Allomones

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Allomones are allelochemicals that evoke in the receiver ofthe signal a behavioral (releaser effect) or physiological response (primer effect) that is adaptively favorable to the sender but not the receiver. This definition includes repellent or toxic compounds, which provide defense against attack or infection (e.g., secondary plant metabolites), suppressants, which inhibit competitors (e.g., antibiotics, allelopathics), and venoms, which poison prey organisms (venoms of predatory animals). Another group ofallomones is ofthose which help an organism to escape from predators, such as the ink used by cephalo-pods to confuse predators (see Fungal Defense Strategies).

Besides functioning as repellent or defensive compound, allomones can be used to manipulate the behavioral response of the receiver. Compounds that fall into this category are, for example, chemical lures, which attract prey to the allomone-producing predator. This principle has been realized, for example, in carnivorous plants. Besides visual signals, pitcher plants of the species Nepenthes albomarginata offer a fringe of edible white hairs to lure and then trap its prey, which consists exclusively of termites in enormous numbers. Other carnivorous plants use a combination of extrafloral nectar and an olfactory cue to attract insect prey from longer distances (e.g., Nepenthes spp., Heliamphora spp.) or soluble attractants to lure unicellular organisms (e.g., Genlisea spp., Utricularia spp.). The latter mechanism may also be used by carnivorous fungi (common genera: Arthrobotrys, Dactylaria, Dactylella, and Trichothecium). In general, olfactory attraction of prey to carnivorous plants or fungi has been rarely investigated but will likely provide a bonanza of respective examples.

Allomones can also be signals, which are normally involved in intraspecific communication (i.e., phero-mones), yet are exploited by a heterospecific sender to its own advantage. An example for an aggressive chemical mimicry is the hunting strategy of the bolas spider (Mastophora spp.) (Figure 11a). The adult female emits chemical attractants that mimic the sex pheromones of its moth prey, thereby exploiting the inherent mate-finding behavior of its victims, which are male moths. The hunting spider produces a specialized web, the so-called bolas, which is held by one ofthe forelegs and consists ofa sticky ball attached to a small thread. When the prey approaches the lurking spider within striking distance, the sticky ball is swung at the moth to catch it.

Mastophora

Figure 11 Examples of allomones involved in chemical mimicry. (a) The bolas spider (Mastophora phyrnosoma) emits a chemical mimic of its prey's sexual pheromones. As soon as the lured male moths get into striking distance, the spider swings the sticky ball of glue which is suspended on a short thread at its prey to catch it. (b) The orchid Ophrys sphegodes lures males of the solitary bee Andrena nigroaenea by emitting semiochemicalsthat resemble the bee's sexual pheromone as well as visual cues that mimic female bees. (c) A sexually excited male of A. nigroaenea pollinates a flower of O. sphegodes during attempts to copulate with it. (a) Photograph by Kenneth F. Haynes, with permission. (b) and (c) Photograph by Manfred Ayasse, with permission.

Figure 11 Examples of allomones involved in chemical mimicry. (a) The bolas spider (Mastophora phyrnosoma) emits a chemical mimic of its prey's sexual pheromones. As soon as the lured male moths get into striking distance, the spider swings the sticky ball of glue which is suspended on a short thread at its prey to catch it. (b) The orchid Ophrys sphegodes lures males of the solitary bee Andrena nigroaenea by emitting semiochemicalsthat resemble the bee's sexual pheromone as well as visual cues that mimic female bees. (c) A sexually excited male of A. nigroaenea pollinates a flower of O. sphegodes during attempts to copulate with it. (a) Photograph by Kenneth F. Haynes, with permission. (b) and (c) Photograph by Manfred Ayasse, with permission.

Another example of chemical mimicry, which is clearly maladaptive to the responder, occurs between Ophrys orchids and their hymenopteran pollinators (Figure 11b). These nectarless orchids mimic sex pher-omones as well as visual and tactile cues to attract particular species of Hymenoptera, mostly bees (Apoidea) and wasps (Scolioidea, Sphecoidea). Lured by the odor of a virgin queen and an insect-like shape of the flower, the pollinating male lands on the labellum and attempts to copulate with it. During these pseudocopulations, the male contacts the pollinia, which become attached to its head or abdomen. Repeated attempts to copulate at different conspecific flowers effect pollination. The flower's odor has been identified as the most important cue for eliciting male mating behavior in this interaction.

The strong reliance of social insects on a chemical communication system opened the door for exploitation of these signals to intraspecific and interspecific parasites. Integration of symbionts into colonies often involves mimicry of chemicals that are characteristic to the social species. Many insects live inside colonies of social insects where they prey upon colony members, parasitize food resources, or just enjoy social benefits, such as protection from enemies. The myrmecophilous scarabaeid beetle Myrmecaphodius excavaticollis (Blanchard), for example, penetrates ant nests of the genera Solenopsis and Iridomyrmex by mimicking the cuticular hydrocarbons of the host colony. The beetle moves freely inside the nests and feeds on ant larvae, dead ants, and regurgitated food from ants. Many other examples for inquilines in social insect nests, which use chemical mimicry, are known from ants and termites and to a lesser extent also from social bees and wasps.

Another example for deception based on chemical cues can be found in the parasitic ant genus Formicoxenus. These ants are called xenobiotic because they live in close association with their Myrmica host from which they obtain food by tropholaxis. In this case, the parasitic ants mediate integration into the host society by acquiring cuticular hydrocarbons from the hosts. Such a case, in which an organism does not biosynthesize the chemical mimics by itself but acquires them from the model, is called chemical camouflage and is not an example for an allomone according to the definition presented above.

See also: Fungal Defense Strategies; Plant Defense Strategies; Pheromones.

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Responses

  • Hamid Girmay
    Is allomones a example of intraspecific hormone?
    4 years ago

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