Interactions between janus and bcd cortical pattern mutants in Tetrahymena thermophila : An investigation of intracellular patterning mechanisms using double-mutant analysis

An analysis of bcd, janA; bcd, janB; and bcd, janC double-mutant phenotypes in Tetrahymena thermophila has allowed us to examine patterning processes affected by two different classes of mutations. bcd brings about a broadening of the oral and contractile vacuole pore domains in the ciliate cortex, while the janus mutations generate a mirror-image duplication of the ventral cortical pattern. We observed both bcd and janus characteristics expressed in the double mutants, as well as features unique to the double-mutant. Temperature-shift experiments employing the temperature-sensitive janB mutation in a double-mutant (bcd, janB) combination allowed us to observe the changes in pattern as a mirror-image geometry was brought into expression and subsequently removed within the bcd, janB double homozygote. These experiments suggest that there are multiple pattern-mechanisms at work with differing kinetics of expression in the ciliate cortex. We discuss how the bcd mutation could influance expression of the janus mutations in light of a model previously proposed to account for the janus phenotype.

Intracellular pattern reversal in Tetrahymena thermophila. I. Evidence for reverse intercalation in unbalanced doublets

Homopolar doublets of Tetrahymena thermophila possess two sets of similar cell surface structures, the most prominent of which are the complex and asymmetrical oral apparatuses. These initially are located on opposite surfaces of the duplex cell, but tend to shift so that they are no longer directly opposite each other. The two sets of oral structures are then separated by one wider and one narrower arc of cell surface. When one arc becomes sufficiently narrow, a new third oral apparatus with partially reversed internal asymmetry frequently becomes interposed between the two preexisting oral apparatuses, always within the narrower arc. After this happens, the reliability of development of new oral structures, particularly of the interposed ones, is reduced. Contractile vacuole pores, typically present within both arcs of homopolar doublets, tend to disappear from the narrower arcs. This anomalous partial triplet condition appears to be a transient intermediate stage in the reversion of homopolar doublets to normal singlets. We interpret the interposition of a transient third oral system in doublets that are regulating toward the singlet state as being a consequence of reverse intercalation of new positional values subsequent to excessive crowding of the preexisting positional values. This interpretation is an adaptation of the shortest-distance intercalation rule of the polar coordinate model applied in an intracellular and morphallactic context.

How the mirror-image pattern specified by a janus mutation of Tetrahymena thermophila comes to expression

The initial changes of cell-surface organization that occurred as the recessive janA1 (janus) mutation of Tetrahymena thermophila first became expressed were elucidated in a special mating scheme in which old macronuclei homozygous for janA+ were synchronously replaced by new macronuclei homozygous for janA1. During this period of onset of expression, the number, regularity, and asymmetry of the ciliary rows remained unchanged. New normal (primary) oral apparatuses (OAs) continued to be formed posterior to old OAs, as in normal cells. At about four fissions after conjugation, abnormal (secondary) OAs with a partial reversal of asymmetry began to appear nearly opposite to the primary OAs, close to but not at the eventual circumferential position of janA1 secondary OAs. The array of contractile vacuole pores (CVPs), normally located adjacent to two ciliary rows centered near 22% of the cell circumference to the right of the primary oral meridian, underwent a two-step transformation: first, the number of adjacent ciliary rows bearing CVPs increased to 3, 4, and sometimes 5, then "skipped" rows appeared within this broadened CVP-arc to split the single set of CVPs into two separated subsets. The CVP transformations occurred gradually and progressively. They began prior to the expression of secondary OAs but accelerated as secondary OAs appeared. As the CVP arc became broader, its midpoint shifted somewhat to the right, away from the primary oral meridian, but ended up close to halfway between the primary and secondary oral meridians. The data provide a better fit to an intercalation model than to an alternative double-gradient model, suggesting that the janA1 mutation alters the large-scale organization of positional values by preventing the expression of a subset of these values and thus provoking reverse-intercalation of the remainder.

Inter-strain variability of structural proteins in Tetrahymena

The high molecular weight proteins found in isolated pellicles of Tetrahymena have been compared in several individual strains within the genus using SDS polyacrylamide gel electrophoresis. Three variants of the beta-protein of epiplasm (MW 174,000; 155,000; 145,00) and 2 of the C-protein (MW 140,000; 122,000) were found among the strains examined. No variation was observed in the major kinetodesmal fiber protein (MW 250,000). The variation found between strains in the proteins of a structure which is (as far as we know) the same in all strains indicates a disjunction between evolutionary change at the 2 levels of organization. The taxonomic implications of the observed variation in structural proteins in Tetrahymena are discussed.

Form and pattern in ciliated protozoa: analysis of a genic mutant with altered cell shape in Tetrahymena pyriformis, Syngen 1

A single cell isolated from the sexual progeny of mutagenized parents gave rise to a clone of cells with an abnormal, conical shape. Breeding analysis revealed that this shape results from the action of a single recessive gene, co (conical). Homozygous mutant cells are shorter and wider than wild type cells, and have their widest point at a more posterior position. Nonetheless, cortical parameters such as number of ciliary rows, number of ciliary units within these rows, and positions of contractile vacuole pores remain essentially unchanged in conical cells, suggesting a considerable degree of mutual independence of pattern and form. Shape changes prior to cell division bring about some convergence in form of dividing conical and wild type cells. However, in conical cells the new oral apparatus and fission line form well posterior to the cell equator, so the opisthes are invariably smaller than proters. Macronuclei nonetheless undergo constriction at the normal central location, and the characteristic inequality in the DNA content of the two macronuclear division products is not increased by the conical condition. Generation times are, on the average, nearly the same in the two wild type daughter cells and in conical proters, while the small conical opisthes have generation times averaging one-third longer. This prolongation explains why population doubling times are always somewhat longer in cultures of conical cells than in parallel cultures of wild type cells. The unusually long generation times of conical opisthes allows for the compensation of initial size differences. Observations on shape changes in conjugating cells of various genotypic combinations suggest that the wild type gene product is not freely exchangeable across the conjugation bridge. The implications of the conical phenotype for problems of cellular patterning and positioning of organelle systems are considered in the discussion.