Summary

We start by distinguishing mutualism, symbiosis and commen-salism and emphasizing that mutualism is best seen as reciprocal exploitation not a cosy partnership.

Mutualisms are examined in a progression: from those where the association is behavioral, through intimate symbioses in which one partner enters between or within another's cells, to those where organelles have entered into such intimate symbioses within the cells of their hosts that they cannot be regarded as distinct organisms.

'Cleaner' fish feed on ectoparasites, bacteria and necrotic tissue from the body surface of 'client' fish. The cleaners gain a food source and the clients are protected from infection. Many ant species protect plants from predators and competitors, while themselves feeding from specialized parts of the plants, though careful experiments are necessary to show that the plants themselves benefit.

Many species, including humans, culture crops or livestock from which they feed. Ants farm many species of aphids in return for sugar-rich secretions, though experiments demonstrate that there can be both costs and benefits for the aphids. Many ants and beetles farm fungi that give them access to otherwise indigestible plant material, and in some cases a three-way mutualism is established with actinomycetes that protect the fungi from pathogens.

Very many plant species use animals to disperse their seeds and pollen. We emphasize the importance of insect pollinators and the coevolutionary pressures generating a range from gen-eralists to ultraspecialists. We also discuss brood site pollinations, of figs and yuccas, by fig wasps and yucca moths that rear their larvae in the fruits of the pollinated plant.

Many animals support a mutualistic microbiota within their guts, especially important in the digestion of cellulose. We outline the range of active sites, and the complex community of mutu-

alists, within the guts of a variety of vertebrates and of termites, focusing especially on the ruminants and noting the importance in many cases of refection. We also describe insect mycetocyte symbioses, especially those between aphids and Buchnera species, through which microorganisms, mostly bacteria, living in specialized cells bring nutritional benefits to their insect hosts.

A number of aquatic invertebrates enter into mutualistic associations with photosynthetic algae, perhaps the most important of which are the reef-building corals. We focus especially on 'coral bleaching' - the whitening of corals as a result of the loss of the endosymbionts - and its possible relationship with global warming, and we emphasize the multi- (not two) species nature of these and many other mutualisms.

A wide variety of symbiotic associations are formed between higher plants and fungi. We concentrate on the mycorrhizas -intimate mutualisms between fungi and root tissues - possessed by most plants. We describe ectomycorrhizas, arbuscular mycorrhizas and ericoid mycorrhizas, noting the range of benefits that they can confer.

The biology is outlined of lichens, discussing the intimate associations between mycobiont fungi and phytobionts, mostly algae. Parallels with higher plants are particularly emphasized.

Mutualisms between plants and nitrogen-fixing bacteria are of enormous importance. We outline the range of these bacteria but focus mainly on the mutualisms of rhizobia and leguminous plants, describing the steps involved in establishing the liaison, the costs and benefits to both parties, and the role of the mutualism in determining the outcome of competition between legumes and other plants. This leads to a discussion of the part played by nitrogen-fixing plants in ecological successions.

We examine briefly some mathematical models of mutualisms, which re-emphasize the importance of looking beyond two focal species to the broader context.

Finally, we discuss the possibility that the origin of the various sorts of eukaryotes from more primitive ancestors has progressed at least in part through the inextricable merging of partners in mutualistic symbioses.

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