Kairomones are allelochemicals that evoke a behavioral (releaser effect) or physiological response (primer effect) in the receiver of the signal that is adaptively favorable to the receiver but not the emitter.
The most well known group of allelochemicals that act as kairomones are probably those involved in the location of food. Such foraging kairomones are exploited by predators, parasites, parasitoids, herbivores, and fungivores during their search for food and/or oviposition sites (Table 2). Depending on the sensory capabilities of the
Table 2 Examples of some allelochemicals that act as kairomones
Origin of kairomone
Predator Hemiptera: Anthocoridae Elatophilus hebraicus
Homoptera: Matsucocidae Matsucoccus josephi
Sex pheromone of M. josephi
Parasite Diptera: Psychodidae Lutzomyia longipalpis
Carnivora: Canidae Vulpes vulpes
Anal and caudal glands of V. vulpes
Parasitoid Hymenoptera: Encyrtidae Acerophagus coccois Aenasius vexans
Homoptera: Pseudococcidae Phenacoccus herreni
Body surface of P. herreni
Herbivore Lepidoptera: Elachistidae Depressaria pastinacella
Apiales: Apiaceae Pastinaca sativa
Essential oil components found exclusively within tissues consumed by D. pastinacella
Fungivore Coleoptera: Cantharidae Malthodes fuscus
Polyporales: Polyporaceae Fomes fomentarius Fomitopsis pinicola
Volatiles emitted from F. fomentarius and F. pinicola receiver and the telltale odor of the sender, a large variety of simple and complex structures can function as kairo-mones. Similar to synomones, kairomones often act in concert with other visual or auditory cues. A strong selection pressure acts on the host/prey organisms to minimize the production of such signals and on the parasitoid/ predator to use the most reliable and detectable cues. These evolutionary constraints are likely responsible for the fact that kairomones involved in food location often fulfill other crucial functions (e.g., sex or trail phero-mones) or cannot be avoided (e.g., feces). The process of host/prey location is usually divided into the following three main steps: (1) locate the correct habitat, (2) locate the host/prey, and (3) evaluate the host/prey in terms of species, developmental stage, and nutritional quality/ suitability for oviposition. Kairomones are particularly important for location and acceptance of the host/prey.
In contrast, habitat location over longer distances may rather be accomplished by using cues, which are associated with the habitat in general than with the host/prey itself.
Kairomones are not only involved in the food location of foraging organisms, but are also used by potential prey or host organisms to detect the presence of natural enemies. This type of kairomones, which elicit certain antipredator responses, has been described for a broad diversity of taxonomic groups including vertebrates (mammals, amphibians, reptiles, and fish) and invertebrates (arthropods, mollusks, cnidarians, and rotifers). Due to the better experimental accessibility, however, the majority of such responses has been recorded in freshwater and marine invertebrates, fish, and mammals; yet, nearly nothing is known for birds and terrestrial insects. The chemical basis of predator-released kairomones is
\ "7 Cladoceran
Thick aperture with tooth
until 50% of typical morphs devoured
Figure 10 Predator-induced polyphenisms. Shown are typical (green) and predator-induced (red) morphologies of various organisms. Numbers beneath each column represent the percentage of organisms surviving predation when both induced and uninduced individuals were presented to predators. (a) Cladocerans (e.g., Daphnia sp.) develop neckteeth and enlarged crests (helmets). (b) Rotifers (e.g., Keratella slacki) produce spined progeny in response to predator-derived kairomones. (c) Exposed to predatory gastropods, barnacles (e.g., Chthalamus sp.) develop asymmetrically, making it more difficult for the gastropod to open the opercular plates. (d) Bryozoans (Membranipora sp.) rapidly grow spines when exposed to waterborne cues from predatory nudibranchs. (e) The mollusk Thais lamellosa develops a thickened shell and a 'tooth' on its aperture when exposed to carps. (f) The carp Carassius carasius responds to piscivorous pike that has already eaten fish by growing into a hunchbacked morph that will not fit into the pike's jaw.
not as well understood as in the case of, say, synomones. Chemosensory risk assessment in prey animals can evoke both behavioral and morphological responses. Behavioral reactions caused by releaser kairomones relate in many cases to escape or avoidance behavior including the increased use of refugia. An example of the prey leaving a high-risk situation once a predator has been detected can be found in sea anemones (Stomphia coccinea) that detach from the substrate and 'swim' in response to kairomones released from predatory sea stars (Dermasterias sp.). Besides such short-term responses, changes in the daily activity pattern can also be observed. The freshwater copepod (Diaptomus kenai), for example, normally descends to deep water (>8 m) at night and ascends to shallow water during daytime (<8 m). These vertical migrations cease in the absence of a predator (Chaoborus trivittatus) and can be reinduced after adding water from a tank holding these predacious fly larvae. Other behavioral responses, which can be provoked by predator-released kairomones, include reduced feeding activity, aggregation (e.g., schooling and shoaling in fish and invertebrates), alarm signaling, defensive body posturing, or an increase in the sensory/detection behavior.
Kairomones released by foraging predators have also been shown to stimulate the production of prey morphologies that effectively inhibit predation and consequently would be termed primer kairomones. Such chemically induced morphologies are common in protozoa, rotifers, cladocerans, and bryozoans and are sometimes called predator-induced polyphenisms (Figure 10). Morphological changes, which are induced by kairomones, usually hamper the predator's ability to ingest or handle its prey (Figure 10) or allow the prey to escape faster (e.g., tadpoles develop a larger tail fin that allows faster swimming). Among the few cases studied, predator-induced polyphenisms have been shown to convey a fitness benefit in the presence of a predator (Figure 10), yet also impose considerable costs under predator-free conditions; the hunchbacked carp morph (Figure 10f), for example, cannot swim as efficiently as the normal phenotype.
Finally, some plant volatiles, which are emitted upon herbivore damage, can also function as kairomones. Such plant-derived compounds can influence the sexual communication of their insect herbivores directly by synergistically increasing the male response to female-derived sex pheromones or by stimulating the production or release of pheromones in the female. Also, aggregations of insect herbivores can be mediated by chemicals that are derived from the host plant. Bark beetles, for example, convert host plant terpenes into oxygenated products that can serve as aggregation pheromones. Such aggregation pheromones may facilitate a better exploitation of the host plant, the finding of mates, or be part of the organism's defense strategy. Since from the insect herbivore's perspective, the benefit of these host plant-derived compounds lies exclusively on its own side, these volatiles meet the definition of kairomones.
Synomones are allelochemicals that evoke a behavioral (releaser effect) or physiological response (primer effect) in the receiver of the signal that is adaptively favorable to both sender and receiver.
The probably best-studied group of chemicals that function as synomones is of those involved in pollination (Table 3). These compounds attract insects, bats, birds, or other animals, which visit the flower to feed on pollen or nectar, thereby pollinating the flower. Usually, both partners benefit from this mutualistic interaction. Pollination mutualisms are widely distributed within the plant kingdom and occur in three phylogene-tically separated groups of plants: cycads, gnetalian conifers, and angiosperms. The vast majority of animal-pollinated plants, however, involves flowering plants (i.e., angiosperms, ^250 000 species). An effective pollination necessitates a certain degree of specialization on the sides of both the visitor and the flower, which is reflected by the pollination syndrome. The pollination syndrome is a suite of floral traits including rewards, which is associated with the attraction of a specific group of pollinators. As resulting from a coevolutionary process with their pollinator(s), animal-pollinated plants show a combination of flowering time, shape, color, and size of flowers, floral scents, as well as type, composition, and amount of a reward (pollen, nectar), which is specifically tailored to fit a specific pollinator or group of
Table 3 Examples for flower colors and chemical compounds involved in attracting different species of animal pollinators
Generalist pollination Diverse insects: drosophilid flies, nitidulid beetles, small beetles
Rubus spp. Ranunculaceae Ranunculus spp.
Arecaceae Chamaedora linearis
White, yellow, red, purple, orange Cream colored
Hymenoptera Bombus spp.
Ranunculaceae Cimicifuga simplex Polemoniaceae Polemonium foliosissimum
Nocturnal moths Sphingidae Manduca sexta
Solanaceae Nicotiana alata
Lime green, red, white, yellow
Necrophilic insects Calliphoridae Lucilia spp. Calliphora spp.
Araceae Helicodiceros muscivorus
Purple pattern (resembles flesh of a dead mammal)
Purple pattern (resembles flesh of a dead mammal)
Microchiroptera Pteropodidae Eidolon helvum Rousettus aegyptiacus
Adansonia digitata pollinators (Table 3). Synomones involved in this kind of interaction are the floral scents, which cover an odor spectrum from weak/moderate pleasant (butterflies) over fruity (beetles), mushroomy (flies), and sweet (bees) to decaying protein (carrion flies) and stale/redolent of fermentation (bats). Also, the chemical compounds that are responsible for flower coloration may be termed synomones. These are mainly flavonoids (orange, red, blue, yellow, white) and carotenoids (yellow, orange, red). The true reason, however, that causes pollinating animals to visit a flower is, of course, the reward, which can be offered as nectar or pollen. Floral nectar consists mainly of glucose, fructose, and sucrose, which are in many cases accompanied by nitrogen-rich amino acids that complement the pollinator's diet. Some plants (e.g., Scrophulariaceae, Malphigiaceae) offer lipids and fatty acids instead of sugars. Also, the pollen itself represents a valuable food source, since it is rich in protein (16-30%), carbohydrates (1-7%), free sugars (0-15%), and fat (3-10%).
A second important group of synomones is volatile compounds that indicate the presence of toxins within a given organism to a predator. Repellent odors emitted from distasteful or potentially dangerous organisms benefit both the predator and the prey. The prey benefits because it is less frequently attacked by experienced predators and the predators profit by not expending time and energy on suboptimal prey. In animals, these odors often accompany a conspicuous warning coloration or striking sounds such as harsh rattles, clicks, or buzzes. Such multimodal warning signals are known from many insect groups including Hemiptera, Coleoptera, Lepidoptera, Orthoptera, Hymenoptera, and Phasmatisdae, and are believed to enhance avoidance learning where there is genuine toxicity. Pyrazines, for example, which are associated with many aposematic, chemically defended insects, have been shown to be involved in the development of a conditioned aversion reaction in naive hatching chicks. Obviously, birds, which are a major group ofinsect predators, can interpret pyrazines as alerting or warning signals and detect it from a distance, probably by olfaction. In analogy to warning of predators by visual or acoustic means, repellent warning odors have been termed olfactory aposematism.
Similar phenomena can also be found in plants, where certain volatiles advertise noxious plant tissues to approaching herbivores. However, the few studies that addressed this issue in plants experimentally detected a rather transient volatile-mediated avoidance reaction, thus suggesting that ongoing toxicosis is necessary for a prolonged protection. These findings make it unlikely that nontoxic plants can mimic the odors of toxic plants (Batesian mimicry) to avoid herbivory, because herbivores constantly sample leaftissues.
The final group of synomones is compounds that are involved in so-called indirect plant defenses, in which plants attract members of the third trophic level (i.e., parasitoids and predators) to increase the predation pressure on herbivores (see Fungal Defense Strategies).
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