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ScienceWeek
EVOLUTIONARY BIOLOGY: ON SOCIAL PARASITISM
The following points are made by J.A. Thomas and J. Settele (Nature 2004 432:283):
1) Ants are such formidable predators that perhaps 100,000 other species of insect have evolved mechanisms to coexist with them[1]. Adaptations include armor to resist attack, mimicry to avoid detection, and secretions such as honeydew to feed or appease them[2]. In general, both partners benefit: in return for honeydew, the ants protect aphids from enemies. But natural selection can also favor cheats. It is a short evolutionary step from possessing the attributes to live safely among ants to deploying them against a colony.
2) Thus, among insects as diverse as butterflies, crickets, beetles and flies are specialist "social parasites", perhaps 10,000 species in all, equipped to penetrate the highly protected chambers inside ant nests and feed, isolated from enemies, on the rich resources concentrated there. New work[3] has provided the first molecular-genetic reconstruction of one such evolutionary pathway, that of large blue butterflies (genus Maculinea), including the pathway's divergence into two remarkable strategies for exploiting ants.
3) The large blues form a small genus that has become an icon for conservation across Europe and Asia. The adults fly in summer, laying eggs on specific plants. After two to three weeks of eating flowers, the caterpillar settles beneath its food plant to await discovery by red ants (Myrmica). By secreting hydrocarbons that mimic those made by Myrmica[4], the caterpillar tricks a foraging worker into taking it into the nest, where it is placed among the ant grubs. In most species -- the "predatory" large blues -- the caterpillar then moves to safer chambers, returning periodically to binge-feed on ant grubs. But in two "cuckoo" species, the caterpillars remain among the brood and become increasingly integrated with their society. Nurse ants feed them directly, neglecting their own brood, which may be cut up and recycled to feed the parasites[5].
4) Cuckoo-feeding is an efficient way to exploit Myrmica, resulting in six times more butterflies per nest than is achieved by the predatory species. The downside is that social acceptance is won only through secreting chemicals that so closely match the recognition codes of one host species that survival with any other ant is unlikely. Thus, a typical population of a cuckoo Maculinea species depends exclusively on a single Myrmica species -- which, however, differs in different regions of Europe. Predatory Maculinea are more generalist; nevertheless, each species survives three to five times better with a single (and different) species of Myrmica.
References (abridged):
1. Elmes, G. W. in Biodiversity Research and its Perspectives in East Asia (eds Lee, B. H., Kim, T. H. & Sun, B. Y.) 33-48 (Chonbuk Natl Univ., Korea, 1996)
2. Hoelldobler, B. & Wilson, E. O. The Ants (Springer, Berlin, 1990)
3. Als, T. D. et al. Nature 432, 386-390 (2004)
4. Akino, T., Knapp, J. J., Thomas, J. A. & Elmes, G. W. Proc. R. Soc. Lond. B 266, 1419-1426 (1999)
5. Elmes, G. W., Wardlaw, J. C., Schoenrogge, K. & Thomas, J. A. Ent. Exp. Appl. 110, 53-63 (2004)
Nature http://www.nature.com/nature
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AN INTERESTING CASE OF ANT-PLANT MUTUALISM
Notes by ScienceWeek:
In biology, symbiosis is an intimate and protracted association of individuals of different species, and mutualism is a type of symbiosis in which both participants receive benefits from the association. An intriguing mutualism is that between ants and Acacia trees. In East Africa, one finds Acacia trees that are "ant-guarded": the ants live on the trees inside modified thorns (pseudogalls), patrol the branches, and attack any insect or vertebrate herbivore, thus protecting the plant, but also preserving the plant for the use of the ant. But this plant requires cross-pollination by visiting insects in order to reproduce, and what one observes is that during the pollination periods the ant-guards essentially remain in the guard-house and cross-pollination by visiting insects proceeds without difficulty. Which of course provokes the question of what are the signals involved in this delicate bit of cooperative maneuvering?
P.G. Willmer and G.N. Stone ((Nature 1997 388:165) report that during the pollination period, the young Acacia flowers apparently release a volatile chemical that deters the ant-guards. The ants thus patrol before and after pollination, but not during the pollination period itself.
Nature http://www.nature.com/nature
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ANTAGONISTIC SYMBIOSIS: ABDUCTION OF ONE SPECIES BY ANOTHER
The following points are made by McClintock and Baker (American Scientist May/Jun 1998):
1) Many interactions between species occur on a covert level and are difficult to perceive. These interactions are chemical, not physical, and rely on substances such as pheromones that attract mates, as well as toxins that repel or kill predators, competitors, and other enemies. Chemical interactions can profoundly alter the conventional scenarios posited by ecologists studying predators and their prey.
2) The research of the authors has focused on secondary metabolites, chemicals that do not seem to be required for any of the primary metabolic processes such as energy production, respiration, or photosynthesis. They have found that sessile and sluggish organisms on the Antarctic ocean floor are much threatened by invertebrate predators and competitors, and the threatened organisms have developed chemical defenses to ward off their enemies.
3) In one unusual adaptation, the amphipod crustacean Hyperiella dilatata captures the sea butterfly Clione antarctica (a snail without a shell). The sea butterfly is held alive in a sustained forced attachment on the back of the crustacean, and experiments and analysis reveal that the sea butterfly secretes a previously undescribed beta-hydroxyketone that turns away fish that are normally predators of the crustacean. The authors suggest this unique association -- the abduction of one species by another --is unprecedented in the annals of behavioral and chemical ecology.
American Scientist http://www.americanscientist.org
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INITIAL CHEMICAL SIGNAL IN INSECT-PLANT-INSECT TROPHIC TRIANGLES
Notes by ScienceWeek:
Both corn and cotton plants, when attacked by plant-eating insects, release a volatile substance that specifically attracts other insects that are the natural predators of the plant-eating insects.
The following points are made by J. H. Tumlinson et al (Science 1997 276:945):
1) The authors studied the trophic triangle of the beet armyworm caterpillar (Spodoptera exigua Hubner), corn seedlings (Zea mays L.), and the parasitic wasp (Cotesia marginiventris).
2) The authors have isolated and synthesized the chemical substance responsible for the initial signal. They have named the substance volicitin. It is present in the oral secretions of the caterpillar, and it induces the damaged corn seedlings to release a volatile blend of terpenoids and indole, which calls in the parasitic female wasps that are the natural enemies of the caterpillars. The wasps lay eggs in the caterpillars, and the hatched larvae destroy the caterpillars by eating them.
3) Mechanically damaged plants exposed to synthetic volicitin, in the absence of caterpillar attack, release the usual volatiles that attract the wasps. Plants mechanically damaged but not exposed to volicitin do not release the volatiles.
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
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