Entomology, Parasitology & Disease

63-03 - Cospeciation between birds and their parasitic mites

Project Leader

Spicer

Affiliation

UC San Francisco Dept of Biology

Objective

Host-parasite systems are a powerful arena for evolutionary research because the environment of many parasites is defined almost exclusively by its host. In this way, factors governing evolution are easier to delineate for parasites than for free-living organisms. This is particularly true for parasites which complete their entire life cycle on the body of a single host. In some cases the phylogeny of these parasites is congruent with that of the host, indicating cospeciation between host and parasite. Cospeciating host-parasite systems are unique in that they represent a long history of parallel evolution. This is important because it provides a temporal framework for comparative analyses of the rates of evolution of the host and parasite. Because the life histories of hosts and their parasites are generally extremely different, studying the molecular evolution in a host-parasite system can provide insight into questions relating to the possible effects of generation time, metabolic rate, and other life history parameters on rates of mutation and evolutionary change.

65-86 - Lizard malaria host parasite ecology

Project Leader

Schall

Affiliation

Univ. of Vermont Dept. of Biology

Objective

Malaria parasites infect a broad range of vertebrate hosts, including mammals (four species infect humans), birds, and reptiles. Since 1978, a malaria system has been under study at the HREC: Plasmodium mexicanum in the western fence lizard, Sceloporus occidentalis. This parasite-host association is now among the best known for any malaria parasite of wildlife hosts. A central goal is a long-term study of the prevalence of the parasite over time at numerous locations at the Hopland site. These 26 years of data reveal the parasite's prevalence seems to be following a long-term cycle, unexpected under standard epidemiological models for malaria parasites. These data can now be compared and contrasted with those from our similar long-term studies at three sites in the Caribbean islands. A second major goal is to understand the ecology and evolution of the life history of the parasite and relate those data to the evolution of parasite virulence. Theory predicts that infections should follow different life histories depending on the genetic diversity of the parasites in the vertebrate host. Over the past three years, a series of experiments were conducted to determine if clonal diversity of the infections is associated with differing behavior of the parasites. More recently, a direct measure of clonal diversity has been developed: variable microsatellite markers for P. mexicanum have been developed, the first for any malaria parasite of nonhumans.

67-84 - Tick-borne disease agents in the Pacific Coast

Project Leader

Lane

Affiliation

UC Berkeley ESPM Insect Biology

Objective

This research is intended to clarify the role of the western gray squirrel (Sciurus griseus) as a keystone species for maintaining enzootic foci of the Lyme disease (LD) spirochete Borrelia burgdorferi sensu stricto (Bb ss) in the far-western USA; and to investigate the host-seeking behavior of Ixodes pacificus (Ip) nymphs in relation to environmental parameters and to risk of human exposure to Bb ss at the Hopland Research and Extension Center (HREC). The reservoir competence of S. griseus for Bb ss will be evaluated by determining the infectivity of naturally infected squirrels for uninfected Ip larvae; the capacity of fed larvae to pass the infection transstadially; the ability of infected nymphs to transmit infection to naïve squirrels; and the duration of infectivity in experimentally infected squirrels. Previous research has established that dense woodlands are primary biotopes of Ip nymphs, and that contact with either leaf litter and wood, but especially wood (e.g., logs), can elevate the risk of human exposure to nymphal ticks. Therefore, host-seeking activities of Ip nymphs in relation to biotic and abiotic factors will be investigated quantitatively in oak/madrone woodlands. These will include the diurnal questing cycle; the densities of host-seeking nymphs, and of Bb ss-infected nymphs, on logs and tree trunks versus adjacent leaf litter; and the movements of marked nymphal ticks.

70-06 - Predicting the effects of environmental change and host diversity on the dynamics of insect-vectored generalist pathogens

Project Leader

Borer

Affiliation

University of Minnesota

Objective

Interactions between human-induced environmental change and pathogen dynamics are one of the most pressing and poorly understood issues facing scientists this century. Environmental change can alter pathogen dynamics in ways that increase human disease risk, intensify pathogen pressure on imperiled species, degrade ecosystem services, and endanger agricultural systems. It is crucial to understand the two major mechanisms by which anthropogenic activity affects pathogens: 1. alteration of host-community structure and 2. alteration of the abiotic environment. Vector-transmitted generalist pathogens are of particular concern as leading causes of emerging diseases. Our ability to predict how this group of pathogens will respond to human activity is limited because most theory focuses on specialists but does not explicitly incorporate the abiotic environment, and it is often logistically impossible or unethical to conduct experiments to directly test causation of processes thought to control pathogen transmission. To address these limitations, we will develop mathematical theory of generalist vector-transmitted pathogens that explicitly incorporates host competition for abiotic resources. We will test the predictions of this theory using geographic-scale field experiments with an aphid-vectored plant virus, barley yellow dwarf (BYDV). We will test how changes in moisture and nitrogen availability interact with changes in host community diversity and composition to control disease dynamics. The BYDV system is unique in that it allows us to create spatially- and phylogenetically-replicated experimental communities that mimic five important types of disease systems, thus allowing us to predict the effects of human activity on a variety of host communities and pathogens using a single general theoretical framework.