Scalar constraints in Tetrahymena evolution. Quantitative basal body variations within and between species.

Abstract

Tetrahymenas of 17 species of the T. pyriformis complex have been stained with protargol and analyzed for numbers of basal bodies in half cells just before cell division. At this stage, cells of all strains manifest considerable variation in numbers of basal bodies; the coefficient of variation (sigma/m) is usually between 0.05 and 0.10. Much of this variability is observed in cells in the same nutritional state, at the same stage of the growth cycle, and in the same part of the life cycle. The basal body variability may be related to the variation in macronuclear DNA content that results from the imprecise amitotic macronuclear division. With a few exceptions, strains of different species are difficult to distinguish on the basis of basal body numbers. The species means in the samples examined show a range only from 234 (T. furgasoni) to 481 (T. capricornis), about a twofold difference. This limited variation in the means suggests that these organisms are constrained within narrow limited by some scalar function of their organismic design, which prevents an evolutionary size dispersion--even though molecular scrambling has occurred in the complex at an appreciable rate for a very long evolutionary interval.