370 



( II M'TER 29 



eluded from a nucleus destined to be in a 

 sperm but not from one destined to be in an 



egg, although it seems much more reasonable 

 to attribute the nontransmission of C'(). -sen- 

 sitivity through the sperm to the rather 

 minute amount of cytoplasm in a sperm as 

 compared with the amount in an egg. It is 

 therefore highly probable that CO_.-sensitivity 

 in Drosophila is due to the presence of a 

 particle called sigma. Other studies show 

 that sigma contains DNA, 1 is mutable, and 

 has many of the characteristics of a virus 

 including infectivity by experimental means. - 

 Since sigma is not visible, its location within 

 the cell remains somewhat of a mystery. 

 Certain sigma and episome characteristics 

 are similar. (Melanotic tumor incidence in 

 Drosophila may also depend upon the pres- 

 ence of an episome-like particle. :; ) 



Consider another trait of Drosophila (p. 

 1 10) — females mated to normal males giving 

 rise almost entirely to females. This trait 

 has a genetic basis; is not transmitted by 

 males; is infective; is not linked to the usual 

 chromosomes; and proves to be intimately 

 associated with the presence of a spirochaete 

 in the blood. 



Maize 



None of the examples just mentioned demon- 

 strates conclusively the existence of both in- 

 tracellular and extranuclear genes. They do 

 serve to illustrate, however, possible results 

 of a search for such genes which starts with 

 a study of genetic recombination. The de- 

 sirability of making a direct correlation be- 

 tween potentially extranuclear genes and ob- 

 jects observable in the cytoplasm is clear. 



Continuing the search for extranuclear 

 genes, let us restrict our attention to cyto- 

 plasmic components which seem to be nor- 



1 See N. Plus (1963). 



Much work on sigma has been done by P. L'He- 

 ritier, G. Teissier, and co-workers. 

 3 See C. Barigozzi (1963). 



mal constituents of present-day cells, disre- 

 garding their normality when they or their 

 precursors first arose. 



Many plant cells contain cytoplasmic bod- 

 ies called plastids. Green plastids (due to 

 chlorophyll) are called chloroplasts; white 

 plastids are called leucoplasts. Immature 

 plastids are small and colorless. In the ab- 

 sence of sunlight, chloroplasts lose their pig- 

 ment and become leucoplasts; the process is 

 reversed when the plastids are again exposed 

 to sunlight. 



In corn, mutants of chromosomal genes 

 can interfere with the sequence of reactions 

 leading to the manufacture of chlorophyll. 

 One such nuclear gene prevents plastids from 

 producing any chlorophyll at all, so that a 

 type of leucoplast incapable of becoming 

 green occurs. A seedling that possesses the 

 appropriate mutant nuclear genotype will not 

 be green; will grow only until it exhausts the 

 food supply in the seed; will die because 

 photosynthesis of sugar cannot occur in the 

 absence of chlorophyll. Nuclear genes that 

 produce albino seedlings act, therefore, as 

 lethals. 



Certain corn plants have mosaic leaves, 

 with stripes of green and white (Figure 

 29-1 ). 4 Although the leucoplasts of the 

 white parts are incapable of becoming green, 

 the white parts survive by receiving nourish- 

 ment from the green parts. Is this mosai- 

 cism based upon nuclear genes causing differ- 

 ent portions of the leaf to follow different 

 paths of development? Were striping due to 

 a nuclear gene acting upon differentiation, 

 such a gene would have to be transmitted 

 through the male or female gamete inde- 

 pendent of the whiteness or greenness of the 

 tissue giving rise to the reproductive struc- 

 tures. 



Sometimes an ear of corn is derived from 

 an ovary that is expected to be mosaic be- 



4 The following account is based primarily upon 

 work ot M. M. Rhoades. 



