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In the ciliate Tetrahymena thermophila, the BCD1 pattern-gene encodes a Beige-BEACH-domain protein that defines cortical organelle dimensions through regulated endocytic activity. Tetrahymena cells homozygous for a bcd1 loss-of-function mutation exhibit supernumerary cortical organelles including oral apparatuses, cytoprocts and contractile vacuole pores. Elements of the broadened cortical domain phenotype can be phenocopied by disrupting clathrin-mediated endocytosis, suggesting that exocytic membrane delivery is balanced by endocytic retrieval of cortical pattern determinants.. 

Abstract As single cells, ciliates build, duplicate, and even regenerate complex cortical patterns by largely unknown mechanisms that precisely position organelles along two cell‐wide axes: anterior–posterior and circumferential (left–right). We review our current understanding of intracellular patterning along the anterior–posterior axis in ciliates, with emphasis on how the new pattern emerges during cell division. We focus on the recent progress at the molecular level that has been driven by the discovery of genes whose mutations cause organelle positioning defects in the model ciliateTetrahymena thermophila. These investigations have revealed a network of highly conserved kinases that are confined to either anterior or posterior domains in the cell cortex. These pattern‐regulating kinases create zones of cortical inhibition that by exclusion determine the precise placement of organelles. We discuss observations and models derived from classical microsurgical experiments in large ciliates (includingStentor) and interpret them in light of recent molecular findings inTetrahymena. In particular, we address the involvement of intracellular gradients as vehicles for positioning organelles along the anterior‐posterior axis.