Effects of nullisomic chromosome deficiencies on conjugation events in Tetrahymena thermophila: insufficiency of the parental macronucleus to direct postzygotic development.

Conjugation fails postzygotically after mating of Tetrahymena cells that have wild-type parental macronuclei but harbor noncomplementing nullisomic parental germline deficiencies. Failures begin shortly after formation of the new macronuclear precursor (anlage) and completion of the first step in elimination of the parental macronucleus (pycnosis). Conjugants fail to complete pair separation, to eliminate one new micronucleus, and to amplify anlage DNA, and they eventually die. Some deficiencies block resorption of the pycnotic parental macronucleus, but we find no evidence for its regeneration. Some deficiencies cause aberrant anlage DNA loss. Those that do not cause DNA loss are epistatic to those that do, indicating that normal anlage development requires the dependent function of at least two types of genes. The possibility that these genes are involved in developmentally regulated anlage DNA rearrangements is discussed. Each observed conjugation defect indicates insufficiency of the parental macronucleus to direct postzygotic development and can be explained by the deficiency of essential conjugation genes that are expressed from the anlage. The failure of nullisomic conjugants to complete pair separation indicates a requirement for gene products, expressed from the early anlage or its precursors, soon after anlage first differentiate.