SEROTYPIC VARIATION AMONG ISOLATES OF ICHTHYOPHTHIRIUS-MULTIFILIIS BASED ON IMMOBILIZATION

Efforts have been made to determine whether surface antigens could be used as biochemical markers to define strain differences in the parasi tic ciliate Ichthyophthirius multifilus. In previous studies, a wild-t ype isolate designated G1 was found to have surface proteins analogous to the immobilization antigens of Paramecium and Tetrahymena; rabbit antiserum against this strain immobilizes homologous cells in vitro. I t has now been shown for two additional Ichthyophthirius isolates (des ignated G1.1 and G2) that immobilization antigens are both present and serologically distinct. Proteins of similar size, which cross-react i n Western blots with rabbit antisera against immobilization antigens o f the G1 strain, are nevertheless found in the G1.1 and G2 isolates. A s shown by Southern blotting analysis, the G1.1 and G2 strains also co ntain genomic DNA sequences which hybridize with an immobilization ant igen cDNA from G1 when probed under conditions of reduced stringency. The serotypic differences in immobilization between l. multifilus isol ates appear to be stable over time and provide a means of discriminati ng strains. In addition to providing a basis for comparative studies, the work described here has implications for the development of vaccin es against this important fish parasite. Supplementary key words. Anti -protozoan vaccine, ciliate, fish parasite, immunity, molecular biolog y,