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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.

Trichogramma pretiosum Riley is a generalist egg parasitoid that is mass released in biological control programmes to control various insect pests on different plant species. The parasitism rates achieved by T. pretiosum vary across host species and across plant species, which suggests that the species of host and its host plant could both influence the host searching performance of this parasitoid. A comparative behavioural study across the eggs of two moth species, Helicoverpa armigera Hübner and Spodoptera litura Fabricius, was conducted because these two host eggs have different characteristics. Helicoverpa armigera eggs are laid singly, whereas S. litura eggs are laid in clusters and are covered with fine scales from the moth abdomen. I tested how these and other differences in host species affect the behaviour of T. pretiosum females towards their host species. The ovipositional responses of T. pretiosum females, from different natal hosts and of different ages since emergence, to eggs of H. armigera and S. litura were studied in choice and no choice tests. Helicoverpa armigera eggs were consistently preferred over S. litura eggs, regardless of the natal host and adult age. When only S. litura eggs were available as hosts, they were parasitized at statistically similar rates to H. armigera eggs. The adult lifespan and lifetime fecundity of T. pretiosum were variable but were significantly affected by natal host species and/or host species to which they were exposed. The ovigeny index (OI) was significantly lower in the parasitoids exposed to H. armigera eggs than in those exposed to S. litura eggs, regardless of the natal host, indicating that H. armigera eggs sustain the adult parasitoids better, through host feeding, than S. litura eggs. Trichogramma females, like other egg parasitoids, use chemical cues that originate not only from their host plants and host eggs, but also from the adult insects that deposit the hosts. These last include the scales left by the ovipositing moths. These host-associated cues are used by parasitoids over long and short distances to find their hosts. Olfactometer tests demonstrated that the odours from moth scales and host plants attracted T. pretiosum females significantly, and thus function as long range cues. By contrast, the odour from host eggs did not attract them in this way. The strength of attraction was similar across the scales from H. armigera and S. litura moths. The female parasitoids responded most strongly to odours from tomato and cabbage, whereas odour from corn was not such a strong attractant. A non-host plant tested did not attract the parasitoids at all. The odours of moth scales from both H. armigera and S. litura were more attractive to T. pretiosum females than was the odour of the host plants tested. When parasitoids have followed these long range cues to the nearby vicinity of their hosts, they respond to short range cues to localise the hosts. Moth scales did not attract female parasitoids over a short distance, but they did arrest their movement so that they remained longer in the area with scales. Trichogramma pretiosum females took significantly more time to localise S. litura eggs than it took for them to locate H. armigera eggs. Time to locate hosts did not increase significantly when the eggs had been washed with hexane, which suggests that cues other than chemicals are associated with the eggs and play a more significant role in host finding over short distances. On their emergence, parasitoids are confronted by several options, including mating, searching for hosts, and seeking food. Season influenced the daily pattern of T. pretiosum emergence, with cooler winter temperatures extending emergence over two days. In summer they emerged earlier in the day and all parasitoids in the tests emerged during the first day. Parasitoids left their emergence sites by walking before their wings expanded fully. Adults of T. pretiosum visit flowers, presumably for nectar. Olfactometer tests showed that both males and females of T. pretiosum were attracted to the odor of alyssum flowers. The attraction to flower odour was significant for males when they had had no access to nutrients for 24h, but was less significant when they had just emerged. On the contrary, the attraction to food was more significant in newly emerged females. For 1 day starved females, cues associated with host plants (tomato leaves) and food (alyssum flowers) were equally important. Nutritional sources influenced the adult lifespan and reproductive performance of the parasitoids. Honey was the best source of nutrition for adults of T. pretiosum, resulting in the longest lifespan and the highest reproductive output, followed by alyssum flowers, tomato leaves and water. The results are discussed in relation to (i) understanding the entire mechanism of host searching behaviour of egg parasitoids, (ii) interpreting the behavioural interaction between generalist parasitoids and their different host insects and plants, and (iii) the better use of egg parasitoids in biological control. Throughout the discussion suggestions are made for further research.

In this thesis, I study a radiation of brood-parasitic finch species, the indigobirds and whydahs (genus Vidua), that occur across Africa. Host colonisation is tightly linked to speciation in Vidua because of their remarkable capacity to imprint on their hosts, with mating traits and host preferences being influenced by the parasite's early environment. The challenge of explaining why the radiation has diversified to the extent it has therefore simplifies to understanding why only certain potential host species have been successfully colonised. Following on from the introduction (Chapter 1), I begin by critically examining the logic with which mimicry in the natural world can be conceptually organized (Chapter 2). This creates a “mimicry landscape” in which to situate the mimetic adaptations of hosts exhibited by Vidua. The framework can be used to contrast and draw parallels between these and other mimetic adaptations present in the natural world. In Chapter 3, I review the literature on begging call mimicry and development across all avian brood parasite species. I outline the conditions under which we expect begging call mimicry to evolve, and when we expect it to develop primarily through genetic or environmental cues. This provides clear predictions for what we expect to occur in Vidua finches, which are tested in Chapters 4 and 5. In Chapter 4, I quantify the mimicry of host nestlings by Vidua in detail. I provide the first quantitative evidence that Vidua nestlings mimic the begging calls and show for the first time that Vidua are imperfect mimics of their hosts. In Chapter 5, I simulate the colonisation of a new host by transferring Vidua eggs into the nest of a new host species. I monitor Vidua survival in the foreign host environment and test several hypotheses about what explains differences in chick survival. I find that Vidua survive poorly in the new nest environment and that they do not show adaptive plasticity in begging calls or head movements. This poor survival occurs despite there being minimal differences in the diets each host species feeds their young. Finally, in Chapter 6, I carry out a comparative analysis on the evolution of estrildid mouth markings. Estrildid finches are the hosts of Vidua and so provide the landscape of potential ecological niches that Vidua may colonise and adapt to. I demonstrate that the host family shows strong phylogenetic signal in mouth marking traits, and find no evidence that ecological factors such as light environment or predation pressure has shaped estrildid mouth marking evolution. The work in this thesis highlights how difficult successfully colonising new hosts is for Vidua finches. Vidua must mimic hosts in multiple traits (mouth markings, begging calls, head movements) to obtain sufficient amounts of food from host parents. Overall, habitat filters, the complex and diverse begging displays of estrildid nestlings, the discriminatory behaviour of estrildid parents against mismatching chicks and the lack of adaptive plasticity in begging displays by Vidua together help explain why the Vidua radiation consists of only 19 species rather than many more or fewer.

Thesis (Ph.D.)--University of Alabama at Birmingham, 2009. / Title from PDF title page (viewed on Feb. 10, 2010). Includes bibliographical references.

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For vector-borne parasites such as malaria, how within- and between-host processes interact to shape transmission is poorly understood. In the host, malaria parasites replicate asexually but for transmission via mosquitoes to occur, specialized sexual stages (gametocytes) must be produced. Once inside the mosquito vector, gametocytes immediately differentiate into male and female gametes, and motile male gametes must swim through the hostile environment of the bloodmeal to find and fertilise female gametes. Despite the central role that gametocytes play in disease transmission, explanations of why parasites adjust gametocyte production in response to in-host factors remain controversial. Furthermore, surprisingly little is known about the mating behaviour of malaria parasites once inside the mosquito. Developing drugs and/or vaccines that prevent transmission by disrupting sexual stages are major goals of biomedicine, but understanding variation in gametocyte investment and male gamete behaviour is key to the success of any intervention. First, I propose that the evolutionary theory developed to explain variation in reproductive effort in multicellular organisms provides a framework to understand gametocyte investment strategies in malaria parasites. I then demonstrate that parasites appear to change their reproductive strategies in response to environmental cues and in a manner consistent with our predictions. Next, I show how digital holographic microscopy can be used to characterise the morphology and motility of male gametes. I then provide evidence for non-random movement of male gametes and that gamete interactions with red blood cells appear to hinder mating success in a bloodmeal. Finally, I discuss the variation in gametocyte differentiation and fertilisation success when exposed to a number of factors implicated in gametocyte activation. The data presented here provides important information on the basic biology of malaria parasite reproductive stages and demonstrates considerable variation in parasite traits and behaviours in response environmental changes; both in the host and in the mosquito vector.

Gastrointestinal parasitism in grazing lambs adversely affects animal performance and welfare, causing significant production losses for the sheep industry. Control of gastrointestinal parasitism using chemotherapeutic treatment is under threat due to the emergence of anthelmintic resistance, thus stimulating research into alternative control strategies. Whilst investigating control strategies experimentally can be costly and time consuming, using a mathematical modelling approach can reduce such constraints. A previously developed model which describes the impact of host nutrition, genotype and gastrointestinal parasitism in a growing lamb, provided an appropriate starting point to explore control strategies and their impact on host-parasite interactions. Two contrasting mechanisms have previously been proposed to account for the occurrence of anorexia during parasitism. These were reductions in either intrinsic growth rate or relative food intake. Thus, the existing individual lamb model was modified to evaluate these mechanisms by exploring the relationship between anorexia and food composition (Chapter 2). For foods that did not constrain food intake, published data was found to be consistent with the predictions that arose from anorexia being modelled as a reduction in relative food intake. Reported genetic parameter estimates for resistance and performance traits appear to vary under differing production environments. In order to explore the impact of epidemiological effects and anthelmintic input on genetic parameter estimates the model was extended to simulate a population of lambs in a grazing scenario (Chapter 3). Whilst estimates of heritabilities and genetic correlations for drenched lambs remained constant, for lambs given no anthelmintic treatment, the heritability of empty body weight (EBW) reduced and the genetic correlation between faecal egg count (FEC) and EBW became increasingly negative with increasing exposure to infective larvae. Thus differences in anthelmintic input and pasture larval contamination (PC) may provide plausible causes for the variation in genetic parameter estimates previously reported. To investigate the interactions between host resistance and epidemiology (Chapter 4) a population of 10,000 lambs were simulated and FEC predictions used to assign the 1,000 lambs with the highest and lowest predicted FEC to ‘susceptible’ (S) and ‘resistant’ (R) groups, respectively. R and S groups were then simulated to graze separate pastures over 3 grazing seasons. The average FEC and PC predictions of these groups diverged during the first 2 grazing seasons and stabilised during the third, such that the difference in FEC predictions between R and S groups were double those predicted when grazed with the population. This was found to be consistent with experimental data. Further, anthelmintic treatment and grazing strategies were predicted to have no impact on the EBW of resistant lambs, suggesting that control strategies should be targeted towards susceptible animals. Targeted selective anthelmintic treatment (TST) has been proposed to reduce risks of anthelmintic resistance with minimal impacts on performance. To describe the short- and long-term impacts of TST and drenching frequency on sheep production and the emergence of anthelmintic resistance, the model was extended to include a description of anthelmintic resistance genotypes within the nematode population (Chapter 5). Reducing the proportion of treated animals was predicted to increase the duration of anthelmintic efficacy, whilst reducing the drenching frequency increased the long-term benefits of anthelmintic on sheep production. Various determinant criteria for use in TST regimes were compared (Chapter 5) including performance traits such as live weight and growth rate, and parasitological traits such as FEC. Using FEC as the TST criterion was predicted to allow the greatest reduction in the number of anthelmintic treatments administered whilst maintaining the highest average EBW, whilst live weight and growth rate were predicted to give little to no improvement in comparison to selecting animals at random for TST. Using estimated breeding values (EBVs) for FEC as the determinant criterion for TST regimes was compared to using measured FEC (Chapter 6). The EBV for true FEC across the entire growth period, akin to perfect genomic selection, was predicted to be a better criterion than measured time-specific FEC (including a sampling error) for a TST regime. EBVs calculated using measured time-specific FEC showed little benefit compared to measured FEC. The information gained from these simulation studies increases our understanding of control strategies and their impact on host-parasite interactions under various scenarios that may not have been possible using experimental methods. It is important to remember that the aim of alternative or complimentary control strategies is to maintain the sustainability of sheep production systems, and as such the production gain of any control strategy needs to be weighed against the financial, labour and time costs involved in implementation.