268 PROTOZOA 



Viable exconjugants from a cross in variety 3 were also obtained 

 by Gruchy, but I cannot find information as to whether the 

 clones crossed were from the same or different populations. Some 

 comment is in order on the meaning of their statements about 

 obtaining viable conjugants. Apparently this is not intended to 

 mean that all or even a large proportion of the exconjugant clones 

 lived, only that some did. This is inferred because Elliott and 

 Hayes (1953) reported that variety 1 exconjugants were viable, 

 while Nanney and Caughey (1953) and Nanney, Caughey, and 

 Tefankjian ( 1955 ) emphasize that crosses of the same strains 

 as those used by Elliott and Hayes yielded a high frequency of 

 abnormalities and few viable clones. In the absence of detailed 

 mortality data on the other varieties, it is difficult or impossible 

 to know the extent to which different natural populations are 

 isolated by genetic barriers to gene flow operating through mor- 

 tality in the Fl and later generations. 



Ray's ( 1955, 1956 ) observations on the cytology of conjugation 

 in varieties 1 and 9, however, are very suggestive. In both va- 

 rieties, he found chromosomal abnormalities at the first meiotic 

 division. They were abundant in variety 9, which gave no viable 

 progeny, and less common in variety 1, which gave some viable 

 progeny. Among the abnormalities noted were clumping of chro- 

 mosomes, lagging on the spindle, multivalent associations, and 

 chromosome bridges at anaphase. The last two abnormalities of 

 course indicate that the strains were structural heterozygotes. 

 These abnormalities were observed when strains from nature 

 conjugated. The natural strains thus might have been hybrids 

 themselves, but another interpretation to be given in the next 

 paragraph is not excluded. Whether the strains being crossed, 

 which were from different sources, also had different chromo- 

 somal arrangements remains unknown. 



The interpretation of the preceding observations is rendered 

 difficult if we keep in mind the complications that have been 

 exposed in the fuller studies on other Ciliates. In both P. aurelia 

 and P. bursaria age is a most important factor influencing sur- 

 vival or death after conjugation. Moreover, chromosomal aberra- 

 tions are induced by age in P. aurelia, and further study may 



