What do genic mutations tell us about the structural patterning of a complex single-celled organism?

Structural inheritance in the ciliate cortex. Ciliates make up a distinctive group of unicellular organisms characterized by dualism of germ line and somatic nuclei, sex by reciprocal exchange of gametic nuclei with subsequent replacement of the somatic nucleus, and an extraordinarily complex organization of the cell surface layer. This organization is dominated by cytoskeletal structures, including rows of basal bodies, cilia, and accessory fibrillar structures, and is perpetuated by longitudinal extension of these structural ensembles during clonal growth. This mode of perpetuation provides an ideal opportunity for the demonstration and analysis of cellular heredity at a nongenic level. This opportunity was seized by three gifted biologists, the comparative zoologist Emmanuel Fauré-Fremiet, the experimental embryologist Vance Tartar, and the geneticist Tracy Sonneborn, who together led the way in establishing the principle that structural variations of nongenic origin in the ciliate cortex can indeed be faithfully transmitted to progeny over numerous cell generations. Although this review is primarily devoted to the contribution of genic mutations to the understanding of the ciliate cortex, the interpretation of some of the more interesting of these mutations becomes more meaningful against a background of the three major arenas of nongenic structural inheritance in the ciliate cortex, all of which were solidly established before the genetic approach was initiated. The best-known form of structural inheritance is the organization of the longitudinal ciliary rows that cover the surfaces of most ciliates. This was first demonstrated by Beisson and Sonneborn (12), who showed that an inversion (180° rotation) of one or more ciliary rows of Paramecium tetraurelia (then Paramecium aurelia, syngen 4) (Fig. 1A) could be nongenically inherited; Sonneborn gave the name “cytotaxis” to “this ordering and arranging of new structures under the influence of pre-existing cell structure” (137). This demonstration was later repeated for Tetrahymena thermophila (then Tetrahymena pyriformis, syngen 1) (118) and for other ciliates (62, 63, 71). It was extended by Nanney’s observation that the preexisting number of ciliary rows in T. thermophila tended to be conserved (103, 104), presumably due to the same structural constraints that conserve the geometrical organization of these rows. A second form of structural inheritance is demonstrated by the propagation of the number of complete sets of cortical structures. This was investigated in detail by Fauré-Fremiet (31, 32), who noted that ciliates that became fused side by side, as a consequence of blockage of division followed by anterior sliding of the presumptive posterior daughter cell, could propagate their duality. Later, Vance Tartar (147) demonstrated that microsurgically constructed Siamese-twin doublets in the large ciliate Stentor coeruleus could perpetuate their doublet condition. In both cases, the way in which the doublets were created virtually rules out the hypothesis that this condition had arisen from a genic mutation; instead, it was due to a “contrainte structurale” (32). Sonneborn (136) completed the demonstration in Paramecium tetraurelia (Fig. 1B) by proving with results of appropriate crosses that the difference between the singlet and doublet conditions was not caused by differences either in nuclear genes or in exchangeable internal cytoplasm and hence had to reside in the cortical layer. There is good reason to believe that the same conclusion applies to doublets induced in other ciliates, including Tetrahymena thermophila (103, 109). This structural constraint within the cell cortex does not influence the number of macronuclei in doublets; these cells typically reverted from possession of two macronuclei to one, while the cortex continued to propagate its duality (14, 109). The third form of structural inheritance was discovered by Fauré-Fremiet (31). He noted that whereas the great majority of the doublets that he studied manifested a twofold rotational symmetry in the organization of their two normal sets of cortical structures, one exceptional clone of a ciliate named Urostyla trichogaster displayed a mirror image symmetry in the arrangement of its two sets of cortical structures. This initial discovery was followed up by a Chinese group (153) and then was confirmed and extended by investigators worldwide for a variety of ciliates (44, 64, 70, 142, 155, 166). In these doublets, the arrangement of cortical structures in the “reversed” partner was close to a mirror image of that in the “normal” partner (Fig. 1C), but the internal organization of each individual ciliary structure in the “reversed” component was normal, though sometimes rotationally permuted relative to the cellular axes (13, 64, 70, 131, 142). The mirror image arrangement could be reliably generated by certain microsurgical operations (132, 153) and was not caused by any relevant genic difference (114, 151). When such mirror image doublets were bisected midlon* Mailing address: Department of Biology, The University of Iowa, 143 Biology Bldg., Iowa City, IA 52242. Phone: (319) 335-1110. Fax: (319) 335-1069. E-mail: [email protected]. Published ahead of print on 25 July 2008.