Chapter *29 

 EXTRANUCLEAR GENES 



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(he DNA rabbit poxvirus and 

 certain RNA viruses, which 

 are restricted to the cytoplasm 

 of cells normally possessing a definite nuclear 

 membrane (and hence a definite nucleus), 

 are clearly autonomous extranuclear genes. 

 To what extent do extranuclear genes occur 

 and what is their relationship with particular 

 chromosomal genes? 



Various operational tests — chemical, re- 

 combinational, mutational, phenotypical, and 

 replicative — can be applied to identify an 

 extranuclear component as genetic material. 



Recombination is the first operational 

 method used in our search for extranuclear 

 genes. To detect recombination, the extra- 

 nuclear gene must produce a recognizable 

 phenotypic effect. To provide the required 

 phenotypic alternatives, changes involving 

 either the kind or the quantity (or both) 

 of such a gene must occur. 



Drosophila 



How do we actually proceed to look for an 

 extranuclear gene in Drosophila? Starting 

 with different nonoverlapping phenotypic al- 

 ternatives which occur generation after gen- 

 eration under the same environmental con- 

 ditions, a series of crosses is made to test 

 whether the occurrence of the alternatives is 

 associated with the presence of one or more 

 particular chromosomes (X. Y, II, III, IV). 

 If it is, the phenotypic alternatives are prob- 

 ably due to some genetic factor linked to, and 

 hence located in, a chromosome. (Addi- 

 tional appropriate crosses and cytological 

 369 



studies will reveal the precise nature of the 

 nuclear gene change. Since the vast ma- 

 jority of carefully analyzed gene-based traits 

 are located in chromosomes, the search for 

 an extranuclear gene will usually be unsuc- 

 cessful. ) 



But consider the genetic alternatives for 

 resistance and susceptibility to C0 2 (gas). 

 Although wild-type Drosophila adults can be 

 exposed to pure CO L » for as long as 1 5 min- 

 utes and recover without apparent effect, flies 

 of other strains almost invariably are killed 

 by such exposure. Using marked chromo- 

 somes, COo-sensitivity is found to be un- 

 linked to any chromosome of the normal 

 genome. In fact, by appropriate crosses it 

 is possible to replace each of the chromo- 

 somes in the sensitive strain by a correspond- 

 ing chromosome of the resistant strain. 

 After this is done, the flies produced are 

 still sensitive to COo! Possibly, the sensitive 

 strain carries an additional nonhomologous 

 nuclear chromosome which it transmits inde- 

 pendent of the usual ones. In the progeny 

 of hybrids derived from sensitive and resist- 

 ant lines the COo-sensitivity trait does not 

 segregate, which indicates that if such a 

 supernumerary chromosome exists, it cannot 

 occur singly (in the individual hybrid for 

 sensitivity) or as a pair (in flies of the pure 

 sensitive strain). Although cytological 

 examination reveals no additional nuclear 

 chromosome, this finding is not a conclusive 

 argument against a nuclear locus for COo- 

 sensitivity, for, according to recombinational 

 evidence, "chromosomes" so small they es- 

 cape cytological detection are known to exist. 

 (A phenotypic change in corn is associated 

 with the presence of readily detectable, su- 

 pernumerary, heterochromatic, "B" chromo- 

 somes.) 



Although the sensitive female regularly 

 transmits CO L >-sensitivity to some progeny, 

 the sensitive male does so only under special 

 circumstances. It might be possible that a 

 nuclear gene for sensitivity is somehow ex- 



