374 



( II Vl'TF.R 29 



FIGURE 29-4. Simplified representation of mi- 

 cronuclear 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 re- 

 maining nucleus divides once mitotically ( D ) . 

 The conjugants exchange one of the haploid 

 mitotic products (E), after which fusion of 

 haploid nuclei occurs (E) so that each of the 

 conjugants, which later separate, contains a 

 single diploid micronucleus. 



Paramecium 8 



Kappa particles (and the similar lambda or 

 mate-killer particles) are located in the cyto- 

 plasm of certain strains of the protozoan 

 Paramecium. Hundreds of kappa particles 

 can easily be seen in a single cell (Figure 

 29-3). They contain DNA (and very prob- 

 ably RNA) and are self-reproducing. Indi- 

 viduals containing kappa are called killers, 

 since animal-free fluid obtained from cultures 

 of killer paramecia will kill sensitive (kappa- 

 tree ) individuals. 



'■ The following discussion of Paramecium is based 

 mainly upon the work of T. M. Sonneborn and 

 co-workers. 



Mutant kappa particles are known to pro- 

 duce modified poisons. Kappa is liberated 

 into the medium once it develops a highly 

 retractile granule which sometimes appears 

 as a "bright spot"" under the microscope. 

 One "bright spot" kappa particle is enough 

 to kill a sensitive individual. Kappa has a 

 specific relationship to its host, in that a par- 

 ticular dominant host gene (K) must be 

 present for kappa to maintain itself, that is. 

 reproduce. Killer individuals homozygous 

 for the recessive host allele (A ) cannot main- 

 tain kappa, and after 8 to 15 divisions, kappa 

 particles are lost and sensitive individuals 

 result. Because it is infective and not typi- 

 cally found in paramecia, kappa seems to be 

 a foreign organism of some kind. Since 

 lambda can be grown in vitro, lambda and 

 kappa are considered to be bacterial endo- 

 symbiotes of Paramecium. 7 



Although the cytoplasmic bacterial endo- 

 symbiote kappa can be transmitted from one 

 generation of Paramecium to the next, its 

 distribution to the next generation depends 

 upon the mechanism by which the new gen- 

 eration is initiated. Two such mechanisms 

 — asexual and sexual — are described briefly 

 with special reference to kappa-transmission. 



A typical Paramecium contains a diploid 

 micronucleus and a highly polyploid (about 

 1000N) macronucleus (or megamaleus) . 

 When the parent divides asexually by fission, 

 two daughter paramecia are produced. Both 

 micronucleus and macronucleus replicate and 

 separate; when fission is completed, both 

 daughter cells are chromosomally identical to 

 each other and to their parent cell. Although 

 the cytoplasmic contents are not equally ap- 

 portioned to the daughters, a killer parent 

 will normally produce two killer daughters 

 since each receives some of the hundreds of 

 kappa particles present in the parental cyto- 

 plasm. Successive fissions by the killer 

 daughters will produce a clone of chromo- 



7 See W. J. van Wagtendonk, J. O. D. Clark, and 

 G. A. Godoy (1963). 



