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CHAPTER 45 



FIGURE 45-4. Simplified representation of micronuclear events occurring during 

 conjugation in Paramecium. Each conjugant has a single diploid micronucleus {A), 

 which following meiosis produces four haploid nuclei {B) . Three of these disintegrate 

 (C), and the remaining nucleus divides once mitotically (D). The conjugants ex- 

 change one of the haploid mitotic products (E), after which fusion of haploid nuclei 

 occurs (F) so that each of the conjugants, which later separate, contains a single 

 diploid micronucleus. 



fission also, etc., a clone of chromosomally 

 identical killer individuals will be produced. 

 Successive fissions of a sensitive Paramecium 

 produce, naturally, a clone of sensitive indi- 

 viduals. 



A second process for forming new genera- 

 tions is sexual. The members of a clone are 

 all found to be of the same mating type. But 

 when clones of different mating type are 

 mixed together, there is a mating reaction. 

 This involves the sticking together of indi- 

 viduals of diff'erent mating types so that 

 larger and larger clumps of paramecia are 

 formed. This is followed by conjugation by 



pairs, each member of a pair being of a 

 diff'erent mating type. During conjugation 

 (Figure 45-4), the micronucleus of each mate 

 undergoes meiosis, at the conclusion of 

 which, three of the four haploid nuclei pro- 

 duced disintegrate. The remaining nucleus 

 divides mitotically once, producing two hap- 

 loid nuclei. Then one of the two haploid 

 nuclei in each mate migrates into the other 

 mate and there joins the nonmotile haploid 

 nucleus to form a single diploid nucleus in 

 each conjugant. During conjugation the 

 macronucleus disintegrates. 

 After conjugation the two paramecia sepa- 



