204 PROTOZOA 



tion. Second, they serve to initiate new life cycles. In the absence 

 of sexual processes, paramecia grow old and either die or become 

 genetically extinct by losing their germ plasm ( micronuclei ) . 

 Survival depends upon the occurrence of fertilization before 

 senility has gone too far. The fertilized animals are reinvigorated 

 and start new, young clones. 



Different sexual processes may occur in different stages of the 

 life cycle when food is depleted. This sequence of responses to 

 famine is a clue to the relative values and functions of the differ- 

 ent sexual processes. Invariably, the response of ripeness for con- 

 jugation precedes the response of autogamy when both occur in 

 the life cycle. In varieties that undergo selfing instead of autog- 

 amy, ripeness for cross conjugation precedes ripeness for selfing. 

 Cross conjugation thus has priority over selfing, and conjugation 

 has priority over autogamy. These invariable priorities indicate 

 the judgment of natural selection as to which sexual processes 

 best fulfill their functions. Since all are equally operative in initi- 

 ating new life cycles, the advantage of conjugation must lie in its 

 greater effectiveness in bringing about genetic recombination. 



A priori it might have seemed possible to question this verdict 

 of natural selection. Autogamy results in conditions approaching 

 those associated with haploidy: there are no hidden recessives 

 masked by dominant alleles; all loci are homozygous in the one 

 case and hemizygous in the other. Selfing also approaches the 

 same condition, but less efficiently. Why then would not autog- 

 amy or even selfing work as well as haploidy? The answer lies 

 in the decisive differences from the conditions in haploids. In a 

 diploid with autogamy, the clone is the unit of expression of a 

 genotype, not the individual animal. This results in a considerable 

 lapse between the occurrence and expression of a mutation. More 

 important, the number of units or clones in a population must 

 be exceedingly small in comparison with the number of units or 

 individuals in the usual successful haploid organism. Hence, 

 neither selfing nor autogamy in Paramecium can be as efficient 

 as haploidy in large and rapidly produced populations. To achieve 

 the necessary genetic variability, natural selection has in P. 

 aurelia given first priority to cross conjugation, which however 



