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Plant-pollinator associations often are a form of mutualism - an association between organisms of two different species in which each member benefits. Pollinators benefit from this association by obtaining food resources when visiting a flower and the plant benefits because its pollen is transported between flowers. An example of mutualism is the yucca moth (Family: Incurvariide) and yucca plants (Genus: Yucca) - the yucca moth is genetically programmed for stuffing a ball of pollen into the cup-shaped stigma of each yucca plant flower. Neither the plant nor the moth can survive without the other (Reference: The Yucca and Its Moth: Pollination That Depends on a Small Moth, W.P. Armstrong In Zoonooz, Vol. 72 (4), pp. 28-31, April 1999).
Pollinator-Plant Associations
Pollination by animals will only occur if it is advantageous to the pollinator or if the plant capitalizes upon other adaptive pollinator traits, such as foraging or mating behavior, in such a way that is not detrimental to that pollinator. Therefore, to ensure pollination by animals, a plant must attract the pollinating animal, either by offering a reward (often food) or by mimicking cues of a reward, such as the appearance and/or odor of a food item or mate. The plant must also ensure that the pollen from one flower makes its way to another flower of the same species.
Some plants have evolved specialized flower parts and other traits that appear to promote more efficient transfer of pollen by pollinators; these traits are sometimes called pollination syndromes. In some cases, the pollinator also has specialized features that make it especially suitable for pollinating that particular plant. Such specialized traits of the plant and the pollinator are thought to have come about through an escalating process of coevolution.
Specialization is sometimes so extreme that the plant has become utterly dependent upon a single species of pollinator; in some cases the pollinator is also dependent upon one species of plant. Such plant-pollinator associations can be advantageous for the plant by promoting maximum efficiency of pollination and minimal expenditure of a plant's energetic resources. The pollinator, too, can benefit if its specialized traits allow for more efficient foraging or superior protection from predators. However, specialization can also be dangerous: if one member of the mutualistic association declines, the other will decline as well.
Highly specialized plant-pollinator associations are not the rule. Most plants are visited by more than one pollinator, and most pollinators visit and pollinate more than one plant. It is often the most obvious pollinators (e.g. large diurnal species), or those thought to be most efficient, that are most commonly thought of in association with a particular plant. However, other pollinators can play important pollination roles as well. Many insects and other animals visit flowers but most are not well understood in terms of their basic biology and ecology, much less in terms of their potential role as pollinators. In other cases, pollinators that are known to generally have little impact on a plant's reproductive success gain importance when the primary pollinator is not available. Finally, some pollinators are important primarily because they interact with the primary pollinator in such a way that its pollination becomes more efficient.
Pollination by Deception
A male thynnine wasp (Neozeleboria cryptoides) attempts to mate with an orchid flower (Chiloglottis trapeziformis). Photo Rod Peakall.
Pollinators are attracted to plants that appear to provide a reward. Most often, the pollinator does receive a reward, such as food or shelter for growing young. But looks (and scents) can be deceiving. Some plants attract pollinators with deceptive cues such that pollination is achieved for the plant but the pollinator gets nothing in return.
Well-known examples of pollination by deception involve orchids and the wasps and other insects that pollinate them. Some orchid flowers attract male insects by mimicking the odor and appearance of female insects. Males attempt to mate with one flower, and then another, transferring pollen among flowers in the process. Though some orchid flowers closely resemble the appearance of females of the primary pollinating species, odor may be the most important cue. In at least some cases, the odor compound emitted by the orchid is identical to the female sex pheromone emitted by the pollinator species (Reference: Pollination by Sexual Deception in Australian Terrestrial Orchids, Rod Peakall, Australian National University, 2007).
Other orchids mimic the pollinators' food plants. For example, the southern African orchid Disperis capensis produces flowers that closely resemble those of another plant, Polygala bracteolata. Bees pollinating P. bracteolata (which does provide a food reward) inadvertently pollinate the lookalike as well.
There is yet another twist on pollination by deception. Certain insects are saprophilous, meaning they live on decaying material, or coprophilous, meaning they live or grow on excrement. Several plant species (e.g. Arum spp., Symplocarpus spp.) have taken advantage of this by mimicking the odors of dung or carrion to attract their fly and beetle pollinators. For example, Malaysian dung beetles of the genus Onthophagus (Family: Scarabaeidae) are the major pollinators of Orchidantha inouei, a plant related to gingers, bananas, and bird-of-paradise plants. The flowers of O. inouei appear to attract the beetles by emitting a dung-like (i.e. food) odor, but do not provide any food reward or protected space for the beetle.
You Complete Me: The Fig Wasp - Fig Obligate Pollination Mutualism
Female fig wasp (Pleistodontes froggatti) laying eggs in a Moreton Bay fig (Ficus macrophylla) fruit, Australia 2004. Photo by James M. Cook.
The relationship between fig trees (
Ficus
spp.) and their wasppollinators (Family: Agaonidae, Family: Chalcidoidea) is one of only about a half dozen documented obligate pollination mutualisms between plants and their seed-consuming pollinators. Other examples include the yucca - yucca moth and the recently documented tree (
Glochidion
spp.) - moth (
Epicephala
spp.) associations. In an obligate pollination mutualism, the plant and its pollinator are totally dependent upon one another to complete reproduction.
The fig fruit is actually a specially adapted inflorescence called a syconium, which conceals many tiny flowers. Pollination begins when a female wasp, already covered with pollen from the fig in which she hatched and developed, flies to a new fig syconium and enters a tiny hole at one end. In the process, the wasp's fragile wings often break off. Inside the syconium, the female wasp crawls among the female flowers, of which there are two types - one with a short style into which her ovipositor fits, and one with longer styles, in which she cannot lay eggs. The wasp deposits an egg inside the ovary of each of several short-styled flowers; the long-styled flowers are fertilized by the wasp's pollen load as she climbs over them in her search for oviposition sites. Once she has laid her eggs, the wasp remains inside the syconium, where she eventually dies.
The wasp eggs develop within the flowers. As an adult, the male wasp will chew its way out of its own flower and will then create a hole in a female's flower from which she can escape. They mate and the female then moves toward the tiny opening at the end of the syconium. To reach the hole, she crawls over male flowers and becomes covered with pollen. The male wasp enlarges the opening, allowing the female to escape the syconium and to fly to another, ripening inflorescence to begin the process again. The male remains inside where he dies.
Until recently, most research on the fig - fig wasp mutualism suggested that each species of fig is pollinated by a single wasp species. Now, however, it appears that at least some fig species may be pollinated by more than one wasp species. Non-pollinating wasp species also reproduce inside fig fruits.
Mutualisms in nature are not all about cooperation. Each member of a
mutualistic association is attempting to maximize its Darwinian
fitness, or genetic representation in the next generation. Therefore,
any mutualism is subject to a kind of push-pull tension because the
reproductive "interests" of the two cooperators, though closely
aligned, are not completely in sync. In the case of the fig and the fig
wasp, this tension can be seen in a negative relationship between the
production of viable seeds by the fig and the production of pollinator
wasp offspring: in other words, the greater the number of wasp
offspring, the fewer the number of viable fig seeds.
Additional information on the fig wasp - fig mutualism and research in this area:
Researcher Profile for Dr. James M. Cook (Imperial College London),
a researcher focusing on ecology and evolution of intimate species
interactions (parasitism and mutualism) and in the evolution of animal
reproductive behaviour; and contributor of the photo used in this
highlight.
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