A new chytridiomycete parasitizing the tardigrade <i>Milnesium tardigradum</i>

<jats:p> The life cycle of a chytridiomycete, Sorochytrium milnesiophthora gen. et sp. nov., infecting the tardigrade Milnesium tardigradum, is described. The zoospores are posteriorly uniflagellate and ovoid, and possess a central mass similar to a nuclear cap. To initiate an endobiotic infection they attach to the cuticle of the host, encyst, and generate an appressorium. The appressorium forms a penetration tube which crosses the cuticle to the epidermis and enlarges at the tip into a spherical thallus. Concomitantly, vacuoles replace the cytoplasm of the cyst and appressorium. As the thallus enlarges, it moves into the body cavity and cleaves into segments. The segments separate and round up into incipient sporangia. The incipient sporangia develop branching rhizoids in conjunction with host death. After a period of growth the sporangia form inoperculate exit papillae which penetrate the host cuticle. Zoospores exit individually and fully formed. The fungus can develop a polycentric phase when freshly collected, dead hosts containing sporangia are cultured in habitat water or on nutrient agar. The growth is extramatrical, covering the surface of the old host. It is rhizoidal, branching, and nonseptate with numerous intercalate incipient sporangia bearing rhizoids. On nutrient media the thallus grows indefinitely while in habitat water the incipient sporangia mature and discharge motile spores. The spores frequently have two to five flagella and are larger than those of the endobiotic colonial phase. A similar polycentric growth develops when motile spores are isolated on nutrient medium and suggests that the extramatrical growth on the host originates from encysted spores. </jats:p>