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CHAPTER 29 



sensitive individual with a killer produces 

 exconjugants whose type depends upon the 

 occurrence or nonoccurrence of cytoplasmic 

 mixing. 



Kappa has special significance because ii 

 shows how a symbiotic microorganism can 

 become so well adapted to its host, that it 

 becomes part o\' the host's genetic system 

 and determines some of the host's traits. 

 1 ike kappa, the rickettsial organism causing 

 Rocky Mountain spotted fever is visible and 

 transmitted through the cytoplasm of carrier 

 cells. These rickettsiae, as well as sigma 

 and the spirochaete already discussed, also 

 determine certain traits of their hosts. Even 

 though each of these organisms seems to be 

 foreign to its host, we cannot be sure whether 

 it was originally a parasite or symbiont. 

 Could some of the now-foreign organisms 

 located intracellularly have been originally 

 part of the normal gene content of a cell? 



This question is particularly pertinent 

 when viruses are considered. From what 

 has been discussed in previous chapters, it is 

 clear that all viruses cannot be classified as 

 either being or arising from foreign infective 

 agents. Present-day virulent phages seem to 

 be acting as foreign organisms when they 

 lyse their bacterial hosts. But, the lytic ca- 

 pacity of a phage depends upon both its 

 genotype and its host's, and, under some 

 genotypic conditions, lysis is quite rare. 

 Determining the normality or abnormality of 

 present-day viruses is even more difficult 

 when temperate phages are considered; not 

 only are they less lytic (yet capable of trans- 

 duction), but the very genes characterizing 

 their prophages seem to be associated with 

 part of a normal bacterial chromosome. As 

 more is learned about viruses, and phage in 

 particular, our understanding of what is ge- 

 netically "normal." and what is "foreign," 

 will undoubtedly undergo drastic revision. v 

 As knowledge of the genetics of viruses and 



• Sec A. Campbell (1961). 



their "hosts" increases, we will be in a better 

 position to postulate how they originated. 



Chlamydomonas 



Chlamydomonas reinhardi is a unicellular 

 plant with two flagella and a single chloro- 

 plast. By means of mitotic cell division, it 

 can reproduce ascxually to produce clones. 

 No sexual reproduction is observed between 

 members of a clone, but if members of two 

 different clones are mixed together, individ- 

 uals from different clones may pair, fuse, and 

 produce zygotes. After two divisions, the 

 zygote produces four cells, each of which can 

 be isolated to give rise to separate clones. 

 When a sample from each of the four clones 

 is mixed with different portions of a fifth 

 clone, two of the four show mating (and are 

 called sexual type + ) and two do not (being, 

 therefore, of — sex). Moreover, when por- 

 tions of the four sibling cultures under test 

 are combined in pairs, we find that individu- 

 als of any -f- culture can mate with individ- 

 uals in any — culture. Combinations of 

 two + or two — cultures, however, show 

 no mating. No morphological difference 

 between -f and — individuals has been 

 detected. 



Among the first four cells produced from 

 a zygote, two are + and two are — . This 

 outcome suggests that the zygote is diploid; 

 that it carries a pair of genes for sex (which 

 we can call mt+ mt~); and that meiosis oc- 

 curs in the next two divisions. As a conse- 

 quence of this meiosis, a 1 : 1 ratio of mt + : 

 ml ~ is found among the haploid products. 

 Mating type, therefore, behaves like a trait 

 based upon two different alleles of a single 

 nuclear gene. 



The wild-type Chlamydomonas is sensi- 

 tive to streptomycin. After exposure to 

 streptomycin, a number of preadaptive mu- 

 tants are found which make the individual 

 resistant to 100 p.g streptomycin per vial, al- 

 though they are still sensitive to 300 /*g per 

 vial. Since crosses with a streptomycin- 



