514 Heredity in Protozoa 



The micronucleus in conjugation 



The behavior of the micronucleus and its derivatives must be con- 

 sidered in relation to the potential genetic effects of conjugation. For 

 instance, it is often assumed that the two gametic nuclei in a conjugant 

 are derived from the same parental haploid nucleus. If this is the case, 

 the nuclear contributions of a heterozygous conjugant to the two zygotic 

 nuclei of the conjugating pair would be identical. So far as cytological 

 evidence goes, this is not necessarily true in Paramecium aurelia because 

 "two to five products of the second division continue to divide" (11), 

 and thus produce a number of potential gametic nuclei. Therefore, it is 

 possible that the two successful gametic nuclei of a heterozygous conju- 

 gant could originate from different nuclei and thus be genetically differ- 

 ent (Fig. 9. 1). In P. caudatinn also, a variable number of nuclei undergo 

 the third pregamic division to produce more than two potential gametic 

 nuclei, and both cross-fertilization and self-fertilization (cytogamy) are 

 believed to occur in conjugation (12). Two products of the second matu- 

 ration division normally undergo the third division in Euplotes, so that 

 there are four potential gametic nuclei (37, 94). Are the functional 

 gametic nuclei derived from one second-division nucleus or from two? 

 Genetic data indicate that both methods of origin occur in Euplotes (8). 



Behavior of mating types in conjugation 



The effects of conjugation on mating types apparently vary with 

 the species and the variety of ciliates. In variety I of Paramecium bursaria 

 (31) the descendants of each pair of conjugants have belonged to the 

 same mating type in most cases. The few exceptional pairs show various 

 results. In some cases, the two exconjugants may produce clones of dif- 

 ferent mating types. In other pairings, a single exconjugant has produced 

 two different mating types. In some cases, these two types were parental 

 types; in others, they were not. Five crosses, of the six possible for the 

 four types in variety I, have produced descendants belonging to all four 

 mating types. Jennings concluded that mating types are controlled by the 

 genetic composition of the synkaryon and that the appearance of non- 

 parental types might represent new nuclear combinations. Chen (6) has 

 suggested that inheritance of mating types in P. bursaria is probably in- 

 dependent of the chromosomes count, which may vary as much among 

 races within the same mating type as it does among different types. 



Inheritance of mating types in group A of P. aurelia (81, 86) — types I 

 and II (variety 1), V and VI (variety 3) and IX and X (variety 5) — may 

 be illustrated by crosses between types I and II (74). Three results are 

 possible (Fig. 9. 2). All of the exconjugant lines may be type I; all may 

 be type II; or an individual exconjugant may differentiate into types I 

 and II, usually at the first exconjugant fission. The third type of result 



