288 RADIATION HIOLOGY 



S('h()enl)oni (1949) reports the median lethal dose for Astasia longa to be 

 between 20,000 and 40,000 r. 



There have been a iiunil)er of investigations of the killing of parasitic 

 protozoa by radiation; and in most of these the criterion was the abiUty to 

 infect the host after irradiation in vitro. Thns chromosomal changes may 

 have been invohed. Ilaberstaedter (1938) fonnd that a dose of 12,000 r 

 of X rays to Trypanosoma gatnbiense, in vitro, was sufficient to prevent 

 the infection of mice, whereas doses of more than 100, 000 )• were necessary 

 to produce any obvious changes in the motility of the trypanosomes. 

 Other investigations with parasitic protozoa are summarized in Table 8-1. 



There are a few cases of delayed death in which a genetic explanation is 

 more difficult. Holweck and Lacassagne's case (1931a, b) with Polytoma 

 in which death occurred after division has alicady been mentioned, and 

 the interpretation has been suggested that chromosomal aberrations were 

 involved. Using a particles, they distinguished other kinds of effects 

 as follows: (1) the cells remain motile and grow in size but finally lyse 

 without division; (2) the cells are immobilized but grow, yet they finally 

 lyse without division; (3) the cells become immobile, fail to grow, and 

 lyse without division. These workers (see also Lacassagne, 1934a, b; 

 Holweck, 1934) made sensitive volume calculations for the first and the 

 last two effects and found for the first a volume approximately the equiva- 

 lent of the centrosome, whereas for the last two, the volume was approxi- 

 mately that of the kinetosomes. These rather remarkable identifications 

 presumably would offer an explanation for the failure to divide and for the 

 immobilization, but would not in themselves account for the eventual 

 death. Holweck and Lacassagne (1931b) suggest that death is due to the 

 suppression of reproduction and of motihty. However, Polytorna cells are 

 probably haploid and so lethal mutational changes could be invoked as an 

 explanation on the assumption that a mutation can express itself in the 

 cell in which it arises. Nevertheless, the possibility must be borne in 

 mind that this is a case of delayed death due to causes other than muta- 

 tional change. 



In the ciliate protozoan, Paramecium, a nongenetic explanation for 

 delayed death appears by far the most likely one. Sonneborn (1947) has 

 shown that the macronucleus in this ciliate contains many sets of genes. 

 It is therefore improbable that loss of chromosomal material or gene 

 mutation in either the macronucleus or micronucleus would have much 

 effect on ciliates multiplying vegetatively. In order to allow for such an 

 effect the unlikely assumption would have to be made that a given muta- 

 tion or deficient chromosome is dominant over many normal genes or 

 chromosomes. Alternatively, the radiation would have to be assumed 

 to cause sufficiently extensive damage to the chromosomes to lead to 

 death even with many sets. 



For this reason, Paramecium and other ciliates might be expected to be 



