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ABSTRACT The mechanisms that govern pattern formation within the cell are poorly understood. Ciliates carry on their surface an elaborate pattern of cortical organelles that are arranged along the anteroposterior and circumferential axes by largely unknown mechanisms. Ciliates divide by tandem duplication: the cortex of the predivision cell is remodeled into two similarly sized and complete daughters. In the conditional cdaI-1 mutant of Tetrahymena thermophila, the division plane migrates from its initially correct equatorial position toward the cell's anterior, resulting in unequal cell division, and defects in nuclear divisions and cytokinesis. We used comparative whole genome sequencing to identify the cause of cdaI-1 as a mutation in a Hippo/Mst kinase. CdaI is a cortical protein with a cell cycle-dependent, highly polarized localization. Early in cell division, CdaI marks the anterior half of the cell, and later concentrates at the posterior end of the emerging anterior daughter. Despite the strong association of CdaI with the new posterior cell end, the cdaI-1 mutation does not affect the patterning of the new posterior cortical organelles. We conclude that, in Tetrahymena, the Hippo pathway maintains an equatorial position of the fission zone, and, by this activity, specifies the relative dimensions of the anterior and posterior daughter cell.
KEYWORDS Hippo; Tetrahymena; cytokinesis; ciliate; polarity
CILIATES are among the most structurally complex cell types known, due to their nuclear dualism and the intricate organization of their surface. Ciliates have two structurally and functionally distinct nuclei that operate in one cytoplasm: the germline micronucleus that undergoes mitosis (or meiosis), and the somatic macronucleus that divides by amitosis-a unique mode of nuclear division that does not involve chromatin condensation or a bipolar spindle formation (reviewed in Ruehle et al. 2016). During the vegetative cell cycle, these two nuclei divide at different times, with the mitosis of the micronucleus preceding the amitosis of the macronucleus (Figure 1). In addition, during cell division, ciliates duplicate the cortex, to produce a tandem ofdaughter cells (reviewed in Wloga and Frankel 2012).
The tandem duplication of the cortical pattern has to be precisely coordinated with cytokinesis in time and space. The cortical regions immediately anterior and posterior to the fission zone undergo vastly different morphogenetic routines, to develop new cortical ends (see Figure 1). The division plane...