HEREDITY IN SOMATIC CELLS 371 



most suitable conditions of the genetic material for multiplication in an 

 environment in which the normal genotype is at a selective disadvantage. 

 Such selections of favorable genetic constitutions have been demon- 

 strated to take place in fungal heterokaryons where new environments 

 bring about changes in the ratios of pre-existing nuclei, allowing the 

 organism to adapt. But such changes do not provide a useful model 

 for somatic cell variation, except insofar as the vegetative fungal mass 

 may be considered a soma. Heterokaryons are unique; rarely is there 

 more than one nucleus per somatic cell in higher plants and animals. 

 Moreover, in fungi, heterokaryons may be established by the migration 

 of nuclei from other individuals. They also arise by mutation of one 

 of the several contained nuclei. When this occurs in a multinucleate 

 cell such as a bacterium, a heterokaryon is also formed, but the conse- 

 quence is different, as the number of nuclei is small and the process of 

 nuclear assortment in the daughter cells does not regularly bring together 

 the products of different nuclei. As shown in Figure 12.8, a segregation 

 of the mutant nuclei occurs after a number of generations determined 

 bv the number of nuclei per cell. The time of onset of increase in a 

 mutant clone is therebv determined, and the pattern of increase is a 

 function of whether the mutant gene dominates in the heterokaryotic 

 condition. Unicellular forms like bacteria provide better models than 

 fungi for the consequences of mutation and selection in populations of 

 somatic cells. What is known about their behavior has not been con- 



(^ ^ i ^ %) 



C^A) (Wlf) 



/ \ / \ 



(gig) (W^ (A~^ (V~X) 

 I i I \ 



I ' ' (#% !#%) 



Increase in mutant / \ 



clone begins at *" ' \ 



this division (A3) (y3) 



FIGURE 12.8. Diagram of the segregation of a mutant nucleus from its nonmutont 

 sisters in a heterokaryotic bacterial cell (from Ryan and Wainwright, 1954, J. Gen. 

 Microbiol., 11:364). 



