Biogeography of ubiquitous Labyrinthula zosterae and emergence of a novel seagrass pathogen

The biogeography of host–pathogen interactions shapes disease dynamics by influencing pathogen presence, host susceptibility and environmental factors. This study investigates the genetic diversity of the protist Labyrinthula in eelgrass (Zostera marina) ecosystems, a model for host–pathogen research. Labyrinthula zosterae (Lz), the pathogen responsible for eelgrass wasting disease, causes leaf lesions that lead to necrosis, shoot mortality and habitat loss. We collected eelgrass samples at six nodes from Alaska to southern California and performed high-throughput 18S gene sequencing. We identified two dominant pathogenic strains: one in the Lz clade, previously linked to Pacific and Atlantic outbreaks, and a novel strain associated with crescent-shaped lesions. While pathogenic Lz dominated 10 sites, non-pathogenic strains were more prevalent at 11 other sites. Notably, the novel crescent strain dominated northern California sites. The observation that 67% of amplicon sequence variants were unique to individual regions suggests localized environmental or ecological drivers of diversity, while the widespread occurrence of the remaining 33% indicates the ability of certain Labyrinthula taxa to persist across diverse habitats. This research enhances our understanding of the diversity of Lz, supporting eelgrass ecosystem conservation and restoration efforts, and it increases our understanding of biogeographic patterns of host–pathogen systems.