Gene expression and phenotypic change inParamecium tetraureliaexconjugants

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Abstract
SUMMARY: A study of the patterns of phenotypic change in exconjugants using the recessive behavioural mutant pawn (pwA) and its wild-type allele shows that both cytoplasmic and nuclear factors contribute to phenomic lag. Following loss of the wild-type allele from the macronucleus, phenomic lag lasts for 6–11 cell cycles in various sublines of a single clone. Inherited cytoplasmic material is estimated to be responsible for phenomic lag of no more than 5–6 cell cycles. Longer persistence of the parental phenotype is due to continued gene activity in macronuclear fragments carrying the wild-type allele. Genes in fragments remain active and can result in maintenance of the parental phenotype as long as fragments are present (up to 11 cell cycles).Phenomic lag in the other direction, from pawn to wild type, varies from 0 to 2 cell cycles. The major cytoplasmic factor involved is the amount of wild-type material acquired from the mate during conjugation. Extensive cytoplasmic exchange often occurs during normal conjugation and can lead to change of phenotype as early as the first meiotic division. Phenotypic change due to gene expression in macronuclear anlagen brings about phenotypic change near the end of the first cell cycle in +/+ cells and about a cell cycle later in heterozygotes.