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

Abstract

Keystone species are integral for the functioning of ecosystems and 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 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. Answers to fundamental questions regarding host-pathogen interactions for both diseases are currently unknown. To address some of these questions, experiments were conducted to 1) determine virulence and sequence 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 oystersÕ 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 ÒµvarsÓ) via challenge experiments; and 4) investigate the viral shedding of tolerant and susceptible Cg exposed to multiple OsHVs-1 µvars. Pathogenic and non-pathogenic isolates of Labyrinthula spp. were isolated from Zm plants in the Pacific northwest, and differed based on partial 18S sequencing. In an infectious dose 50 experiment, only 6 Lz cells mL-1 were needed to cause SWD infection in 50% of exposed plants at both 15?C and 7.5?C. Co-culturing Cg with Zm exposed to Lz significantly decreased severity and intensity of SWD at both 11?C and 18?C. For OsHV-1, Cg lines accumulated high viral loads within their tissues, but had a range of tolerance to infection determined by a range in survival. Survival differed between spat and larger juveniles of the same family. 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. Overall, oysters shed the most virus from 24-120 hours post infection. These results provide crucial information about disease in two foundation species, Zm and Cg, and highlight the potential for these two species to benefit from one another. These findings provide information about marine disease systems of keystone species which will aid monitoring, management, and future research.