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In various polarized cells, positions of nuclei are often off-center. However, extrinsic signals regulating nuclear off-centering and its biologic roles remain to be elucidated. In Caenorhabditis elegans, polarity of the EMS cell undergoing asymmetric division is regulated by the MOM-2/Wnt and MES-1 signals from its posterior neighbor P2 cell. We show that after divisions of different cells including EMS, the nuclei of the posterior but not anterior daughter cells are anchored to the posterior cell cortex via centrosomes. We also show that this nuclear anchoring is regulated by components of the Wnt pathway and SRC-1 that functions in MES-1 signaling. To understand the biologic roles of nuclear anchoring, we analyzed its effects on asymmetric nuclear localization of POP-1/TCF that is also regulated by Wnt and Src signaling. We found that in mom-2 mutants where the nuclear anchoring and POP-1 asymmetry is partially inhibited, the proximity of the nucleus to the cell cortex correlated with POP-1 asymmetry. Furthermore, in mutants of mom-2, the defect in the anchoring is clearly correlated with that of asymmetric fate determination. These results suggest that the asymmetric nuclear anchoring functions in asymmetric division by enhancing POP-1 asymmetry.

Background: The PAR proteins are part of an ancient and widely conserved machinery for polarizing cells during animal development. Here we use a combination of genetics and live imaging methods in the model organism Caenorhabditis elegans to dissect the cellular mechanisms by which PAR proteins polarize cells. Results: We demonstrate two distinct mechanisms by which PAR proteins polarize the C. elegans zygote. First, we show that several components of the PAR pathway function in intracellular motility, producing a polarized movement of the cell cortex. We present evidence that this cortical motility may drive the movement of cellular components that must become asymmetrically distributed, including both germline-specific ribonucleoprotein complexes and cortical domains containing the PAR proteins themselves. Second, PAR-1 functions to refine the asymmetric localization of germline ribonucleoprotein complexes by selectively stabilizing only those complexes that reach the PAR-1-enriched posterior cell cortex during the period of cortical motility. Conclusions: These results identify two cellular mechanisms by which the PAR proteins polarize the C. elegans zygote, and they suggest mechanisms by which PAR proteins may polarize cells in diverse animal systems.

Conjugation following pair formation in Tetrahymena can be divided into three distinct sequences of events: prezygotic development, postzygotic development, and exconjugant development. The decision to proceed with postzygotic development is governed by a developmental checkpoint occurring sometime during the middle stages of conjugation. A second developmental decision is made to initiate pair separation and exconjugant development. This paper examines the phenotypes of five newly isolated conjugation mutants ( cnj6–cnj10 ) which affect middle and late events within the conjugation program. cnj6 mutants exhibit normal nuclear behavior throughout development up to and including differentiation of new macronuclear anlagen. Pairs arrest at this developmental endpoint, unable to dissociate. cnj7 and cnj8 eliminate the third prezygotic nuclear division and the first postzygotic nuclear division. All subsequent developmental events appear normal. cnj9 eliminates the second postzygotic nuclear division, and subsequently, new macronuclei fail to develop despite parental macronuclear degradation. cnj10 results in a pleiotropic phenotype characterized by failure of numerous events which all appear to involve nuclear–cytoskeletal interactions. These defects include nuclear selection (anchoring nuclei to the exchange junction), pronuclear exchange, pronuclear fusion, and anchoring postzygotic nuclear division products to the posterior cell cortex. These mutant phenotypes are used to draw inferences regarding developmental dependencies that govern a cell's entry into the postzygotic and exconjugant developmental programs.

Summary During the conjugation of Tetrahymena thermophila two anterior products of the second postzygotic division develop into macronuclear anlagen and two posterior nuclei remain micronuclei. This differentiation depends either upon some soluble cytoplasmic factor(s) or upon the short range inhibition of the posterior nuclei due to their connection with the cell cortex. Two different configurations of the conjugation fusants were studied: i.) side-by-side fusants in which the posterior end of the cell of one pair was fused with the posterior end of the cell of another par, and ii.) metameric fusants in which the posterior end of the cell of one pair was fused with the anterior pole of the cell of another pair. In the latter case, the macronuclear anlagen of the posterior pair developed in close proximity to the incipient micronuclei of the member of the anterior pair within the cytoplasm of the fused cells. This result is consistent with the hypothesis that the postzygotic nuclei are committed to the macronuclear development except those posterior nuclei which are arrested in this development due to a short range interaction with the posterior cell cortex.