Labyrinthula sp. diversity in North America

We used targeted amplicon-based high throughput sequencing (HTS) to examine the phylogeography of Labyrinthula species associated with two seagrass hosts (Zostera marina, Thalassia testudinum) collected from across North America, including locations in the USA and Mexico. After testing 189 seagrass blades from 16 locations across North America, Labyrinthula spp. were detected at 14 sites with PCR prevalence ranging from 20 - 100%. We generated 10,936,628 sequences for analyses, which yielded 25 unique Labyrinthula amplicon sequence variants (ASVs), which were phylogenetically grouped into three species of Labyrinthula. All three species detected were previously identified as pathogenic, indicating that Labyrinthula species capable of causing wasting disease are present across these regions, including in locations where disease outbreaks have not been reported. Assessing the phylogeography of strains was limited by sample size in each location, but indicates that the genetic connectivity of strains appears high, with location and seagrass host being the most important factors explaining the variation in Labyrinthula strains detected. The high connectivity, but low overall species diversity of Labyrinthula, is consistent with a parasite that is a host generalist.