Considerations of symmetry in the cortical integration of Tetrahymena doublets

Abstract

Homopolar doublets of syngen 1, T. pyriformis, may be induced by treatment of conjugating pairs with immobilizing antiserum. These doublets have geometric properties and basal body populations generally indicative of separate autonomous integrative systems in the two halves. The duplex system, though metastable, is transformed through a process of "simplification" back to the simplex state. The transformation is not a single event, but a series involving changes at different times for different structures and processes; for the micronuclei and the macronuclei; for the capacity to generate two oral apparatuses through stomatogenesis and the capacity to develop them through oral replacement; for the structures at the anterior and posterior ends of the cell; for the numbers of ciliary rows and for the numbers of basal bodies which make up the rows. Although the two semicells composing a doublet are in important respects independent of each other, they are coordinated in significant ways. The positions of the contractile vacuole pores and their numbers depend not only on the number of ciliary rows in a semicell, but also on the number of ciliary rows in the opposing twin. Most notably, the probability for dual stomatogenesis, and hence the perpetuation of the doubled oral apparatus, depends on the symmetry of the semicells. The cell as a whole is maintained as an integrated unit over a prolonged interval as various aspects of duplex structure and function are progressively consolidated.

What Do Genic Mutations Tell Us about the Structural Patterning of a Complex Single-Celled Organism?

“…The mutation was aptly named 'doublet-former' (dbf1-1), and when homozygous it produced Siamese-twin doublets with a variable penetrance at restrictive temperatures, 75% in one sample and not much above zero in another. These were perfectly normal Siamesetwin doublets with twofold rotational symmetry, like the doublets studied by Nanney et al (109). The modal ciliary row number of doublets was 28, which was near the lower limit of the expected row number for doublet cells; this fit with the observation that doublet cells tend to lose rows rapidly shortly after their formation but maintain the doublet condition until the number of ciliary rows falls into the range of 25 to 28 rows (103,109).…”

“…These were perfectly normal Siamesetwin doublets with twofold rotational symmetry, like the doublets studied by Nanney et al (109). The modal ciliary row number of doublets was 28, which was near the lower limit of the expected row number for doublet cells; this fit with the observation that doublet cells tend to lose rows rapidly shortly after their formation but maintain the doublet condition until the number of ciliary rows falls into the range of 25 to 28 rows (103,109). The only unusual aspect of the dbf1-1 doublet clones was that they were selected as recessive homozygotes following mutagenesis.…”

“…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).…”

“…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.…”

“…In this case, the mode of origin of the doublets has not been pinpointed, but it is likely to resemble that which Fauré-Fremiet used to create his doublets: blockage of division followed by an anterior migration of the posterior daughter cell. Variations in the frequency of occurrence of this initiating event would ac-count for the highly variable penetrance of this mutation, since doublets, once formed, propagate their condition for a while but tend to revert to singlets by loss of ciliary rows and possibly also become diluted by overgrowth by singlets in a mixed culture (109).…”