Propagation of cortical differences in tetrahymena.

Progeny clones were derived from crosses arranged so that the number of ciliary meridians (corticotype) was unusually high in one partner, and normal in the other. An analysis of the propagation of corticotypes during maintenance of these clones for up to 1,000 fissions indicated that corticotypes above 21 undergo a rapid downward shift, while corticotypes in the range of 18 to 21 change slowly. Although these observations are consistent with Nanney's earlier deduction of a "stability center" at corticotype 19, there appears to be little if any difference in the stability of perpetuation of corticotypes 18, 19 and 20. Within this "stability range," the inertia of maintenance of pre-existing corticotypes is sufficiently strong that sister clones derived from an exconjugant pair can remain different for 1,000 fissions. These findings are consistent with observations made earlier, and those in the present study, indicating that cells in stock cultures express a substantial range of corticotypes even when maintained with frequent transfer. The results suggest that mechanisms of spatially ordered structural assembly within the cell can show sufficient fidelity to allow long-term vegetative perpetuation of phenotypic differences without artificial selection.