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The unique feature of the "B system" of mating-type determination found in Paramecium tetraurelia is the existence of a cytoplasmic difference between odd (O) and even (E) cells created and maintained by the action of their macronuclei. Thus far, the presence of a determining factor that controls the differentiation of the developing zygotic macronucleus for O mating type has not been verified. Results of crosses between cells of differing clonal age and complementary mating type suggest that, for one to two fissions after autogamy, O cells produce some factor that determines the gametic nucleus (micronucleus) as mating type O. Direct evidence for the production of O-determining factor by the young O macronucleus was obtained by transplanting young O macronuclear karyoplasm (a part of the macronucleus) into E cells: 32-35% of E exautogamous clones transformed to O; transformation of E exautogamous clones to O reached as high as 72% by transfer of young O macronuclear karyoplasm from a conjugant, 3-4 hr after mixing. This indicates that O determinants produced by the O macronucleus can also act during the sensitive period of development of the new macronucleus. These O-determining factors may be produced or activated at the sexual stage and then decrease in activity in subsequent fissions after new macronuclear reorganization.

A STUDY OF ODD MATING-TYPE DETERMINATION FACTOR BY TRANSFER OF MACRONUCLEAR KARYOPLASM IN PARAMECIUM TETRAURELIA