Hereditary variation of ciliary patterns in ciliated protozoa

Summary— In ciliates, the basic pattern of ciliature consists of longitudinal polarized ciliary rows. This basic pattern is expected to be retained through successive binary fissions by means of the so-called cytotactic mechanisms, as a consequence of autogenetic proliferation of basal bodies within each row. This idea is supported by the hereditary transmission of 180° rotated rows in Tetrahymena and Paramecium. These mechanisms should theoretically ensure intraclonal homogeneity for ciliary row number. In fact, some alterations are responsible for either loss or addition of rows. Such alterations are not strictly random processes, since they have been shown to be controlled by nuclear genes. Cytotaxis does not account for exact positioning of primordia for complex structures (buccal ciliature, for instance): the sites of basal body proliferation are determined by certain “positional information” which, in turn, is controlled by chromosomal genetic material more or less independently of pre-existing structures, as illustrated by many spontaneous or induced mutations. All deviations from the wild-type phenotype seem to be associated with very reduced fitness, at least in laboratory conditions. On the other hand, the currently known variants cannot account for intraspecific diversity. Thus, the evolutionary significance of these phenomena remains somewhat obscure.