IMMOBILIZATION ANTIGEN VARIATION IN NATURAL ISOLATES OF TETRAHYMENA-THERMOPHILA

In Tetrahymena thermophila alternative forms of the major cell surface glycoprotein (the immobilization antigen) are specified by both allel ic and non-allelic genes. The differential expression of non-allelic g enes is affected primarily by temperature and culture medium. This rep ort describes expression and genetic variation of immobilization antig ens among 2,600 clones isolated from natural populations. The temperat ure regulated L (< 20-degrees) and H (20-degrees-C-36-degrees-C) antig ens and two new antigens J and K were present among approximately 57% of isolates; the remaining 43% appear to have unknown antigens. Geneti c and Southern analyses show that J and K are due to genes with domina nt epistasis over H and that the gene for K is also epistatic over tha t for J. This is the first reported instance of naturally occurring ep istasis involving immobilization antigen expression in T. thermophila. In ponds, the frequencies of J and H vary inversely in a manner consi stent with dominant epistasis. The frequencies of J and H also show se asonal variation, with J more common in the late spring, and H more co mmon in the late summer and fall. L and H (and J) also show seasonal v ariation, with L more common in the early spring and late fall. Alleli c variation was also found among the H antigens. Immunodiffusion showe d that the H3 protein of natural isolates is partially identical to H3 of inbred strain B. In addition, two Hind III restriction fragment le ngth polymorphisms were found among the natural SerH3 genes. New SerH3 genes also appeared to segregate in crosses. The genetic and seasonal variation in i-antigen frequencies suggests an important biological r ole for these surface proteins.