164 I The Process of Evolution 



made. Spontaneous breakage of the transferring chromosome also 

 occurs. Hybrids of E. coli and Shigella dysenteriae, as well as of 

 E. coli and Salmonella typhimurium, have also been studied. 



Microbial Genetics and Evolution 



The occurrence and significance of such phenomena as transforma- 

 tion, transduction, and sexual recombination in many microorganisms 

 in nature are unknown. The extent to which these processes are 

 distributed among microorganisms also is largely unknown. It is 

 obvious that population genetics of these forms is apt to be rather 

 different from that known in larger organisms. The possibility of 

 some sort of "genetic" relationship between very small replicating 

 systems and larger organisms is only beginning to be studied. Most of 

 what has been observed in this category has been termed infection. 

 At this level, of course, the distinction between genetics, infection, 

 and development breaks down. Many viruses, particularly the RNA 

 viruses of plants (which may be transmitted by sucking insects), 

 produce morphological effects similar to those controlled by chro- 

 mosomal genetic material. 



Some strains of Parameciiwi are known as killer strains, since 

 they secrete into the medium in which they grow a substance poison- 

 ous to sensitive strains of Paramecium. It has been found that killer- 

 strain individuals contain particles of DNA called kappa particles. 

 In order for kappa particles to be maintained in the cell, the geno- 

 type of the strain must contain a dominant nuclear factor K. In the 

 course of asexual fission the kappa particles reproduce in such a way 

 that all members of the resulting clone contain the particles. Should 

 the rate of cell reproduction exceed that of particle reproduction, 

 KK individuals lacking kappa may appear. Such individuals are not 

 killers, but sensitives, and can regain kappa particles only by con- 

 tamination from a cellfree suspension of killer animals. They are 

 not able to initiate the formation of particles despite their chromo- 

 somal genotype. 



Sexual reproduction in Paramecium involves conjugation of cells 

 and cytoplasmic exchange, during which kappa particles may or 

 may not be transferred. In the cross between KK and kk individuals, 

 the resulting Kk paramecia will be killers if conjugation has lasted 

 long enough that kappa particles are transferred. Paramecia that 

 are homozygous for the recessive factor k may inherit kappa par- 

 ticles in the cytoplasm, but these are lost during subsequent genera- 

 tions of fission. 



It is obvious that there is a resemblance between this situation in 



