemony, musky, clove-, cabbage-, carrion-like and others. Scent seems to be addressed to special pollinators, e.g., Sinningia tubiflora, with powerful lemony fragrance, to moths, or Capanea cf. grandiflora, with cabbage-like scent, to bats. Fragrance has also been noted in some paleotropical species, such as Streptocarpus vandeleurii. See the article Fragrant Gesneriads (Boggan).
Bee flowers providing nectar are usually broad-tubed, infundibuliform or (zygomorphic-)campanulate, with a wide open mouth (gullet or bell flowers) and of light color (white, blue, violet, yellow in various shades and combinations). Personate flowers (mouth closed by a vault of the palate) are rare, examples include Rhynchoglossum, Didymocarpus antirrhinoides and D. corchorifolius. Pollen deposition is mostly on the head or back of the insect. In Drymonia serrulata glandular trichomes inside the corolla secrete an oil which is deposited on the anthers and transferred to the thorax of visiting bees, facilitating pollen adhesion. The remarkable “salt-shaker”-mechanism of the coherent anthers of Drymonia is described under Floral structure.
In the pollination of Gesneriaceae a wide range of bees is involved. The dominant group is the Apidae, from which family the euglossine bees (tribe Euglossini, an exclusively neotropical group of bees, related to bumble bees) play an important role in the neotropics. The Anthophoridae and the Xylocopinae may also have some significance. Regarding the pollination by euglossines, two different pollination syndromes must be distinguished. These are associated with flowers providing either nectar or perfume as a floral reward.
In general, flowers polllinated by butterflies provide nectar as a floral reward, have a hypocrateriform corolla (with a long, narrow tube and a flat limb) and exhibit red or blue flower colors. These characters apply to some species of Achimenes, Sinningia and Episcia. From that it may be inferred that they are pollinated by butterflies, but factual evidence is lacking so far. Moth-pollinated flowers are generally long- and narrow-tubed, white-, cream- or yellow-colored, and are strongly fragrant. Sinningia tubiflora is a striking example exhibiting this syndrome, and there seems to be little doubt that this species is moth-pollinated. The “key-hole flowers” of some Streptocarpus species (tube narrow, limb flat, tube entrance a small, laterally compressed hole) are probably visited by long-proboscid flies.
Pollination by birds
Pollination by birds (ornithophily) occurs both in the paleotropical and neotropical Gesneriaceae, as well a in the Southern hemisphere Gesneriaceae (Rhabdothamnus; Mitraria, Sarmienta, Asteranthera). It is rather rare in the paleotropical Gesneriaceae, where only a few genera (Aeschynanthus, Agalmyla) and particular species of species-rich (essentially bee-pollinated) genera (Henckelia, Streptocarpus, Cyrtandra) exhibit an ornithophilous pollination syndrome (flowers bright red, tubular or salverform, with abundant nectar). Almost all species of Agalmyla and Aeschynanthus are climbers and epiphytes, and display their bright red flowers (often in dense bunches) high above the ground. In some Malayan species of Aeschynanthus the nectar sugar composition has been studied. The low sucrose content and the roughly 1:1 ratio fructose to glucose proved to be in accordance with typical bird-flowers (1991). Observations or special studies of actual pollination are largely lacking. The probable bird pollinators in Africa and Asia are sunbirds (Nectariniidae) and, in the area from Sulawesi to New Zealand, honey-eaters (Meliphagidae).
In contrast, bird pollination is apparently very frequent in the neotropical Gesneriaceae (perhaps 60% of the species). Here the pollen vectors are essentially hummingbirds (Trochilidae). It is remarkable that the distribution centre of neotropical Gesneriaceae and that of hummingbirds coincide (Colombia and Ecuador). There is a strong correlation of ornithophily and an epiphytic habit of the plants, but also many terrestrial species are pollinated by hummingbirds.