The development of sexual maturity has been studied in Tetrahymena hegewischi. Progeny lines d o not typically change from immaturity to mating with all different mating types during a single test interval, but about 30% d o mature abruptly. Some testers are more likely than others to participate in the earliest mating reactions of progeny lines which d o not mature abruptly. Subcaryonidal vegetative pedigrees of 10 pairs from 4 crosses revealed considerable intrapair variation in the time, measured in fissions, of maturity. The average intrapair coefficient of variation was 20%. A nested ANOVA revealed significant genomic effects on the immaturity interval, but no significant cytoplasmic or caryonidal effects; 56% of the total variation was non-genomic. Growth in different environments had highly significant effects on the immaturity interval. Subclones grown at 27°C with alternate day transfers took on the average 2 to 3 times as many fissions to mature as sister subclones grown at 27°C with daily transfers. Subclones grown at 18°C or 34°C and transferred on alternate days had intermediate maturation times. The greatest range in the immaturity interval among lines of the same genotype was from 34 to 143 fissions. The development of maturity in this species involves genetic control of timing, but the genetic differences are obscured by a large amount of intraclonal variation and sensitivity to the environment.

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