Hypoangular: a gene potentially involved in specifying positional information in a ciliate, Tetrahymena thermophila

In Tetrahymena, two unique cell-surface structures, the oral apparatus and the cytoproct, are formed at opposite ends of one ciliary row, the reference meridian, which is propagated longitudinally during clonal growth. A third set of unique structures, the contractile vacuole pore(s) (CVP), is located at a nearly constant proportion of the cell circumference to the cell's right of the reference meridian. Three allelic recessive temperature-sensitive mutations, collectively named hypoangular (hpo), alter both the geometry of propagation of the reference meridian and the location of the CVPs. In mutant cells, the reference meridian typically undergoes a steady rightward shift in successive cell generations ("cortical slippage"); concomitantly, CVP sets come to lie closer to the reference meridian. Although CVP location is still proportional to the cell circumference, the constant of proportionality (the "CVP angle") is reduced. Another effect is an alteration in the widths of morphogenetic domains within the cortex. As the temperature is raised (made more restrictive), these effects are accentuated and the CVP angle becomes reduced further. At the extreme, the CVP angle collapses to zero and less, i.e., there is a topological switch such that CVPs come to lie to the left of the reference meridian, and the direction of cortical slippage reverses from rightward to leftward. These observations are hard to reconcile with existing formal models of pattern specification in this system and suggest that the hpo locus might specify a key component of the intracellular positional system.