90 Reproduction and Life-Cycles 



becomes attached near the aboral end of a macroconjiigant. Fusion then 

 occurs and the endoplasm of the microconjugant giadually flows into the 

 macroconjugant, leaving the pellicle behind. Pregamic divisions and for- 

 mation of a synkaryon then occur much as in other ciliates. 



Conjugation is often considered an orderly process which, once started, 

 goes through a fixed series of nuclear activities. This is not always the 

 case and variations are striking in several species. Furthermore, conjuga- 

 tion between particular strains of a species may be abnormal. For instance, 

 in conjugation of certain Russian strains (variety IV) with several Amer- 

 ican strains of P. biirsaria, the first pregamic division is usually not com- 

 pleted and all conjugants die before or after separation. The lethal effect 

 is produced after cytoplasmic fusion, but before the exchange of pro- 

 nuclei (57, 132). Mixtures of certain abnormal strains of P. hursaria 

 with normal strains undergo typical pairing, but separation occurs after 

 a few hours. The micronucleus enlarges slightly but does not start the 

 first pregamic division (56). Polyploidy seems to have arisen frequently in 

 P. hursaria, probably through the fusion of more than two pronuclei in 

 conjugation (52). Chromosomal variations also are produced by matings 

 between diploid and polypoid strains, as well as between micronucleate 

 and amicronucleate races. In the latter case, each exconjugant contains a 

 single haploid nucleus which undergoes three divisions and probably 

 produces a new nuclear apparatus (53). 



Nuclear behavior varies also in Parameciinn trichium (72, 73). Micro- 

 nuclei are sometimes transferred just after the second or even the first 

 pregamic division. Occasionally only one of the migratory pronuclei ac- 

 tually migrates, so that conjugants sometimes contain one and three pro- 

 nuclei. There also may be no exchange of pronuclei, with resulting 

 autogamy in each conjugant. After the second pregamic division, three 

 haploid nuclei sometimes degenerate and the fourth, without dividing 

 again, migrates into the other conjugant. Each exconjugant thus contains 

 a haploid nucleus which undergoes postzygotic divisions. Heteroploidy 

 occurs frequently in P. trichium and has been noted also in P. aiirelia and 

 P. caudatum (71). The exchange of macronuclear fragments has been ob- 

 served in P. trichium (72) — but not in other species of Paramecium — and 

 also in several species of Chilodonella (166, 167). 



Factors inducing conjugation 



The possible causes of conjugation have been discussed for many 

 years. Diverse ancestry was one of the prerequisites suggested by Maupas 

 (176) and the more recent discovery of mating types has proven that 

 apparently hereditary differentiation of potential conjugants does exist 

 in certain species. However, conjugation has been observed within single 

 clones, and also among the descendants of a single exconjugant after only 

 a few fissions. Some of these matings between closely related conjugants 



