Parasiteinduced changes in growth and behavior

Some parasites induce a new programed change in the development of the host. The agromyzid flies and cecidomyid and cynipid wasps that form galls on higher plants are remarkable examples. The insects lay eggs in host tissue, which responds by renewed growth. The galls that are produced are the result of a morpho-genetic response that is quite different from any structure that the plant normally produces. Just the presence, for a time, of the parasite egg may be sufficient to start the host tissue into a morphogenetic sequence that can continue even if the developing larva is removed. Amongst the gall-formers that attack oaks (Quercus spp.), each elicits a unique morphogenetic response from the host (Figure 12.4).

Fungal and nematode parasites of plants can also induce morphogenetic ga"s responses, such as enormous cell enlargement and the formation of nodules and other 'deformations'. After infection by the bacterium Agrobacterium tumefaciens, gall tissue can be recovered from the host plant that lacks the parasite but has now been set in its new morphogenetic pattern of behavior; it continues to produce gall tissue. In this case, the parasite has induced a genetic transformation of the host cells. Some parasitic fungi also 'take control' of their host plant and castrate or sterilize it. The fungus Epichloe typhina, which parasitizes grasses, prevents them from flowering and setting seed - the grass remains a vegetatively vigorous eunuch, leaving descendant parasites but no descendants of its own.

Most of the responses of modular organisms to parasites (and indeed (sometimes dramatic) other environmental stimuli) involve changes in host changes in growth and form, but in behavi°r unitary organisms the response of hosts to infection more often involves a change in behavior: this often increases the chance of transmission of the parasite. In worm-infected hosts, irritation of the anus stimulates scratching, and parasite eggs are then carried from the fingers or claws to the mouth. Sometimes, the behavior of infected hosts seems to maximize the chance of the parasite reaching a secondary host or vector. Praying mantises have been observed walking to the edge of a river and apparently throwing themselves in, whereupon, within a minute of entering the water, a gordian worm (Gordius) emerges from the anus. This worm is a parasite of terrestrial insects but depends on an aquatic host for part of its life cycle. It seems that an infected host develops a hydrophilia that ensures that the parasite reaches a watery habitat. Suicidal mantises that are rescued will return to the riverbank and throw themselves in again.

Figure 12.4 Galls formed by wasps of the genus Andricus on oaks (Quercus petraea, Q. robur, Q. pubescens or Q. cerris). Each figure shows a section through a gall induced by a different species of Andricus. The dark colored areas are the gall tissue and the central lighter areas are the cavities containing the insect larva. (From Stone & Cook, 1998.)

Figure 12.4 Galls formed by wasps of the genus Andricus on oaks (Quercus petraea, Q. robur, Q. pubescens or Q. cerris). Each figure shows a section through a gall induced by a different species of Andricus. The dark colored areas are the gall tissue and the central lighter areas are the cavities containing the insect larva. (From Stone & Cook, 1998.)

12.3.8 Competition within hosts constant final yield?

Since hosts are the habitat patches for their parasites, it is not surprising that intra- and interspecific competition, observed in other species in other habitats, can also be observed in parasites within their hosts. There are many examples of the fitness of individual parasites decreasing within a host with increasing overall parasite abundance (Figure 12.5a), and of the overall output of parasites from a host reaching a saturation level (Figure 12.5b) reminiscent of the 'constant final yield' found in many plant monocultures subject to intraspecific competition (see Section 5.5.1).

However, in vertebrates at least, competition or the we need to be cautious in interpreting immune response? such results simply as a consequence of intraspecific competition for limited resources, since the intensity of the immune reaction elicited from a host itself typically depends on the abundance of parasites. A rare attempt to disentangle these two effects utilized the availability of mutant rats lacking an effective immune response (Paterson & Viney, 2002). These and normal, control rats were subjected to experimental infection with a nematode, Strongyloides ratti, at a range of doses. Any reduction in parasite fitness with dose in the normal rats could be due to intraspecific competition and/or an immune response that itself increases with dose; but clearly, in the mutant rats only the first of these is possible. In fact, there was no observable response in the mutant rats (Figure 12.6), indicating that at these doses, which were themselves similar to those observed naturally, there was no evidence of intraspecific competition, and that the pattern observed in the normal rats is entirely the result of a density-dependent immune response. Of course, this does not mean that there is never intraspecific competition amongst parasites within hosts, but it does emphasize the particular subtleties that arise when an organism's habitat is its reactive host.

We know from Chapter 8 that niche differentiation, and especially species having more effect on their own populations than on those of potential competitors, lies at the heart of our understanding of competitor coexistence. We noted earlier that parasites typically specialize on particular sites or tissues within their hosts, suggesting ample opportunity for niche differentiation. And in vertebrates at least, the specificity of the immune response also means that each parasite tends to have its greatest adverse effect on its own population. On the other hand, many parasites do have host tissues and resources in common; and it

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Figure 12.6 Host immune responses are necessary for density dependence in infections of the rat with the nematode Strongyloides ratti. (a) Overall reproductive output increases in line with the initial dose in mutant rats without an immune response (•; slope not significantly different from 1), but with an immune response (□) it is roughly independent of initial dose, i.e. it is regulated (slope = 0.15, significantly less than 1, P < 0.001). (b) Survivorship is independent of the initial dose in mutant rats without an immune response (•; slope not significantly different from 0), but with an immune response (□) it declines (slope = — 0.62, significantly less than 0, P < 0.001). (After Paterson & Viney, 2002.)

Figure 12.5 Density-dependent responses of parasites within their hosts. (a) The relationship between the number of fleas CeratophyUus gallinae ('founders') added to the nests of blue tits and the number of offspring per flea (mean ± SE). The greater the density, the lower the reproductive rate of the fleas. This differential increased from an initial assessment at blue tit egg hatching, through to the end of the nestling period. (After Tripet & Richner, 1999.) (b) The mean weight of worms per infected mouse reaches a 'constant final yield' after deliberate infection at a range of levels with the tapeworm Hymenolepis microstoma. (After Moss, 1971.)

is easy to see that the presence of one parasite species may make a host less vulnerable to attack by a second species (for example, as a result of inducible responses in plants), or more vulnerable (simply because of the host's weakened state). All in all, it is no surprise that the ecology of parasite competition within hosts is a subject with no shortage of unanswered questions.

None the less, some evidence for interspecific competition amongst parasites comes from a study of two species of nematode, Howardula aoronymphium and Parasitylenchus nearcticus, that infect the fruit-fly Drosophila recens (Perlman & Jaenike, 2001). Of these, P. nearcticus is a specialist, being found only in D. recens, whereas H. aoronymphium is more of a generalist, capable of infect-

Figure 12.6 Host immune responses are necessary for density dependence in infections of the rat with the nematode Strongyloides ratti. (a) Overall reproductive output increases in line with the initial dose in mutant rats without an immune response (•; slope not significantly different from 1), but with an immune response (□) it is roughly independent of initial dose, i.e. it is regulated (slope = 0.15, significantly less than 1, P < 0.001). (b) Survivorship is independent of the initial dose in mutant rats without an immune response (•; slope not significantly different from 0), but with an immune response (□) it declines (slope = — 0.62, significantly less than 0, P < 0.001). (After Paterson & Viney, 2002.)

ing a range of Drosophila species. In addition, P. nearcticus has the more profound effect on its host, typically sterilizing females, whereas H. aoronymphium seems to reduce host fecundity by only around 25% (though this itself represents a drastic reduction in host fitness). It is also apparent that whereas H. aoronymphium is profoundly affected by P. nearcticus when the two coexist within the same host in experimental infections (Figure 12.7a), this effect is not reciprocated (Figure 12.7b). Overall, therefore, competition is strongly asymmetric between the two parasites (as interspecific competition frequently is; see Section 8.3.3): the specialist P. nearcticus is both a more powerful exploiter of its host interspecific competition amongst parasites

Figure 12.7 (a) Mean size ± SE (mm2, longitudinal section area) of Howardula aoronymphium motherworms in 1-week-old hosts, Drosophila recens, in single and mixed infections. Size is a good index of fecundity in H. aoronymphium. The hosts contained either one, two or three H. aoronymphium motherworms, having been reared on a diet contaminated with either H. aoronymphium (dark bars) or mixed infections (H. aoronymphium and Parasitylenchus nearcticus; light bars). Size (fecundity) was consistently lower in mixed infections. (b) Number of P. nearcticus offspring (i.e. fecundity) ± SE, in single (dark bars) and mixed (light bars) infections. Numbers above the bars indicate sample sizes of flies; treatment numbers refer to the numbers of nematodes added to the diet. Fecundity was not reduced in mixed infections. (After Perlman & Jaenike, 2001.)

Figure 12.7 (a) Mean size ± SE (mm2, longitudinal section area) of Howardula aoronymphium motherworms in 1-week-old hosts, Drosophila recens, in single and mixed infections. Size is a good index of fecundity in H. aoronymphium. The hosts contained either one, two or three H. aoronymphium motherworms, having been reared on a diet contaminated with either H. aoronymphium (dark bars) or mixed infections (H. aoronymphium and Parasitylenchus nearcticus; light bars). Size (fecundity) was consistently lower in mixed infections. (b) Number of P. nearcticus offspring (i.e. fecundity) ± SE, in single (dark bars) and mixed (light bars) infections. Numbers above the bars indicate sample sizes of flies; treatment numbers refer to the numbers of nematodes added to the diet. Fecundity was not reduced in mixed infections. (After Perlman & Jaenike, 2001.)

(reducing it to lower densities through its effect on fecundity) and stronger in interference competition. Coexistence between the species occurs, presumably, because the fly host provides the whole of both the fundamental and the realized niche of P. nearcticus, whereas it is only part of the realized niche of H. aoronymphium.

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