Several intracellular motility events in the Caenorhabditis elegans zygote (pseudocleavage, the asymmetric meeting of the pronuclei, the segregation of germ line-specific granules, and the generation of an asymmetric spindle) appear to depend on microfilaments (MFs). To investigate how MFs participate in these manifestations of zygotic asymmetry, the distribution of MFs in oocytes and early embryos was examined, using both antibodies to actin and the F-actin-specific probe rhodamine-phalloidin. In early-stage zygotes, MFs are found in a uniform cortical meshwork of fine fibers and dots or foci. In later zygotes, concomitant with the intracellular movements that are thought to be MF mediated, MFs also become asymmetrically rearranged; as the zygote undergoes pseudocleavage and as the germ line granules become localized in the posterior half of the cell, the foci of actin become progressively more concentrated in the anterior hemisphere. The foci remain anterior as the spindle becomes asymmetric and the zygote undergoes its first mitosis, at which time fibers align circumferentially around the zygote where the cleavage furrow will form. A model for how the anterior foci of actin may participate in zygotic motility events is discussed. Phalloidin and anti-actin antibodies have also been used to visualize MFs in the somatic tissues of the adult gonad. The myoepithelial cells that surround maturing oocytes are visibly contractile and contain an unusual array of MF bundles; the MFs run roughly longitudinally from the loop of the gonad to the spermatheca. Myosin thick filaments are distributed along the MFs in a periodic manner suggestive of a sarcomere-like configuration. It is proposed that these actin and myosin filaments interact to cause sheath cell contraction and the movement of oocytes through the gonad.

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