A mendelian mutation affecting mating-type determination also affects developmental genomic rearrangements in Paramecium tetraurelia

Abstract

In Paramecium tetraurelia, mating type is determined during the differentiation of the somatic macronucleus rom a zygotic nucleus genetically competent for both types, O and E. Determination of the developing macronucleus is controlled by the parental macronucleus through an unknown mechanisms resulting in the maternal inheritance of mating types. The pleiotropic mutation mtF(E) affects macronuclear differentiation. Determination for E is constitutive in mutant homozygotes; a number of unrelated mutan characters are also acquired during development. We have examined the possibility that the mutation causes a defect in the developmental rearrangements of the germ-line genome. We show that the excision of an IES (internal climinated sequence) interrupting the coding sequence of a surface antigen gene is impaired in the mutant, resulting in an alternative macronuclear version of the gene. Once established, the excision defect is indefinitely transmitted across sexual generations in the cytoplasmic lineage, even in a wild-type genetic context. Thus, the processes of mating-type determination and excision of this IES, in addition to their common sensitivity to the mtF(E) mutation, show a similar maternal inheritance of developmental alternatives in wild-type cells, suggesting a molecular model for mating-type determination.