Analyses of inverted ciliary rows in Paramecium. combined light and electron microscopic observations

Many visible cytoskeletal and membranous differentiations organized around the basal bodies of ciliate protozoa reveal asymmetry (left-right) and/or polarity (anteriorposterior) in their organization. These cortical differentiations serve as orientation markers in analyses of cellular patterning and morphogenesis. We performed an integrated light and transmission electron microscopic analysis of 180° rotated (inverted) regions of the cortex of Paramecium. The internal organization of cortical units, and the ciliary rows they form, is unaffected by the inversion. Nevertheless, cortical structures in an inverted region show a reorientation (with respect to cellular axes) consistent with a planar 180° rotation of the affected ciliary rows. Typically-oriented regions adjacent to the inversion are unaffected by the inversion. A cell with an inversion displays two boundaries (junctures), with characteristic morphology, between normally-oriented cortex and the inversion. Therefore, the inversion is expressed as a reorientation of the affected cortical units relative to the cell's orthogonal axes, but does not disrupt the internal organization of those cortical units. Cortical inversions are clonally stable during continued asexual propagation, and also through sexual exchanges. These observations demonstrated that significant differences in cellular phenotype can be sustained in cell lines in the absence of genotypic differences.