The Janus A gene encodes a polo-kinase whose loss creates a dorsal/ventral intracellular homeosis in the ciliate, Tetrahymena.

Genetic studies on the protist, Tetrahymena thermophila provide a glimpse into the unexpectedly rich world of intracellular patterning that unfolds within the ciliate cell cortex. Ciliate pattern studies provide a useful counterpoint to animal models of pattern formation in that the unicellular model draws attention away from fields of cells (or nuclei) as the principal players in the metazoan pattern paradigm, focusing instead on fields of ciliated basal bodies serving as sources of positional information. In this study, we identify JANA, a Polo kinase of Tetrahymena, that serves as an important factor driving global, circumferential pattern. Loss of function of JanA results in global, mirror-duplication of ventral organelles on the dorsal surface: a kind of intracellular homeosis that has been named the janus phenotype. Gain of function (over-expression) reduces or even eliminates cortical organelles within the ventral hemi-cell. GFP-tagging reveals that JanA decorates basal bodies predominantly within the left-dorsal hemi-cell. These results led us to propose a model in which the default state of cortical patterning is a mirror-image assemblage of cortical organelles including oral apparatus, contractile vacuole pores and cytoproct. JanA normally suppresses organelle assembly in the dorsal hemi-cellular cortex, resulting in a simple, ventral assemblage of these organelles, a half-pattern as it were. PLK inhibitors produce a janus phenocopy, but reveal other unanticipated roles for PLK activities involving more local patterning events that control organelle dimensions and organization. We discuss results in light of metazoan studies in which PLK activity links cell cycle control to intracellular symmetry breaking.