Research Overview:

This lab studies primarily the interactions between different types of parasites and their insect hosts. Most of our projects involve parasitoid wasps that develop as immatures in or on the bodies of other insects but that are free-living as adults. Parasitoid wasps are among the most species-rich group of organisms on earth. Many parasitoids are also very important economically because of their use as biological control agents of insect pests.

One study area in the lab focuses on understanding how the host's immune system protects insects from parasitoid invasion and reciprocally, how parasitoids overcome host defenses. This includes the study of microbial symbionts like polydnaviruses that are carried by many parasitoids and that play a key role in suppressing the immune system of host insects.

Another study area focuses on life history evolution and how parasitic lifestyles have affected developmental processes. We are particularly interested in polyembryonic parasitoids that exhibit many dramatic developmental adaptations as well as a sophisticated caste system that resembles in many ways the social systems of bees, ants and termites.

We also study vector arthropods like mosquitoes that transmit many important human diseases. Here our interest is primarily in how the mosquito immune system responds to invasion by different pathogens including the parasite that causes malaria in humans.

Overall, our work is strongly integrative as it uses approaches that range from molecular and biochemical methods to ecological and behavioral studies. Current projects in the laboratory are funded by competitive grants from the National Institutes of Health (NIH), National Science Foundation (NSF) and US Department of Agriculture National Research Initiative Program (USDA NRI).

Our home department at the University of Georgia is Entomology. However, the lab is also affiliated with several interdisciplinary campus programs including the Center for Tropical and Emerging Global Diseases (CTEGD) and Interdisciplinary Program in Evolutionary Biology.

These programs provide a rich environment for discussion and interaction with labs working on other biological systems. Through avenues like training grants, they also offer additional funding opportunities to support graduate student and post-doctoral training.

Current Projects

1. Regulation of cellular defense responses in insects
2. Disruption of cellular defense responses by polydnavirus-carrying parasitoids
3. The evolution of caste formation and polyembryonic development in parasitoids
4. Characterization of the mosquito cellular immune response

1. Regulation of cellular defense responses in insectsCirculating blood cells (hemocytes) play an essential role in defending insects against invading organisms. The response of host hemocytes often defines whether a given species is a compatible host for a given parasite. Parasitoids are usually killed by an immune response called encapsulation in which certain classes of hemocytes attach and spread across the surface of the foreign target to form a multilayered sheath of cells. Despite the fundamental importance of this defense response, the cellular and molecular mechanisms coordinating capsule formation are poorly understood. The insect species being used in our studies is the lepidopteran, Pseudoplusia includens (Lepidoptera:Noctuidae).

Studies completed to date demonstrate that encapsulation involves a cooperative response between specific populations of hemocytes. We have developed panels of markers that identify hemocyte subpopulations, in vitro assays for capsule formation, and have identified several products involved in capsule formation.

One such factor is Plasmatocyte Spreading Peptide (PSP1). PSP1 belongs to a multifunctional family of peptides involved in hemocyte activation and cell growth. We recently solved the secondary structure of PSP1 and showed that it shares significant features with the mammalian growth factor EGF.

2. Disruption of cellular defense responses by polydnavirus-carrying parasitoids

This project addresses how parasites evade encapsulation and other host immune responses. Parasites survive in permissive hosts by either possessing surface features that passively protect the insect from encapsulation or by immunosuppressing the host's ability to mount an encapsulation response. Of particular interest to immunosuppression is the role of polydnaviruses (PDVs). PDVs are associated with selected parasitoid taxa and are characterized by having double stranded, circular DNA genomes that are heterologous in size. PDVs replicate in the ovaries of female wasps. When wasps oviposit, they inject a quantity of virus and one or more eggs. Viral products then immunosupress the host. Our studies focus primarily on the moth Pseudoplusia includens, the wasp Microplitis demolitor, and M. demolitor bracovirus (MdBV).

Current goals include analysis of the MdBV genome to identify features that influence MdBV function during parasitism. Studies are also in progress to understand at the cellular and molecular level how MdBV gene products interact with the host immune system to disrupt encapsulation and other defense responses. The goal of comparative studies in collaboration with colleagues at other institutions is to discern patterns in the diversification and origins of polydnavirus-parasitoid associations.

3. The evolution of caste formation and polyembryonic development in parasitoids

Social animals are characterized by the development of castes in which some colony members reproduce while others function as altruistic helpers. Most social species are insects like ants, bees, and termites.
Caste systems have also evolved in selected groups of aphids, thrips, and polyembryonic wasps. The conditional switch controlling caste formation usually involves environmental stimuli that act upon processes that regulate development of individuals. Unlike other social species, embryos of polyembryonic wasps develop clonally to produce large numbers of genetically identical offspring and two morphologically distinct castes. All embryos in a clone exist in an identical environment, the host, yet develop into either reproductive larvae that mature into adult wasps or soldier larvae whose function is defense. Previously conducted background studies indicate that caste determination in the polyembryonic wasp <em>Copidosoma floridanum</em> involves the inheritance of germ cells.

4. Characterization of the mosquito cellular immune response

The insect immune system consists of both cellular and humoral elements that innately recognize broad classes of foreign intruders. While great progress has been made over the last several years in identifying humoral factors like antimicrobial peptides and the signaling pathways that regulate their synthesis, much less is known about control of cellular defense responses. This is due in large part to the small size of many insects which makes collection of hemocytes and identification of hemocyte-produced effector molecules difficult. It is also often difficult to conduct manipulative experiments on hemocyte-mediated defense responses in vivo or to isolate defined populations of hemocytes for use in experiments in vitro. This is particularly true for mosquitoes where little information exists on the origins, differentiation or functions of different hemocyte types. Mosquito hemocytes are classified primarily on the basis of morphology with only limited data relating these morphological characters to functional features. In the case of critically important vectors, like <em>Anopheles gambiae</em>, virtually nothing is known about hemocyte ontogeny in any life stage. Here, our goal is to use the fully sequenced An. gambiae genome in combination with primary immune cells and established cell lines to develop markers for An. gambiae immune cells. These markers in turn are being used to understand the role of specific immune cell populations in defense against malaria and other vector borne parasites.