Figure 1.

Nuclear events during conjugation of Tetrahymena thermophila. Conjugation in T. thermophila is a complicated process that is initiated by interaction between complementary mating types, which form a conjugating pair. A. Vegetative phase. B. Meiotic prophase. C. Meiosis. D. Nuclear exchange. One of four meiotic products mitotically divides, forming two pronucei. Subsequently, one of the pronuclei is reciprocally exchanged between mating partners. E. Fertilization (synkaryon formation). F. PZD (postzygotic division). Fertilized nucleus successively divides twice. G. Mac I. Anteriorly-located nuclei differentiate into the new macronuclei, while posterior nuclei remain the micronuclei. H. Mac IIp. The parental macronucleus migrates posteriorly and begins to degenerate. I. Mac IIe. Pair separates (exconjugants). J. Mac III. One of two micronuclei is eliminated. Progeny of T. thermophila do not undergo conjugation during the first ~100 vegetative fissions after conjugation called "immature." A: macronuclear anlagen. m: micronuclei. pMa: parental macronucleus. The scale bar in photograph indicates 10 μm.