| Summary: | Scuticociliatosis is an invasive external or systemic infection caused by ciliated protozoa, mainly those within the subclass Scuticociliatia (scuticociliates). Many scuticociliates are fish pathogens, including <i>Miamiensis avidus</i>, <i>Philasterides dicentrarchi</i>, <i>Pseudocohnilembus persalinus</i>, and <i>Uronema marinum</i>. Our previous study showed that hemolysis-related genes derived from bacteria through horizontal gene transfer (HGT) may contribute to virulence in <i>P. persalinus</i>. Hemorrhagic lesions are a common feature of scuticociliatosis, but it is not known whether other scuticociliates also have bacteria-derived hemolysis-related genes. In this study, we constructed a high-quality macronuclear genome of another typical pathogenic scuticociliate, <i>U. marinum</i>. A total of 105 HGT genes were identified in this species, of which 35 were homologs of hemolysis-related genes (including hemolysin-like genes) that had previously been identified in <i>P. persalinus</i>. Sequencing of an additional five species from four scuticociliate families showed that bacteria-derived hemolysis-related genes (especially hemolysin-like genes) are widely distributed in scuticociliates. Based on these findings, we suggest that hemolysin-like genes may have originated before the divergence of scuticociliates.
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