EFFECTS OF NULLISOMIC CHROMOSOME DEFICIENCIES ON CONJUGATION EVENTS IN TETRAHYMENA-THERMOPHILA - INSUFFICIENCY OF THE PARENTAL MACRONUCLEUSTO DIRECT POSTZYGOTIC DEVELOPMENT

Conjugation fails postzygotically after mating of Tetrahymena cells th at have wild-type parental macronuclei but harbor noncomplementing nul lisomic parental germline deficiencies. Failures begin shortly after f ormation 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 mic ronucleus, and to amplify anlage DNA, and they eventually die. Some de ficiencies block resorption of the pycnotic parental macronucleus, but we find no evidence for its regeneration. Some deficiencies cause abe rrant anlage DNA loss. Those that do not cause DNA loss are epistatic to those that do, indicating that normal anlage development requires t he 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 indicate s insufficiency of the parental macronucleus to direct postzygotic dev elopment and can be explained by the deficiency of essential conjugati on genes that are expressed from the anlage. The failure of nullisomic conjugants to complete pair separation indicates a requirement far ge ne products, expressed from the early anlage or its precursors, soon a fter anlage first differentiate.