A Novel Phagomyxid Parasite Produces Sporangia in Root Hair Galls of Eelgrass (Zostera marina).

The objective of this study was to identify the parasite causing the formation of root hair galls on eelgrass (Zostera marina) in Puget Sound, WA. Microscopic and molecular analyses revealed that a novel protist formed plasmodia that developed into sporangia in root hair tip galls and released biflagellate swimming zoospores. Root hair galls were also observed in the basal section of root hairs, and contained plasmodia or formed thick-walled structures filled with cells (resting spores). Phylogenetic analyses of 18S rDNA sequence data obtained from cells in sporangia indicated that the closest relative of the parasite with a known taxonomic identification was Plasmodiophora diplantherae (86.9% sequence similarity), a phagomyxid parasite that infects the seagrass Halodule spp. To determine the local geographic distribution of the parasite, root and soil samples were taken from four eelgrass populations in Puget Sound and analyzed for root hair galls and parasite DNA using a newly designed qPCR protocol. The percent of root hairs with galls and amount of parasite DNA in roots and sediment varied among the four eelgrass populations. Future studies are needed to establish the taxonomy of the parasite, its effects on Z. marina, and the factors that determine its distribution and abundance.