Michelle Tseng

Interactions between the Parasite's Previous and Current Environment Mediate the Outcome of Parasite Infection

The study of parasite virulence has generally focused on the conditions under which virulence is expected to increase or decrease over time and how the interactions between hosts and their environments may mediate the outcome of infection. Recently, parasite traits such as transmission, offspring production, and development have also been shown to be influenced by environmental variation. What is unclear is how variation in the parasite’s environment may impact virulence. Recent theory demonstrates that plasticity can promote the evolution of decreased virulence; thus, understanding whether the parasite’s environment can mediate virulence can improve predictions regarding the outcome of parasite infection. Here, an obligate mosquito parasite was reared in hosts fed high or low levels of food. Parasite oocysts (offspring) produced in these two host environments were subsequently fed to uninfected hosts. Parasites originating from well‐fed hosts were found to be more virulent to these subsequent hosts compared to parasites originating from poorly fed hosts. Additionally, this effect was apparent only when current hosts were food deprived. These results demonstrate that parasite virulence was mediated by a cross‐generational effect of the environment and that the overall outcome of infection was modified by variation in both the parasites and host’s environments.

The effect of food limitation on immunity factors and disease resistance in the western tent caterpillar

Epizootics of nucleopolyhedrovirus characterize declines of cyclic populations of western tent caterpillars, Malacosoma pluviale californicum. In field populations, infection can be apparently lacking in one generation and high in the next. This may suggest an increase in the susceptibility to infection of larvae at peak density or the triggering of a vertically transmitted virus. Here, we test the hypothesis that reduced food availability, as may occur during population outbreaks of tent caterpillars, influences the immunocompetence of larvae and increases their susceptibility to viral infection. We compared immunity factors, hemolymph phenoloxidase and hemocyte numbers, and the susceptibility to nucleopolyhedroviral infection of fifth instar larvae that were fully or partially fed as fourth instars. To determine if maternal or transgenerational influences occurred, we also determined the susceptibility of the offspring of the treated parents to viral infection. Food limitation significantly reduced larval survival, development rate, larval and pupal mass, moth fecundity and levels of hemolymph phenoloxidase, but not the numbers of hemocytes. Neither the food-reduced larvae nor their offspring were more susceptible to viral infection and were possibly even less susceptible at intermediate viral doses. Food reduction did not activate latent or covert viral infection of larvae as might be expected as a response to stress. We conclude that reducing the food intake of fourth instar larvae to an extent that had measurable and realistic impacts on their life history characteristics was not translated into increased susceptibility to viral infection.

Sex–specific response of a mosquito to parasites and crowding

Host–parasite interactions are significantly influenced by the sex of the host and the environment in which the host is found. Sex–specific responses to parasite infection, however, may change according to the host environment. I examine the combined effect of parasite infection and crowding on males and females of the mosquito Aedes albopictus. At a high larval density, infected males experienced a greater relative reduction in body size than did infected females, whereas the pattern was reversed at low density. This experiment demonstrates the importance of the environment on sex–specific responses to parasites and contributes to a growing body of work examining sources of variation in host–parasite interactions.

A Simple Parafilm M-Based Method for Blood-Feeding Aedes aegypti and Aedes albopictus (Diptera: Culicidae)

Abstract A simple device for blood-feeding Aedes (Stegomyia) mosquitoes is described. Parafilm M is stretched and pressed into fiberglass window screen to form a packet for holding warmed blood. The method has been used successfully to maintain Aedes albopictus colonies for >2 yr. In typical blood-feeding trials, 53 ± 0.8% (±SE) of Aedes aegypti (Linnaeus) and 31 ± 6% of Ae. albopictus (Skuse) fed to repletion.

ASCOGREGARINE PARASITES AS POSSIBLE BIOCONTROL AGENTS OF MOSQUITOES

The seemingly ubiquitous presence of gregarine parasites in numerous mosquito taxa (Table 1; Chen 1999) suggests that consideration of these parasites as biological control agents may be a worthwhile endeavor. However, because most studies prior to 1985 did not demonstrate significant fitness (e.g., mortality, adult size) differences between infected and non-infected mosquitoes, Beier and Craig (1985) deemed in their overview of gregarine parasites of mosquitoes that ‘‘There is no evidence that gregarines can be used to control mosquitoes, and no evidence that gregarines in their natural habitat have a significant negative impact on populations of their normal host.’’ (p 182) However, the authors did suggest that because ‘‘conventional strategies for controlling container-breeding mosquitoes are not effective, the possibility of using gregarines in unnatural mosquito hosts should not be ruled out.’’ A number of additional studies have since been published that have examined the pathogenicity of gregarine parasites both for natural and nonnatural hosts, as well as for hosts reared in stressful vs. non-stressful environments. This paper reviews the outcomes of these studies and addresses whether the prospects of using gregarines as mosquito biological control agents have changed with these recent findings.

The relationship between parasite fitness and host condition in an insect--virus system.

Research in host-parasite evolutionary ecology has demonstrated that environmental variation plays a large role in mediating the outcome of parasite infection. For example, crowding or low food availability can reduce host condition and make them more vulnerable to parasite infection. This observation that poor-condition hosts often suffer more from parasite infection compared to healthy hosts has led to the assumption that parasite productivity is higher in poor-condition hosts. However, the ubiquity of this negative relationship between host condition and parasite fitness is unknown. Moreover, examining the effect of environmental variation on parasite fitness has been largely overlooked in the host-parasite literature. Here we investigate the relationship between parasite fitness and host condition by using a laboratory experiment with the cabbage looper Trichoplusia ni and its viral pathogen, AcMNPV, and by surveying published host-parasite literature. Our experiments demonstrated that virus productivity was positively correlated with host food availability and the literature survey revealed both positive and negative relationships between host condition and parasite fitness. Together these data demonstrate that contrary to previous assumptions, parasite fitness can be positively or negatively correlated with host fitness. We discuss the significance of these findings for host-parasite population biology.

Phylogeography of the coastal mosquito Aedes togoi across climatic zones: testing an anthropogenic dispersal hypothesis.

The coastal mosquito Aedes togoi occurs more or less continuously from subarctic to subtropic zones along the coasts of the Japanese islands and the East Asian mainland. It occurs also in tropical Southeast Asia and the North American Pacific coast, and the populations there are thought to have been introduced from Japan by ship. To test this hypothesis, the genetic divergence among geographic populations of A. togoi was studied using one mitochondrial and three nuclear gene sequences. We detected 71 mitochondrial haplotypes forming four lineages, with high nucleotide diversity around temperate Japan and declining towards peripheral ranges. The major lineage (L1) comprised 57 haplotypes from temperate and subarctic zones in Japan and Southeast Asia including southern China and Taiwan. Two other lineages were found from subtropical islands (L3) and a subarctic area (L4) of Japan. The Canadian population showed one unique haplotype (L2) diverged from the other lineages. In the combined nuclear gene tree, individuals with mitochondrial L4 haplotypes diverged from those with the other mitochondrial haplotypes L1—L3; although individuals with L1—L3 haplotypes showed shallow divergences in the nuclear gene sequences, individuals from Southeast Asia and Canada each formed a monophyletic group. Overall, the genetic composition of the Southeast Asian populations was closely related to that of temperate Japanese populations, suggesting recent gene flow between these regions. The Canadian population might have originated from anthropogenic introduction from somewhere in Asia, but the possibility that it could have spread across the Beringian land bridge cannot be ruled out.

EDITORIAL: Editorial: 2008 year in review

2008 was an eventful inaugural year for Evolutionary Applications. We published four issues comprising 45 papers in total, including the special issue: Evolutionary Perspectives on Salmonid Conservation and Management. Wiley-Blackwell sponsored an Evolutionary Applications symposium at the Canadian Society of Ecology and Evolution annual meeting in Vancouver (May), and hosted an official Evolutionary Applications launch reception at the SSE annual meeting in Minneapolis (June). The journal received much press coverage, and was featured in both Nature and New Scientist. The year was successfully capped with the acceptance of the journal for full indexing in ISI. The Editorial team would like to thank the scientific community for their overwhelming support of this new and unique journal. Undoubtedly this support, as well as the official endorsement of the journal by both the Society for the Study of Evolution, and the European Society for Evolutionary Biology (ESEB), contributed to the exceptionally rapid acceptance of the journal into ISI. In 2008, Evolutionary Applications published papers in most of the fields listed on our website (http://www.evolutionaryapplications.org; see Table 1). Published papers also focused on a wide diversity of organisms of applied importance, including plants, animals and microbes (Table 2). Clearly, the journal is fulfilling its primary goal of being the principal outlet for the increasing number of high-quality papers using evolutionary concepts to address issues of economic, medical and social relevance. Table 1 Subjects covered in 2008 in Evolutionary Applications Table 2 Examples of organisms of applied significance featured in Evolutionary Applications In 2009, we are again publishing four issues, including this current special issue on evolutionary medicine, and an upcoming special issue on the impact of harvesting on fish evolution and Darwinian fishery science. Among other events, in August, Wiley-Blackwell will be sponsoring a full-day symposium on evolutionary applications at the 12th ESEB Congress in Turin, Italy. Despite being in our first year of operation, we succeeded in maintaining a very short time from submission to first decision (34 days). Also, our transparent online review and production system allows authors to track the progress of their manuscripts from submission to publication. We will continue striving both to publish the highest quality research on applied evolution, and to make the importance and broad relevance of evolutionary biology more accessible to all interested readers.

The Effect of Parasitism and Interpopulation Hybridization on Aedes albopictus (Diptera: Culicidae) Fitness

Abstract Recent research in mosquito population genetics suggests that interpopulation hybridization has likely contributed to the rapid spread of the container-breeding mosquitoes. Here, I used laboratory experiments to investigate whether interpopulation Aedes (Stegomyia) albopictus (Skuse) F1 and F2 hybrids exhibit higher fitness than parental populations, and whether hybrid mosquito performance is related to infection by the coevolved protozoan parasite Ascogregarina taiwanensis (Lien and Levine). Overall, there were significant differences in development time, wing length, and survival between the two parental mosquito populations, but no difference in per capita growth rate r. Hybrid mosquitoes were generally intermediate in phenotype to the parentals, except that F2 females were significantly larger than the midparent average. In addition, As. taiwanensis parasites produced fewest oocysts when they were reared in hosts of hybrid origin. These data suggest that hybridization between previously isolated mosquito populations can result in slight increases in potential mosquito reproductive success, via increased hybrid body size, and via the temporary escape from coevolved parasites. These findings are significant because studies have shown that even slight hybrid vigor can have positive fitness consequences for population persistence. Although this was a laboratory experiment extending only to the F2 generation, many other invasive insects also carry coevolved parasites, and thus the patterns seen in this mosquito system may be broadly relevant.

A comparison of epifaunal invertebrate communities in native eelgrass Zostera marina and non-native Zostera japonica at Tsawwassen, BC

Abstract The introduction of non-native species can alter the structure of local communities and ecosystems. Although there is concern over the negative effects imposed by introduced species, positive effects are also possible. Introduced species may facilitate local resident species by providing or modifying habitat; these positive effects are sometimes overlooked, but are important for decisions about the local management of non-native species. The introduction of Zostera japonica to the Northeast Pacific coast is a case of a non-native foundation species that appears to expand total eelgrass habitat area, potentially facilitating local eelgrass-inhabiting species. In this study, we asked whether the non-native Z. japonica provides habitat for eelgrass-dwelling invertebrates that is similar to its native congener, Z. marina. We systematically quantified invertebrate assemblages in the two eelgrass species and quantified eelgrass characteristics that are relevant to epifaunal habitat quality (e.g. shoot density and surface area). We found that Z. japonica supports a greater total abundance of the same invertebrate species found in Z. marina meadows; however, the relative abundance of invertebrate species differed between Z. japonica and Z. marina. This result is consistent with differences in the two congeners’ morphological characteristics, density of vegetative and reproductive shoots, and total blade surface area per square metre. We conclude that the expansion of total eelgrass habitat due to the introduction of Z. japonica appears to increase the available habitat for local epifaunal invertebrate communities.