Phylogenetic position of the genus Mesochytrium (Chytridiomycota) based on zoospore ultrastructure and sequences from the 18S and 28S rRNA gene

Ultrastructural features and a molecular phylogeny of Mesochytrium penetrans (strain X-10 CALU) based on 18S and 28S rRNA gene sequences were investigated for the first time. The parasite is strongly specific for the green alga Chlorococcum minutum (Chlorococcales) and did not grow on another 29 strains of algae from the CALU collection. Most attention was paid to the zoospore ultrastructure. Ribosomes are dispersed through the cytoplasm. The nucleus and microbody-lipid globule complex (MLC) is surrounded by rough endoplasmic reticulum. The MLC is composed of a single mitochondrion and a single lipid globule partially covered with a microbody and a fenestrated cisterna, which is most posterior. The non-flagellated centriole is shorter than the kinetosome and lies at an angle of approximately 30 degrees to the latter, the two being connected with a broad, dense fibrillar bridge. The flagellar transition zone contains a spiral fiber. The non-flagellated centriole is Surrounded by a thin fiber, and has a veil. The features that distinguish Mesochytrium are the partial penetration of the sporangium into the host cell, and a zoospore with unique ultrastructural configuration. In our phylogeny, M. penetrans was nested within Lobulomycetales and the Polychytrium clade, but its phylogenetic position is tenuous because of marginal support, and thus its family and order status are considered incertae sedis. Zoospores of M. penetrans are most Similar to those of Synchytrium macrosporm and Rozella allomycis, which belong to different clades. These ultrastructural similarities may be a result of the relative simplicity of zoospore morphology, and do not reflect a phylogenetic relationship between these genera.