GENOMIC EXCLUSION IN TETRAHYMENA: GENETIC BASIS.

Abstract

SYNOPSIS. Genomic exclusion is an aberration that occurs during conjugation in variety 1 of Tetrahymena pyriformis. Instead of containing markers from both parents, the out cross pairs are either homozygous for all the genes of one parent (unilateral genomic exclusion); or, some of the pairs are homozygous for the genes of one parent and other pairs are homozygous for the genes of the other parent (bilateral genomic exclusion). This phenomenon was first demonstrated in the C strain: some stocks evoke unilateral genomic exclusion; others, bilateral genomic exclusion. C*, inbred for 5 generations, was used to explore this phenomenon in some detail since unilateral genomic exclusion of C genes occurs in almost all pairs in outcrosses of C*. In a mating of C*, both exconjugants are recovered, both are dipioid and similar in phenotype. Using morphological markers, C* can be shown to participate in the mating; therefore, C* does not induce illegitimate matings of the normal mate. When the normal mate is heterozygous for alleles (HA/HD) not present in C*, 3 classes of offspring (HA/HA, HA/HD and HD/HD) are produced in a 1:2:1 ratio. These observations indicate that 2 meiotic products of the normal mate unite to form the syn carya. The genetic ratios obtained in 1 and 2 factor crosses limit the possible cytogenetic bases for genomic exclusion. They suggest that 1 of the 4 haploid nuclei replicates and the replica fuses randomly with any 1 of the 4 nuclei. The 2 schemes of nuclear behavior (single fertilization, double fertilization) that would satisfy these requirements have not yet been resolved.