312 RADIATION 1U()I-<JGY 



DippoU (1948) lias reported finding spontaneous mutations of kappa. 

 So far lit) reports of radiation-induced mutations of this entity have been 

 made. 



Lee Oi)4l)) lound that X inaxhatioii of I', hmsnria led to changes in 

 mating type wliich became more fre(iuent as the dose was increased. He 

 suggests that X rays may induce autogamy which may in tiun cause an 

 increased frequency of mating type change. However, as he points out, 

 tlie comicctioii between chang(> of mating type and autogamy has not been 

 fully established for this species. Thus some other pathway of action of 

 the X rays is possible. 



A series of reports on genetic changes in /^ aitrclia induced by 0, ultra- 

 violet, and X radiation and by nitrogen mustard ha\-e been made by 

 Geckler, Kimball, and Powers and their coworkers. The method used 

 by all these workers was basically the same and depended on the fact that 

 autogamy makes the animals completely homozygous. The paramecia 

 were exposed to the radiation, and a number of the exposed animals were 

 isolated. After a period of vegetative multiplication autogamy was 

 induced, and a number of autogamous animals were isolated from the 

 progeny of each treated animal. Each autogamous animal was allowed 

 to multiply for a period of several days and was then checked for survival 

 (Powers) or for survival and amount of growth (Kimball, Geckler). The 

 percentage surx^iving with normal growth can be taken as a measure of the 

 effect. 



When ionizing radiations were used, it was found that at doses much 

 too low to have immediately detectable effects either on survival or rate 

 of division of vegetative animals, many of the autogamous clones were not 

 viable or, if viable, divided more slowly than usual. The dose range used 

 has been from about 300 to about 20,000 r ; above the latter dose almost 

 all the exautogamous clones were affected. Such observations have been 

 reported by Kimball (19-l:9a, b) for /3 particles from P'^' outside the culture 

 medium and for X rays, by Powers (1948) for P'*- and for a mixture of Sr^^ 

 Sr^", and Y^" in the medium, and by Powers and Shefner (1948, 1950) for 

 X rays. Geckler (1950) reported similar findings for nitrogen mustard; 

 Powers and Shefner (1950) and Powers and Raper (1950) reported on 

 doses of X rays and nitrogen mustard which were sufficiently high to have 

 a distinct immediate effect on the animals in addition to the effect after 

 autogamy. Kimball and Gaither (1951) report that doses of 2650 A 

 ultra\iolet, which an^ sufficient to produce a detectable effect after autog- 

 amy, also cause a temporary retardation of the first few cell divisions 

 following irradiation. 



The simplest interpretation for effects which do not appear until after 

 autogamy is that they are due to gene mutations or chromosomal aberra- 

 tions in the micronuclei. Sonneborn has shown that autogamy results in 

 the formation of a completely homozygous synkaryon from which the new 



