Gesneriaceae:
Pollination (part 1)
 

 

 

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).

 

Animal pollinators

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:

  1. Nectar flowers.  Corolla with a distinct (often long) tube, coloration ± conspicuous, limb zygomorphic, nectar-secreting disc present, nectar often accumulating in distinct bulges at the corolla base; pollen sticky.

  2. Deceptive nectar flowers.  The flowers exhibit the general habit of nectar flowers, but the nectary, if present at all, is non-functional. The flowers of some species of Ornithoboea may belong to this type: by the spreading calyx and the corolla shape they strongly resemble orchid flowers; they may indeed mimic orchid flowers (Calanthe?), but at least bear the general habit of nectar flowers.

  3. Pollen flowers.  These exhibit the general character syndrome of oligandric pollen flowers: nodding flower position; corolla short-tubed, campanulate or flat-faced, with (sub)regular limb; stamens sometimes five, anthers ± exposed, large, sometimes forming a cone or coherent, often bright yellow or orange, dehiscing by apical pores or transversal slits, pollen non-sticky, powdery; nectary non-functional or reduced. Flower exploitation is apparently by buzzing. Striking examples include Ramonda, Conandron, Didymocarpus cordatus, Bellonia, Niphaea, Phinea and Napeanthus.

  4. Partially and fully deceptive pollen flowers.  Simulation of copious pollen is by (a) yellow anther walls (e.g., Saintpaulia: by the robust, yellow wall the anthers are attractive for a much longer time than pollen is available), (b) a yellow blotch on the corolla above the anthers (e.g., Petrocosmea kerrii), (c) yellow filament knees (Henckelia caerulea: exserted filaments with yellow knees, the true anthers are of cryptic colour), (d) yellow blotches on the corolla near filament insertion (e.g., Henckelia puncticulata, (e) a patch of yellow, moniliform hairs at the palate (plastic imitation of pollen grains?: Stauranthera), or (e) an exserted yellow style (Henckelia geitleri: style studded with yellow glands).

  5. Perfume flowers.  This flower type is known from Gloxinia perennis (discovered by Stefan Vogel 1966), and (probably) Monopyle. The corolla is rather short- and broad-tubed, and an osmophore (tissue producing the fragrance) is present near the base, replacing the nectary. The osmophore consists of palisade-like, purple-brown epidermal cells that secrete tiny droplets of terpenes that are collected by bees.

 

Fragrance

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

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.

  1. Non-euglossine-pollinated bee flowers.  Bee flowers prevail by far in the Old World Gesneriaceae. However, almost nothing is known about their actual pollinators. Detailed field studies are completely lacking, and even casual observations are very rare. The floral structure suggests that the collected rewards are nectar and/or pollen.

  2. Euglossine-pollinated nectar flowers.  Both male and female Euglossines visit the flowers in search of nectar. The corolla is relatively large, broad-tubed, the limb is subregular, with rounded lobes, color is light (white, light yellow or rose), and a nectary and nectar are present. The frequently encountered “episcioid” flower (which apparently evolved independently in several neotropical genera such as Episcia, Drymonia, Nautliocalyx, Paradrymonia etc.) may represent a special subtype of the general bee pollination syndrome: color white or whitish, corolla lobes conspicuously dentate or fimbriate, corolla tube with a dorsal sac or blunt spur at the base.

  3. Euglossine-pollinated perfume flowers. This flower type is more specialised and exclusively addressed to male Euglossine bees. They collect the fragrance compounds produced by the osmophore (see "Perfume Flowers" above). The perfume is used by the male bees to mark the swarm areas and to attract female bees. Outside Gesneriaceae this pollination type is known from numerous orchids, some Solanaceae, Araceae and Euphorbiaceae.  Note the picture above.

 

(Presumed) pollination by butterflies, moths and flies

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. 

 

 

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