Immune responses are presumed to contribute to host fitness, either by fighting off infections or via immunopathology. Research in this thesis sought to relate the magnitude of a putative immune response to infection and host and parasite fitness. The experiments and field studies presented here all focus on the interactions between the freshwater crustacean, Daphnia magna and its sterilizing bacterial endoparasite, Pasteuria ramosa, using the number of circulating haemocytes as a measure of host immune activity. I found substantial genetic variation in Daphnia’s cellular response to P. ramosa, and that Daphnia genotypes that mount the strongest cellular responses are the most likely to get infected and suffer sterilization. Thus, a strong cellular response is associated with low, as opposed to high host fitness potential. There were also some host genotypes that mounted a weaker cellular response and did not go on to suffer infection, and some that lacked a cellular response and also never suffered infection with P. ramosa. These findings led to a heuristic two-stage model for infection, where the parasite has to (1) pass from the host gut to haemolymph and then (2) successfully overcome haemolymph-based immune effectors to reproduce and achieve fitness. I also demonstrate that both the magnitude of host cellular response and likelihood of infection increases with initial parasite dose in susceptible host genotypes, and that host cellular response is associated with likely infection under both host and parasite genetic variation. Parasitised Daphnia also have substantially more circulating haemocytes than their healthy counterparts in both the laboratory and in the wild, where there is substantial genetic and environmental variation. This is one of the very few examples of how an immune response designates low host and high parasite fitness potential in a wild system. Finally, using a mixture of field study and common garden experiment, I demonstrate evolution in parasite infection traits over the course of an epidemic in a wild population, and that this evolution is associated with a decline in host abundance.

The mammalian intestinal tract is highly colonised with a diverse bacterial microbiota. The importance of this bacterial presence is now recognised; these bacteria contribute both to the nutritional status of their hosts and are required for the development of a competent immune system. In addition, the composition of the microbiota is likely important in influencing how the immune system reacts to antigens, as the presence of specific bacterial species can promote differentiation of T cells towards specific effector or regulatory fates. Though the ability of the microbiota to influence infections with bacterial and viral agents has been reported, whether the microbiota can affect a parasitic infection has not yet been described. It is likely, due to millions of years of co-evolution within mammalian hosts, that helminths have co-opted mechanisms of the microbiota to manipulate the host’s immune system, in order to promote their own survival. In this thesis, the immune parameters required for expulsion of a primary infection with the murine gastrointestinal helminth parasite Heligmosomoides polygyrus are examined, and whether the microflora influence these parameters in order to modulate susceptibility is explored. Firstly, a multiparameter analysis of H. polygyrus infection was performed in two mouse strains which differ in susceptibility to a primary infection, to identify both immune factors and microbial populations which correlate with susceptibility to infection. BALB/c mice exhibited a stronger T helper (Th)2-type response to H. polygyrus excretory-secretory antigen (HES), produced high numbers of intestinal granulomas following infection and were better able to expel H. polygyrus, whereas the more susceptible C57BL/6 strain produced higher levels of inflammatory Th1 cytokines in response to HES. High levels of duodenal Lactobacillus/Lactococcus species positively correlated with H. polygyrus persistence within the BALB/c host, as did high levels of Enterobacteriaceae in the C57BL/6 host. Furthermore, the abundance of both of these bacterial groups was elevated in H. polygyrus-infected C57BL/6 mice compared to naïve controls, and mice given antibiotic treatment to diminish these groups were rendered more resistant to H. polygyrus. Infection persistence was prolonged in BALB/c mice which were administered the single species Lactobacillus taiwanensis, a normal component of the microbiota. Next, the impact of a loss of microbiota signalling by immune cells during H. polygyrus infection was examined, through the use of Toll-like receptor (TLR)- and TLR adaptor protein-deficient mice. MyD88-/- mice were more resistant to H. polygyrus than wildtype (Wt) C57BL/6 mice and exhibited increased granuloma formation: phenotypes which were not recapitulated by individual deficiencies in TLR2, TLR4, TLR5 or TLR9, and not seen in TRIF-/- mice. When MyD88-/- mice were additionally deficient in TRIF, the increased granuloma formation phenotype of MyD88-/- mice was lost. Whether MyD88 controls susceptibility to H. polygyrus infection via a TLR-independent mechanism, and how MyD88 and TRIF antagonistically contribute to granuloma formation remains to be resolved. Finally, the importance of TGF-β signalling during H. polygyrus infection was examined, using mice deficient in TGF-β signalling specifically in T cells (TGF-βRII DN mice). These mice were more susceptible to H. polygyrus than Wt C57BL/6 mice, which was explained by an attenuated Th2 response to infection accompanied by exuberant IFN-γ production. The increased susceptibility to H. polygyrus was lost in TGF-βRII DN IFN-γ-/-mice, in which Th2 responsiveness was partly restored. These data highlight the importance of both immune components, particularly IFN-γ, which promotes susceptibility, and the presence of specific intestinal bacterial populations in controlling the persistence of a primary H. polygyrus infection.

Parasites are important drivers of ecological and evolutionary processes in their hosts. However, hosts often differ in how they are affected by parasitism, which can be important in how parasite effects on individuals scale up to the population level. Hosts may differ intrinsically in their susceptibility to parasitism, and extrinsic factors may impose constraints on how hosts allocate resources between immunity, maintenance and reproduction, thereby further affecting their ability to cope with infection. These extrinsic factors include the host’s ecological environment, for example food availability or weather, and its social environment, that is its interactions with conspecifics. This is particularly true during a reproductive attempt when individuals interact closely with other family members. Not only might immediate impacts of parasitism differ between and within parents and offspring, but the direct effects of parasitism on a host could have further indirect consequences for other family members through their behavioural interactions with parasitised individuals. The distribution of direct and indirect effects among all family members could affect the outcome of the breeding event and individuals’ future performance. However, teasing apart these various avenues of parasite impacts on families may be difficult if parasite burden or susceptibility is correlated between family members. In this thesis, I explore the consequences of parasitism for different family members of the European shag Phalacrocorax aristotelis infected with gastrointestinal nematodes, over a range of ecological conditions. In chapter 2, I demonstrate that chicks’ responses to anti-parasite treatment across four years vary between siblings and with environmental conditions, which may be mediated by resource allocation among siblings. In chapter 3, I explore how costs of parasitism are distributed among the whole family by simultaneously treating chicks and/or parents with an anti-parasite drug and measuring the outcomes for all family members. Treatment has a more marked effect for the non-treated generation than for the treated individuals, suggesting that parasitism may have important indirect costs. In chapter 4, I investigate whether within-brood variability in the effects of anti-parasite treatment and its cross-generational impacts are mediated by behavioural change, and show that chick treatment but not parent treatment influences several aspects of behaviour in the nest. In chapter 5, I demonstrate that the impact of chick anti-parasite treatment on parents persists beyond the breeding attempt, with parents of treated chicks foraging less overwinter and breeding earlier the following year, whereas there is no persistent effect of parents’ own anti-parasite treatment. Lastly, I provide an appendix examining the parasitology of the system in detail, including an assessment of in situ and proxy measures of worm burdens of chicks. This thesis demonstrates that parasitism can be a key component, previously overlooked, of reproductive performance in seabirds, a group that plays an important ecological role as apex predators and thus indicator species of the marine environment.

No description available.

Bibliography: leaves 113-144. Investigates the functions of the maternal, protein secretions of the endoparasitoid wasps Venturia canescens Gravenhorst regarding their role in providing protection against the host's immune system. Also investigates mucinous protein secretions on the egg surface and coagulation reactions and their role in protecting eggs against host cellular attacks.

Most, if not all, animals will at some stage in their lives encounter parasites. Some of the most widespread and abundant parasites belong to the Class Trematoda. Trematodes often have a substantial negative impact on individual intermediate host ecology. In this thesis, I investigate ecological and evolutionary consequences of the interaction between the microphallid trematode, Maritrema novaezealandensis, and its intermediate snail and crustacean hosts. Parasites often show a heterogeneous spatial distribution pattern in natural animal host populations. In this study, factors determining the spatial distribution of larval trematodes in Zeacumantus subcarinatus were investigated at two spatial scales (within and among bays). The distribution of shorebird definitive hosts explained a significant amount of the variation in the distribution of trematodes among bays. However, within a bay, other factors override the effect of bird distribution. The influence of larval trematodes on reproduction, survival and population density of Z. subcarinatus was investigated using laboratory and field studies. In the laboratory, it was found that larval trematodes induce castration and mortality of Z. subcarinatus. The field study revealed that the local prevalence of trematodes had a significant negative effect on population density of Z. subcarinatus. Through castration, trematodes act as strong selective agents on snail host life history. The effect of trematodes on life history characteristics (reproductive effort, juvenile growth, size at maturity and susceptibility to trematode infections) were investigated among natural populations of Z subcarinatus. Reproductive effort was not higher for uninfected females from populations where the risk of becoming infected was high. However, offspring from those populations were significantly larger, and laboratory-reared juveniles grew significantly faster than conspecifics from other populations. In addition, size at maturity was negatively correlated with trematode prevalence across snail populations. Z. subcarinatus thus adapts to a high local risk of trematode infection by reaching maturity early, thereby increasing the chance of reproducing. The influence of M. novaezealandensis on the survial of the amphipod host, Paracalliope novizealandiae was examined using experimental infections and field observations. The experimental infections demonstrated that parasite-induced mortality was intensity-dependent. The number of M. novaezealandensis per amphipod was too low to significantly induce host mortality in the field. However, the transmission strategy of this parasite allows it to affect host populations during weather conditions ideal for trematode transmission. Trematode strategies in the second intermediate host are important to the understanding of host-parasite co-evolution and the evolution of parasite life cycles. In this study, potential density-dependent effects at the metacercarial stage on size and fecundity of in vitro adult M. novaezealandensis was examined in both experimentally infected P. novizealandiae and naturally infected Macrophthalmus hirtipes. For this purpose, a method to excyst and cultivate M. novaezealandensis metacercariae to an egg producing stage, was developed. Naturally infected M. hirtipes also harboured larval stages of three other helminths. Crowding effects in the two crustacean hosts were expressed as a decreased volume and smaller egg production of in vitro adult M. novaezealandensis. In addition, interspecific interactions among parasite species were observed in crab hosts. The work in this thesis provides evidence that M. novaezealandensis significantly and negatively affect intermediate host ecology. The heterogeneous distribution of trematodes causes differential effects among host populations with subsequent effects on the life history of snail hosts. In addition, this study demonstrates that parasites interact within their second intermediate host with possible implications for the way parasites exploit their hosts.

Bibliography: leaves 113-144. / ix, 144 leaves, [56] leaves of plates : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Investigates the functions of the maternal, protein secretions of the endoparasitoid wasps Venturia canescens Gravenhorst regarding their role in providing protection against the host's immune system. Also investigates mucinous protein secretions on the egg surface and coagulation reactions and their role in protecting eggs against host cellular attacks. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied & Molecular Ecology, 2002

Graduation date: 1995

Mast cells (MCs), normally classified into connective tissue MCs and mucosal MCs, are highly granulated cells found in the interface between the interior and the exterior environment of our body, e.g. skin, airways and gastro-intestinal tract. They react to bacteria, parasites, viruses, and allergens by degranulation and release of premade and newly synthesized inflammatory mediators. The MC-proteases (tryptases, chymases and carboxypeptidase A), histamine and serglycin (SG) proteoglycans are premade mediators. Among these, SG is also expressed in a variety of other immune and non-immune cells. Heparin and chondroitin sulphate glycosaminoglycan chains confer highly negative charge to SG, by which MC-proteases are retained in secretory granules. Deletion of SG cause impaired packing and storage of most MC-proteases. During challenge with Toxoplasma gondii the SG-deficient mice showed significant lower inflammatory cytokine levels in comparison to wild-type mice. Results were consistently similar in vitro, bringing forward the importance of SG in inflammatory cytokine and innate immune responses towards T. gondii. Infection with Trichinella spiralis in SG-/- mice caused increased intestinal enteropathy, a tendency of delayed worm expulsion and increased larval burden in the muscle tissue as compared to wild-type animals. An altered TH2 cytokine response was also observed, and all these effects were not repaired by wild-type MC reconstitution of the SG-/- mice. Altogether, our results suggest that SG is important for tissue homeostasis, and that SG expressing cells seem capable of switching from a SG-dependent storage mode to a SG-independent secretory mode upon infection. The chymase (MCPT4) expressed by connective tissue MC has been implicated to have a protective role during infection and in limiting inflammation. We explored a protective role by inducing T. gondii infection in the Mcpt4-null mice, and found MCPT4-mediated recruitment of neutrophils and eosinophils via control of cytokine signaling. Endogenous proteins “alarmins” released by dead cells can trigger tissue and cell damage. We conclusively show that chymase efficiently degrades Hsp70 both in vitro and in vivo and that the degradation of other alarmins, e.g. HMGB1, biglycan and IL-33 may also depend on chymase.

Tarebia granifera is an ovoviviparous and parthenogenetic freshwater snail indigenous to South East Asia. T. granifera was introduced into many areas of the world as a result of human activity and caused detrimental effects on native freshwater snails. T. granifera also served as the first intermediate hosts of many digenetic trematodes. Specimens of T. granifera were collected seasonally in Jinlun River, S.E. Taiwan, from April, 2003 to January, 2004. The embryo composition in the brood pouches was compared among seasons. We found the species was fertile and the embryo composition was similar throughout the year. The maximum embryo number occurred in October, 2003, and the minimum occurred in January, 2004. T. granifera was reared to compare juvenile releasing rates in summer and winter. More juveniles were released per mother-snail in summer than in winter. And low water temperature seemed to inhibit juvenile release in cold days. The survival rates of brooded juveniles outside brood pouches were tested. Except the smallest size group (shell height=0.25~0.50mm), all others (0.5~1.25mm) had high survival rates approaching 100%. About 15% of the snails were infested by unidentified xiphidiocercariae. Larger snails were more likely to be infected and the infected snails were less likely to be fertile than uninfected ones.