Immobilization antigen variation in natural isolates of Tetrahymena thermophila

Abstract

In Tetrahymena thermophila alternative forms of the major cell surface glycoprotein (the immobilization antigen) are specified by both allelic and non-allelic genes. The differential expression of non-allelic genes is affected primarily by temperature and culture medium. This report describes expression and genetic variation of immobilization antigens among 2,600 clones isolated from natural populations. The temperature regulated L (<20 o ) and H (20 o C-36 o C) antigens and two new antigens J and K were present among ∼57% of isolates; the remaining 43% appear to have unknown antigens. Genetic and Southern analyses show that J and K are due to genes with dominant epistasis over H and that the gene for K is also epistatic over that for J. This is the first reported instance of naturally occurring epistasis involving immobilization antigen expression in T. thermophila. In ponds, the frequencies of J and H vary inversely in a manner consistent with dominant epistasis. The frequencies of J and H also show seasonal variation, with J more common in the late spring, and H more common in the late summer and fall. L and H (and J) also show seasonal variation, with L more common in the early spring and late fall. Allelic variation was also found among the H antigens. Immunodiffusion showed that the H3 protein of natural isolates is partially identical to H3 of inbred strain B. In addition, two Hind III restriction fragment length 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 role for these surface proteins