Polymorphism and specificity of positioning of contractile vacuole pores in a ciliate, Chilodonella steini.

The unicellular ciliate Chilodonella steini has a well-defined flat and ciliated ventral field. During divisional morphogenesis two sets of new contractile vacuole pores (CVPs) are formed on this field. During final pattern formation some of these CVP primordia and the old parental set of CVPs are completely resorbed. Primary patterns of distribution of the CVP primordia and final pattern of distribution of the matured CVPs manifest an intraclonal polymorphism. From analysis of this polymorphism some features of mechanism(s) of CVP pattern determination are deduced: 1. There is a strict, short-distance negative control of appearance of CVP primordia at sites of oral morphogenesis and around the ventral field. 2. Certain indeterminacy of large-scale patterning of CVP primordia is observed over the area competent to yield CVP formation. However, within this area three longitudinal sectors with a high probability of occurrence of CVP primordia are alternated with sectors nearly deprived of their occurrence. 3. Positive control of probability of occurrence and of specificity of location is found for certain CVP primordia. An interaction of mechanism of positioning on cellular longitudes and latitudes is proposed to account for these facts. 4. The resorption of supernumerary CVP primordia does not alter the character of the global map of distribution of CVP primordia achieved during primary pattern formation. The primordia located at some latitudes persist, whereas others are resorbed at random. It is suggested that all CVP primordia which do not mature at the time of stabilization of divisional morphogenesis are resorbed. Thus the global map of CVPs distribution would result from the sum of the individual determinations of the fates of each CVP primordium, superimposed on an initial spatially non-uniform distribution of CVP primordia.