Dimensions of control of cortical patterns in Euplotes: The role of pre‐existing structure, the clonal life cycle, and the genotype

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Abstract
Within each of three investigated strains of Euplotes minuta (autogamous — A23; non‐autogamous — K7, VF17) variation was observed in the number of ciliary rows commonly manifested. Subcloning of two of these strains (A23, K7) revealed a strong tendency toward vegetative perpetuation of the preexisting row number; differences within genetically homogeneous material could persist over 200 fissions of serial propagation. However, initially different subclones tended gradually to converge toward a common distribution of row number. The rate of convergence depended on the strain and on the degree of senility.Non‐autogamous strains of E. minuta became senile at a clonal age of 500 to 700 fissions. Senility was manifested by reduced fission rates and by low survival after conjugation. Senile lines showed a reduced average ciliary row number, an increased variation in the row numbers commonly found, and a reduced fidelity in the vegetative propagation of preexisting row number. Initiation of new life cycles by conjugation reversed all of these effects.Young (non‐senile) clones displayed a characteristic range of ciliary row numbers, which remained the same following successive initiations of new clonal life cycles. This strain‐specific “stability range” was eight to nine rows for non‐autogamous strains (K7, VF17, and progeny of crosses between them) and nine to ten rows for the autogamous stock A23. The appearance of progeny lines with substantial numbers of ten‐rowed cells following a backcross of the A23 X VF17 F1 to the A23 parent (Heckmann and Frankel, '68) could therefore only have been due to genie contributions from the A23 parent. Reanalysis of the earlier data combined with results of the present study strongly suggest that the difference in stability range between the autogamous and non‐autogamous strains has a polygenic basis.
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