Mitochondrial genome sequencing and analysis of scuticociliates (Uronema marinum) isolated from Takifugu rubripes

Scuticociliates are dangerous parasitic pathogens for in worldwide mariculture. Scuticociliates cause high mortality to marine fish. After an outbreak of scuticociliatosis in Takifugu rubripes in Liaoning Province, northern China, Uronema marinum, a scuticociliate, was identified. In this study, using Illumina MiSeq next-generation sequencing, the mitochondrial genome of U. marinum was assembled and analysed phylogenetically using mitochondrial genomes of other scuticociliates. The complete U. marinum mitochondrial genome was 39,845 bp; it contained two rRNAs, six tRNAs, and 39 protein-coding genes (PCGs). From the 39 PCGs, 15 PCGs were located on the heavy strand, and 24 PCGs on the light strand of U. marinum mitogenome. The phylogenetic tree showed that there were two main clades, Oligohymenophorea and Spirotrichea. Nine ciliate species were clustered together within Oligohymenophorea; Uronema marinum was a separate cluster sharing a relatively close ancestry with Hymenostomatida. The results of this study will help advance the systematics, and studies of evolution and molecular epidemiology of scuticociliates.