Name: Tori Agnew
Date: 11/15/2023
Time (EST/EDT): 1:30 pm
Location: Rita Rossi Colwell Center 2038 MPR, IMET and Remote
Remote Access: email: mees@umd.edu

Committee Chair: Dr. Hal Schreier
Committee Members: Dr. Colleen Burge, Dr. Eric Schott, Dr. Tsetso Bachvaroff, Dr. Bill Dennison
Dean’s Representative: Dr. Bill Dennison

Title: Disease dynamics in marine keystone species: understanding Pacific oyster-Ostreid Herpesvirus-1 and eelgrass-Labyrinthula spp. interactions

Abstract: Keystone species are integral for functioning of ecosystems, therefore understanding threats to these species is incredibly important especially in an era of global climate change. Marine environments are particularly vulnerable to changes in climate; specifically, marine disease is expected to increase, eliciting questions on how changes in disease dynamics may affect population structure of foundation species. A fundamental understanding of host-pathogen interactions is important to better predict potential changes over time. This dissertation is focused on two disease systems: the virus Ostreid Herpesvirus-1 (OsHV-1), which affects Pacific oysters Crassostrea gigas (Cg), and the protist Labyrinthula zosterae (Lz), which causes seagrass wasting disease (SWD) in Zostera marina (Zm). Although separate disease systems, the two host species often overlap in the intertidal zone and both have a substantial effect on the ecosystems they inhabit. SWD has been associated with massive losses of Zm globally, causing decreases in habitat, carbon sequestration, and sediment stabilization. Similarly, OsHV-1 causes mass mortality events in Cg globally, which has devastating impacts on oyster farms and the ecosystem services oysters provide. Fundamental questions regarding host-pathogen interactions for both diseases are currently unknown or understudied. To address some of these questions, experiments were conducted to 1) determine virulence and phylogenetic differences among Labyrinthula spp. via challenge studies and a partial 18S sequencing analysis; 2) test a mitigation strategy for SWD by co-culturing with Cg to assess the oysters’ ability to filter the pathogen from the water; 3) determine if Cg stocks in the US are resistant to multiple OsHV-1 variants (termed microvariants or “µvars”) via challenge experiments; and 4) investigate the viral shedding of tolerant and susceptible Cg exposed to multiple OsHVs-1 µvars. For SWD, pathogenic and non-pathogenic isolates of Labyrinthula spp. were isolated from Zm plants in the Pacific northwest. Partial 18S sequences of pathogenic isolates were identical to Lz, the causative agent of SWD, while non-pathogenic isolates formed their own SSU ribotype within a non-pathogenic clade and were not genetically similar to any other previously sequenced Labyrinthula. One isolate identified as Lz was used in an infectious dose 50 experiment, in which only 6 cell mL-1 were needed to cause SWD infection in 50% of exposed plants at both a high (15C) and low (7.5C) temperature. The presence of Cg co-cultured with Zm exposed to Lz significantly decreased both severity and intensity of SWD regardless of temperature (11C vs. 18C). For OsHV-1, Cg lines produced by the Molluscan Broodstock Program at Oregon State University were not resistant to OsHV-1 due to high viral loads within their tissues, but instead had a range of tolerance to infection determined by a range in survival. Juvenile Cg lines that had increased survival to OsHV-1 exposure as spat had equal survival and viral shedding as more susceptible Cg lines. Oysters exposed to a French µvar of OsHV-1 shed significantly higher amounts of virus than oysters exposed to a µvar from San Diego, CA. Regardless of susceptiblity or viral exposure, oysters shed the most virus from 24-120 hours post infection. These results provide key aspects of disease in two foundation species, Zm and Cg, and highlight the potential for these two species to benefit from co-culture with one another. These findings are useful to improve laboratory and field studies as well as monitoring of disease over time.