What leads to variation in host susceptibility?
To better understand this question and how this can scale up to impact ecosystems has been the main aim of my research. Ecological immunology seeks to understand traits that can be measured to determine a population's susceptibility to parasites. My research examines host and parasite traits in tandem to do this.
Study system
The early-stages of infection start with spores penetrating into the body cavity and then growing hyphae. These branching structures are important for the parasite to adhere to a surface and anchor itself in the host body cavity.
To understand host and parasite traits that lead to variation in host susceptibility, I use the Daphnia x Metschnikowia bicuspidata model system. Daphnia are a globally-distributed genus of freshwater zooplankton that were originally studied for their fascinating displays of phenotypic plasticity in the 1800s and have since become model systems in both disease ecology and ecotoxicology. Their transparent carapace and cyclical parthenogenesis make them perfect for use in the laboratory.
While filter-feeding in the water column, they ingest needle-shaped Metschnikowia bicuspidata spores. About 50 µm in size, these spores use a fungal cannon mechanism to puncture through the host gut epithelium and into the body cavity. Once inside the host body cavity, the parasite encounters the host immune system which can fight off early-stages (pictured below) of the parasite invasion; however, late-stage infections are rarely fended off by the host and ultimately result in host death. Upon death, spores are released into the environment to be ingested by new hosts.
Hemocytes covering a spore that penetrated into the host body cavity.
Variation in host morphology
In order for infection to occur, a parasite must first penetrate into the host body cavity. However, different host genotypes and species vary in the number of spores that penetrate into their body cavity and susceptibility. This work examines host morphology to determine host traits that could underlie variation in spore penetration using four different Daphnia species as both juveniles and adults. After spores penetrate, there is also a visible cellular immune response, with hemocytes often covering penetrating spores. My research further examines the influence of hemocytes on infection outcomes.
Biomechanics of parasite puncture
Collaborators: Philip S L Anderson
The ability to physically puncture through host epithelial barriers has evolved in a variety of parasite taxa, from parasitic plants to helminths. For this reason, the morphology of parasites can be important in determining which hosts they can infect. The role of these tools can be multi-purpose and different morphologies could provide the parasite with different benefits. In the case of Metschnikowia bicuspidata, the tool can have a barbed morphology or no barb morphology. My research uses a scaled up model with crayfish epithelial tissue and 3D printed spores to test the role of these different morphologies on the parasite’s ability to puncture into the body cavity, and the ability of the parasite to stay in the body cavity without withdrawing back into the gut.
(a) SEM picture of a spore without barbs penetrating through the gut epithelium of Daphnia and (b) the tip of a spore that has barbs. (c) the 3D design of spore with back-barbs based on measurements collected from pictures taken with SEM and light microscopy. (d) One of the 3D printed ascospores.
A Daphnia with a late-stage infection. The spores fill the body cavity and give the host a brownish coloration.
Host-parasite interactions in an ecological context
Collaborators: Tara Stewart Merrill
Animals face trade-offs when allocating energy to different traits. Many previous studies have noted trade-offs between investment in immunity and other host traits. Within Cladocera, it has been noted that there is an association between host eye size and immune response when exposed to a parasite. However, our work found that smaller eyes are not associated with a weaker immune response but rather exposure decreases eye size. This work also found that eye size could change depending on factors like the age of the host and the availability of resources, indicating that this trait is quite flexible and influenced by multiple factors. These findings highlight how complex and context-dependent the relationships between different physiological traits and immune responses can be in ecological systems.