222 SEX IN MICROORGANISAIS 



reported two mating types and conjugation among four races of 

 P. calkinsi, but cytological details were not given. No accounts of 

 conjugation in F. poly cary 21m or of P. nuoodruffi have been found. 

 Gelei (1938) stated that conjugation occurs in Parameciwn nephrid- 

 iatum but gave no details. 



Paramecium "Hybrids" 



Various attempts have been made to hybridize different species 

 of ciliates. In view of the existence of mating types in Paramecium, 

 one would hardly expect successful hybridization to take place. Miil- 

 ler (1932) described conjugation between P. caudatum and P. mul- 

 timicromicleatwn, but none of the exconjugants survived. This was 

 also true of the crosses between P. caiidatum and P. aurelia obtained 

 by DeGaris (1935). Considering that P. caudatuvi has a different 

 type of micronucleus from those of P. aurelia and P. imdtiniicromi- 

 cleatum, these results are not surprising, although autogamy (cy- 

 togamy) or parthenogenesis might take place in such cross-species 

 attachments. It will be recalled that Chen (1946b) found that crosses 

 between two varieties of P. bursaria were also lethal. 



Nuclear Reorganization Other than 

 Conjugation in Paramecium 



''''Endojnixis''^ 



Woodruff was able to maintain a strain of P. aurelia for many 

 years and for thousands of generations by the daily isolation method 

 without conjugation occurring unless induced in side lines. However, 

 cell division rate underwent a decrease or "depression" about every 

 30 days. Erdmann studied the nuclear phenomena during these de- 

 pression periods and found that the old macronucleus broke up into 

 fragments and a new nuclear apparatus developed from products of 

 micronuclear divisions. There are some variations in the process but, 

 as described by Woodruff and Erdmann (1914), the main events are 

 as follows. AVhile the macronucleus is breaking up, two micronuclear 

 divisions produce eight small nuclei. Of these, all degenerate except 

 one or two. A cell division produces two animals, each with one mi- 

 cronucleus. Two micronuclear divisions give rise to four small nuclei, 

 two of which become macronuclear anlagen and the other two remain 

 as micronuclei. A cell division, accompanied by micronuclear division, 

 segregates the two anlagen into different cells, thus restoring the 

 normal complement of one macro- and two micronuclei. These 



