MaynardSmith_Szathmary_Ch9

MaynardSmith_Szathmary_Ch9 - *0 Km" WK“ fr"...

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Unformatted text preview: *0 Km"; WK“. fr". EHEEEEEE ............................................................................................................. .. LIVING TOGETHER One of the themes of this book is that complex organisms depend on a division oflabour between their parts. But this end result can evolve in two very different ways. Compare, for example. an elephant and a plant cell. Au elephant depends on co—operation between different kinds ofcells—epithelial cells. muscle cells. neurones, and so on. These cells have essentially the same gens. They are derived during development by the division of a single fertilized egg. In evolu— tionary time. they are all descended from the same single-celled ancestor. The differences between them arise not from possessing different genes but because influences external to the cells cause different genes to be active in different cells. The division of labour in human society. or between castes in an insect colony. is analogous. Humans. although not genetically identical. are very sim- ilar. and have a recent common ancestor. The differences between a carpenter and an electrician are caused not by their genes but by their training. The evolution of such systems—multicellular organisms. and animal and human societies—is not discussed in this chapter: it is the topic of the remainder of the book. In contrast. a plant cell depends on co-operation between three genetically different entities, the genomes of the nucleus, the mitochondrion, and the chloroplast. As described in Chapter 6, mitochondria and chloroplasts are thought to be descended from once free—living prokaryotes that were engulfed by a primitive eukaryotic cell. This coming together of once-independent repli- cators also happened. we think. in the origin of the first cells from populations of replicating molecules. The process whereby once-independent replicators come to live together in an intimate union is called ‘symbiosis'. Although often used to refer to cases in which the partners co-operate, the word properly refers both to cases of ‘mutualism', in which the partners do indeed co-operate. and of ‘parasitism‘, in which one partner lives at the expense of the other: ‘cornrnensalism’ refers to cases in which there is neither co-operation nor exploitation. As we shall see. it is not always easy to decide to which category a particular relationship belongs, and one can readily evolve into another. We first describe some examples, and then discuss the selective mechanisms responsible for their evolution. \Illl'l\ll 'r ‘I 102 LIVING TOGETHER The natural history of symbiosis Symbioses between bacteria and eukaryotes It is a curious fact that one can kill aphids with antibiotics. Although not rec- ommended as a control procedure for greenfly on one‘s roses. this sensitivity does tell us that aphids are dependent on bacterial symbionts. Aphids live on the fluids circulating in the plants they attack. This fluid lacks certain substances— in effect. insect vitamins—that the aphids need but cannot synthesize for them- selves [in particular. many amino acids). The bacteria help the aphids by synthesizing these substances. They are transmitted to the next generation inside the eggs laid by the aphid: they cannot survive on their own. This sym~ biosis is ancient: a molecular study of different families of aphids. and their symbionts. has shown that the bacteria have been vertically transmitted. from aphid mother to aphid daughter. for over 50 million years. In this example. the host benefits because the symbiont can carry out a bio- chemical synthesis impossible to the host. This is the usual basis of long-term symbiosis between bacteria and eukaryotes. Plants cannot ‘fix’ nitrogen: that is. they need nitrogen in the form ofamrnonia or other nitrogenous compounds. and cannot use the molecular nitrogen so abundant in the atmosphere. Leguminous plants, however. form a symbiotic union with a bacterium, Rhizobium. that can fix atmospheric nitrogen. [t is for this reason that we plant clover in our grasslands. and alfalfa on arable land. The ecosystem of deep—sea vents depends wholly on symbiosis. Most eco- systems depend ultimately on photosynthesis for their energy. Plants trap sun- light and use the energy to synthesize sugars and other organic compounds: ail other organisms in the system depend on plants. The same is true of most deep-sea organisms: it is too dark for photosynthesis so they rely instead on the fallout of dead organisms from the surface layers, which do depend on photo- synthesis. But in deep-sea vents there is an alternative source of energy—the sulphides emerging from the vents. The large worm Riflia that inhabits these vents has no mouth or anus as an adult. and relies on antibiotic bacteria. housed in a special organ, which oxidize the sulphides. Both oxygen and sul- phide are transported to these organs by a special haemoglobin. In effect, deep- sea vents are occupied by a unique ecosystem. not dependent on photosynthesis for its energy but depending instead on symbiosis between animals and sulphur-metabolizing bacteria. Symbioses between animals and single-celled algae 0n the beaches of Brittany. there is a unique flatworm, Camium. M an adult it resembles Rrfiin in lacking both mouth and anus but it contains symbiotic green algae. When the tide is out, it lies on the surface of the sand. and its THE EVOLUTION OF SYMEIIOSIS: MUTUALISM OR PAMSITISM? 103 symbionts photosynthesiae. When shaken by the incoming tide, Convohita burrows beneath the surface. and so avoids being swept out to sea. This has a curious effect: at low tide the sand is green. but if one pats it, it turns golden. Com-alum is perhaps a curiosity. but symbiosis between aquatic animals and algae is widespread. and can he ecologically important: the animals that build coral reefs can do so only with the aid of symbiotic dinoflagellate algae. Symbioses involving fungi Lichens are perhaps the most familiar example ofsymbiosis. They are import- ant in colonizing bare rock. it lichen consists of a host fungus containing sym- biotic ‘algae‘. which may be either symbiotic green algae or prokaryotic granobacteria. The fungi are of many kinds. and it is clear that lichen associa- tions have evolved many times. Almost all the algae found in these associations are found free in nature. The fungi are certainly benefiting from the association. but it is less clear that the algae are getting anything out of it. Many land plant communities are dependent on an association between mycorrhizal fungi and plant roots. These fungi are known from the Devonian period. when the land was first colonized by plants. At that time. soils would have been mineral in composition. lacking organic nutrients. The fungi were probably important in making minerals available to the plants. Today, mycor- rhizal fungi are important in mineral soils. particularly in the tropics. There is a net flow of minerals from fungus to plant. and of organic compounds from plant to fungus1 so it seems that both partners are benefiting. A more recently evolved group of mycorrhizal fungi are associated with ericaceous plants {heather-s. rhododendrons, and so on} growing on acid soils with high organic content {peat}- These ericoid fungi not only supply minerals to the plants but also make available organic compounds that the plants could not acquire with- out them. As a final example. we cannot resist describing the symbiosis between leaf- cutting ants and fungi. leaves. and even flower petals. are cut by the worker ants. and carried to their nest across the floor of the forest. like the banners of a miniature political demonstration. There they are digested by special fungi. farmed by the ants as we farm mushrooms. The are provided with food. and in turn provide food to the ants. This is just the most dramatic of the many ways in which animals that cannot themselves digest cellulose use other organ- isms to do it For them. The evolution of symbiosis: mutualism or parasitism? [n the previous section we gave examples in which either both partners are benefiting. or at least the host organism is benefiting. But there are plenty of 104 LIVING TOGETHER examples of symbionts that damage or kill their hosts. Can we make any predictions about how evolution will proceed? Until relatively recently, the conventional wisdom was that symbionts associ- ated for a long time with a given host species would become relatively harmless. The view was well expressed by Rene Dubos in 1965 in his book Man adopting: 'Given enough time a state of peaceful coexistence eventually becomes estab- lished between any host and parasite’. On this view. serious disease is caused by parasites invading a new host species for the first time. There is little doubt that this is sometimes true. A recent example is the fact that the human immuno- deficiency virus {HW} is almost always fatal to humans. but the simian equiva- lent {SIV}. from which HIV only very recently evolved, is found in a large proportion of African green monkeys. apparently without causing any harm. But this is not always the case. For example, typhoid is caused by the bacterium Salmonella typhi, which is found only in humans. The related bacterium. S. nrphimurium. kills its normal host, the mouse, but is harmless in humans. Of course. it may be that in time a state ofpeaceful coexistence will evolve between these bacteria and their hosts. but the time-stale will be in millions rather than thousands of years. There are reasons why host-parasite systems should often evolve towards commensalism. Essentially. they are the same as the reasons, discussed in the last chapter. why selfish genetic elements have not destroyed all complex organ— isms. The host organism will be selected to evolve so as to control the parasite. and the parasite will evolve so as not to destroy a host on whose survival its own Future may depend. That hosts will be selected to reduce the damaging elfects of parasites is obvious. Studies ofhost—parasite systems often reveal traces of a past arms race between them. Thus the main weapon of vertebrate hosts against parasitic microbes is their immune system: they learn to make antibodies against their parasites. in particular against their surface proteins. These proteins evolve much more rapidly than others. to escape immune attack. Some parasites have evolved special mechanisms for periodically replacing the proteins exposed on their surfaces. Of course. such arms races need not lead to oommensalism: the parasite maylteep one step ahead More interesting is the possibility that some parasites may evolve towards commensalism because it pays them to do so. Whether or not this will happen depends, among other things. on the methods whereby parasites reach new hosts. If transmission is vertical (Fig. 9.1a}, it is in the inter-mt of the symbiont to keep its host alive and fertile. For example, the bacteria symbiotic in aphids are transmitted only in the aphid eggs. Only those mutations in the bacterium that benefit the aphid will be selected. Transmission of symbionts in the host eggs is unusual. but there are other means of vertical transmission. Termites THE EVOLUTION OF SYMBIOSIS: MUTUALISH OR PmSITISM? 105 Figure 9.] Vertical and horixontal transmision of symbionls. (a) 1ul'ertial transmission: symbiontsare passeddirectly to allthedcscendants oftbe host. {b} Horizontal transmission: a hostacquires its syrnbionts. notfiom its parent but from unrelated individuals. (c) Horizontal transmission Iwith double infection: a host acquires symbionts from two or more unrelated individuals. ingest wood. which is digested by a diverse population of symbionts in the termite gut. Larval termites acquire symbionts by licking their mother's anus. Vertical transmission, however. is not a particularly common feature of mutualistic symbioses. Returning to the examples described earlier, the deep- sea worm Riftia. although it lacks a mouth as an adult. has a planktonic larva with a mouth. and acquires sulphur-metabolizing bacteria by swallowing them. The flatworm Camluta also has a mouth when young. and swallows its symbi- otic algae. Mycorrhisae in the soil must find plant roots. as must the nitrogen- fixiug bacterium Rhimbium. The best-studied example of the evolution ofa horizontally trausrrdtted para- site (Fig. 9.1b} concerns the myxorna virus in rabbits. This virus was originally commeural in a South American relative of the rabbit. and was introduced into rabbits as a control measure. Originally. the virus killed rabbits in one or two weeks after infection- Today, thevirus only sometimes kills the rabbit. and takes monthsratherthanweekstodoso. It isknownthattl'iischangehashappenod 1.15“ 1 106 LIVING- TOGETHER THE IMPORTANCE OF MUT'UA 07 this explanation is correct. it has interesting implications. We suggested on pp. 82—3 that there are two rival explanations for the prevalence ofsex: first. that it acceleratm evolution. and. second. that it reduces the mutational load. The loss of sex by many mutuaiists suggests that the first of these explanations must be at least part ofthe truth. because the viruses are less virulent. in addition to the evolution ofresistance in the rabbits. The population biologists Robert May and Roy Anderson have analysed this case. They argue that it will pay the parasite not to kill its host. but it will also pay to be highly efficient in transferring to new hosts: it is the prod- uct ofthese two factors, host survival and infectivity, that will be maximized. [f the parasite can achieve high infectiviry only by damaging the host. we should not expect evolution towards harmlss commensalisrn. Parasites do cause symptoms in their hosts that increase infectivity. as anyone who has sulfered from a common cold knows only too well. There is another reason why parasites may not evolve towards commensal— ism. Suppose that. typically. a host is infected simultaneously by parasites from two sources (Fig. 9.1c). Then selection on the parasite will favour high infect- ivity. at the expense ofhost survival. There is no point in keeping the goose alive if someone else is going to kill it. Comparing different host—symbiont systems does lend support to the idea that the outcome is affected by the mode of transmission. But the cases of mutualism in which transmission is horizontal are common enough to suggest that the most important factor is the opportunity for mutual benefit. (Jo-operation or slavery? In some cases of symbiosis. it is tempting to ask whether co-operation or slavery is the more appropriate analogy. For example, most of the algae involved in lichen associations are also found free-living. Would it be better to regard them as slaves ofthe fungus or as willing co-operators? This is not a question that can be answered. The point ofanalogics ofthis kind is not that they are true but that they suggest questions to ask, and predictions to test. In this case. we can as]: ‘are there features of the host organism (the fungus) or of the symbiont (the algal that have evolved because they help to establish the symbiosis?’ In the case of some ancient symbioses——for example. that between eukaryotic cells and their organelles—we cannot say: there is really no way of choosing between the two analogies. But wecan do better in some more recent examples. For exam- ple. should we regard the fungi fanned by leaf-cutting ants as slaves or partners? The answer seems to be the latter, because the fungi have features that make sense only as attractors for ants: their thread-like hyphae have inflated tips. absent in other fimgi, that serve as food for the ants. and seem to serve no other purpose. Another feature of many mutualistic symbionts is that they have become asexual: few parasites have done this. The explanation. presumably. is that the mutualists do not have to evolve continuouslyto overcome the defences of their hosts: instead. the host species are selected to make it easy for the symbionts. If The importance of mutualism Most of the mutualisms we have described exist because the symbiont can carry out some biochemical process impossible to the host: it can photosynthesize, fix nitrogen. metabolize sulphur. digest cellulose. or synthesize amino acids. Such A“ symbioses have been ecologically important. Today they are the basis of ecosys— . terns in deep-sea vents. coral reefs. tropical forests. and acid moors. Symbiosis between plants and fungi may well have been important in the conquest ofdry! land. Symbiosis. then. played a part in three ofour major transitions—the origin of the first cells. of chromosomes. and of eukaryotic cells—and in heiping host - organisms to adapt to difficult environments It is important. however. not to]. misunderstand or exaggerate its role. Lynn Margulis. who marshalled the evi- dence that persuaded biologists that mitochondria and chloroplasts were once symbionts. has sometimes argued that symbiosis is the main source of evoiu- tionary novelty. and that natural selection has been of minor importance. This will not do. Symbiosis is important because both partners contribute some- thing. In nitrogen-fixing symbioses. for example. Rhizobium contributes the ability to fix nitrogen. and the plant contributes photosynthesis and the whole anatomy of root and shoot required to succeed on land. These are complex adaptations that could only have evolved by natural selection. The motorbike is a symbiosis between the bicycle and the internai combustion engine. It works fine. if you like that kind of thing. but someone had to invent the bicycle and internal combustion. Symbiosis is not an alternative to natural selection: rather, we require a Darwinian explanation of symbiosis. The other important point to bear in mind is the one made at the start ofthis chapter. Not all co-operation between parts arose by symbiosis. In fact, the most complex examples ofco+operation between parts specialized for different functions arose by a process of differentiation between genetically identical. or at least similar. entities. We now turn to such processes: the origins of multi- cellular organisms. and of societies. ...
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