Extranuclear Genes 



375 



somally identical killer individuals. Simi- 

 larly, successive fissions of a sensitive Para- 

 mecium will produce a clone of sensitive 

 individuals. 



A new generation can also be formed sex- 

 ually. All members of a clone are of the 

 same mating type. When different mating- 

 type clones are mixed, a mating reaction 

 occurs which involves individuals of different 

 mating types sticking together to form larger 

 and larger clumps of paramecia. After this 

 clumping, pairs — each member a different 

 mating type — undergo conjugation. During 

 conjugation (Figure 29-4) the micronucleus 

 of each mate undergoes meiosis to produce 

 four haploid products, three of which subse- 

 quently disintegrate. The remaining nucleus 

 divides mitotically to produce two haploid 

 nuclei. Next, one of the two haploid nuclei 

 in each conjugant migrates into the other 

 conjugant where it joins the nonmotile hap- 

 loid nucleus to form a single diploid nucleus 

 in each conjugant. The macronucleus dis- 

 integrates during conjugation. 



After conjugation the two paramecia sepa- 

 rate and produce the exconjugants of the 

 next generation. Since each conjugant con- 

 tributes an identical haploid nucleus to each 

 fertilization micronucleus, both exconjugants 

 are chromosomally identical — as can be 

 proved by employing various marker genes. 

 (When the conjugants are homozygous for 

 different alleles, the exconjugants are identi- 

 cal heterozygotes. ) The diploid micronucleus 

 in each exconjugant divides once mitotically; 

 one product forms a new macronucleus, 

 while the other remains as the micronucleus. 

 Since all conjugants happen to be resistant 

 to killer action, we can study the consequence 

 upon kappa-transmission of mating a killer 

 with a sensitive individual. The cytoplasmic 

 interiors of conjugants are normally kept 

 apart by a boundary probably penetrated 

 only by the migrant haploid nuclei so that 

 little or no cytoplasm is exchanged. Conse- 

 quently, the exconjugants have the same 



kappa-condition as the conjugants; namely, 

 one is a killer and one is a sensitive indi- 

 vidual. Under special conditions, however, 

 a wide bridge forms between the conjugants 

 allowing the cytoplasmic contents of both 

 mates to flow and mix (Figure 29-5). The 

 extent of the cytoplasmic mixing can be con- 

 trolled experimentally. When cytoplasmic 

 mixing between killer and sensitive conju- 

 gants is sufficiently extensive, kappa particles 

 flow into the sensitive conjugant and both 

 exconjugants are killers. 



Consider how specific nuclear genes are 

 distributed in conjugation. If each conjugant 

 is a micronuclear heterozygote, Aa, which 

 one of the four haploid nuclei produced by 

 meiosis — A , A , a, or a — will survive depends 

 on chance. Accordingly, whether the cyto- 

 plasms of the conjugants mix or not, both 

 exconjugants will be A A 25% of the time, 

 Aa 50% of the time, and aa 25% of the 

 time. Note again that both exconjugants 

 are identical with respect to micronuclear 

 genes, and that both will give rise to clones 

 phenotypically identical with respect to the 

 micronuclear gene-determined trait under 

 consideration. When dealing with a trait 

 determined by a cytoplasmic particle like 

 kappa, however, the result can be different. 

 In this particular example, the cross of a 



figure 29-5. Silhouettes of conjugating Para- 

 mecium. A. Normal, no cytoplasmic mixing. 

 B. Wide bridge, permitting cytoplasmic mix- 

 ing. 



