Left-right cortical interactions drive intracellular pattern formation in the ciliate Tetrahymena.

In ciliates, cortical organelles, including ciliary arrays, are positioned at precise locations along two polarity axes: anterior-posterior and circumferential (lateral). We explored the poorly understood mechanism of circumferential patterning, which generates left-right asymmetry. The model ciliate Tetrahymena has a single anteriorly-located oral apparatus. During cell division, a single new oral apparatus forms near the equator of the parental cell and along the longitude of the parental organelle. Cells homozygous for hypoangular 1 (hpo1) alleles, assemble multiple oral apparatuses positioned either to the left or right flanking the normal oral longitude. Using comparative next-generation sequencing, we identified HPO1 as a gene encoding an ARMC9-like protein. Hpo1 colocalizes with the ciliary basal bodies, forming a bilateral concentration gradient, with the high point on the cell's right side and a sharp drop-off that marks the longitude at which oral development initiates on the ventral side. A second Hpo1 concentration drop-off is present on the dorsal surface, where it marks the position for development of a cryptic oral apparatus that forms in the janus mutants. Hpo1 acts bilaterally to exclude oral development from the cell's right side. Hpo1 interacts with the Beige-Beach domain protein Bcd1, a cell's left side-enriched factor, whose loss also confers multiple oral apparatuses on the ventral surface. A loss of both Hpo1 and Bcd1 is lethal and profoundly disrupts the positioning, organization and size of the forming oral apparatus (including its internal left-right polarity). We conclude that in ciliates, the circumferential patterning involves gradient-forming factors that are concentrated on either the cell's right or left side and that the two sides of the cortex interact to create boundary effects that induce, position and shape developing cortical organelles.