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

Abstract

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.

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

“…This gene is the runner-up to cdaA in the number of papers published, 13 in this case (22,25,41,42,43,45,47,50,53,68,69,76,141). Unlike the situation with the circumferential-pattern mutations considered thus far, alleles at three different gene loci generate the janus phenotype-JANA on chromosome 3R (two mutant alleles), JANB on chromosome 2 (one mutant allele), and JANC, probably on chromosome 1R (four mutant alleles) (47,50;Jenkins and Frankel,unpublished).…”

“…Although the janus mutants often have two sets of oral structures (Fig. 19A) and other relevant cortical landmarks, they still have a number of ciliary rows characteristic of singlet cells; when the janA-1 mutation came to expression synchronously, the number of ciliary rows did not change while the two CVP sets bifurcated and the sOA made its later mid-dorsal appearance (43). Thus, janus mutant cells are not true doublet cells; rather, they are singlet cells in which a broad domain has undergone a reversal of circumferential polarity.…”