AbstractThe life styles of ciliated protists are particularly suitable for experimental analyses of certain aspects of developmental and genetic biology. The progression from sexual immaturity tomaturity to senescence represents one of the most intriguing aspects of developmental programs. The extent to which progeny clones, their subclones, and testers used in the assay result in different lengths of immaturity has been investigated inEuplotes crassus.Six subclones from each of 12 progeny clones from a cross between stocks EC1 and EC2 were tested for maturity with stocks EC3, EC4, and EC5 on every transfer. Analysis of variance was used to partition thetotal variation in fissions to maturity into parts due to clones, subclones, and testers and the interactions between these levels. The error, interaction of subclones and testers, corresponds to a standard deviation of only 4.1 fissions, while the within clone within tester means range from 15.2 to 46.7 fissions; all levels except testers contribute significantly to the total variation. Most of the variability is attributable to clones (66%), the next most to error (16%), the next most to interaction of clones by testers (13%), and the least to subclones (5%). An a posteriori analysis examined whether the differences among clones were due to the cytoplasm of the clone ancestor (exconjugant), itsmat(mating‐type) locus genotype, or the mated pair it came from. None of these characteristics was able to interpret simply the large variability among clones. These results provide evidence that the transition from immaturity to maturity is quantitative and complex rather than a jump from one well‐defined state to another. © 1992 Wiley‐Lis