510 Heredity in Protozoa 



various dyes by exposing the ciliates to gradually increasing concentra- 

 tions. The developed resistance was specific except for some reciprocal 

 effects of arsenic and antimony compounds. Similar results have been 

 obtained in P. aurelia and P. caudatiim by a combination of selection and 

 acclimatization (33). For example, a strain was grown in a non-lethal 

 concentration of an arsenical and then subjected to a dosage lethal for 

 most of the ciliates. The survivors were returned to a non-lethal arsenic 

 medium for a time before heavy dosage was repeated. As the procedure 

 was continued, the strain became progressively more resistant. Resistance 

 was inherited for long periods after a return to normal culture media. 

 Such modifications — although inherited through hundreds of fissions, 

 through endomixis (or autogamy?), and in rare cases through conjuga- 

 tion — eventually disappeared after removal of the stimulus. Accordingly, 

 Jollos (33, 34) called such changes "Dauermodifikationen," distinguish- 

 ing them from true mutations. The more recent acclimatization of both 

 amicronucleate and normal strains of Colpoda steinii to arsenicals indi- 

 cates that the micronucleus is not necessarily involved in "Dauermodifi- 

 kationen" (71). 



Comparable acclimatization has been reported in Bodo caudatus, 

 strains of which developed a tolerance to acriflavine in concentrations of 

 1:500, as compared with the normal susceptibility to dilutions of 1:50,000 

 to 1:10,000. This resistance was inherited, in decreasing degree, for at 

 least a year in drug-free media (68). 



Morphological modifications have been reported in several cases. Loss 

 of the kinetoplast, induced in Trypanosoma brucei by inoculating in- 

 fected mice with certain dyes, became an apparently fixed characteristic 

 (98). Loss of the parabasal body also was induced in Bodo caudatus by 

 treatment with acriflavine, but no permanently abnormal strain was ob- 

 tained (68). Various structural changes have been reported in Chlamy- 

 domonas debaryana (49). One type could be transformed into another by 

 maintenance in an appropriate medium for a period varying with the 

 length of time the original strain had been exposed to the conditions 

 which produced it. In view of these findings, Moewus suggested that many 

 of the varieties found in natural populations are merely "Dauermodifika- 

 tionen" induced by specific environmental conditions. 



Morphologically distinct types of Chilodonella uncinatus have been in- 

 duced by ultraviolet irradiation. These changes, believed to be mutations, 

 persisted through fission and conjugation (44, 45). Likewise, a physio- 

 logical change, expressed as a lowered fission rate, has been induced in 

 P. aurelia by treatment with X-rays (40). Homozygous strains were ob- 

 tained in autogamy, and the abnormality was transmitted through both 

 exconjugants in matings between normal and abnormal clones. This in- 

 duced change was attributed to a micronuclear mutation (41). 



Aside from the rare cases which may have involved true mutations. 



