Worm Breeder's Gazette 9(1): 54
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Several intracellular motility events in the C. elegans zygote ( pseudocleavage, segregation of germ-line-specific-F granules, and generation of an asymmetric spindle) appear to depend on microfilaments (MFs). As a first step toward trying to understand the participation of MFs in these manifestations of zygotic asymmetry, I have characterized the distribution of MFs in oocytes and early embryos, using the F-actin-specific probe phalloidin and also antiactin antibodies to stain fixed specimens. In oocytes and newly fertilized zygotes, MFs are found in a uniform cortical meshwork of fine fibers and dots or foci. In the zygote, concomitant with the intracellular movements that are believed to be MF-mediated, MFs also become asymmetrically rearranged; as the zygote undergoes pseudocleavage and as P granules become localized in the posterior of the zygote, the foci of F-actin become progressively more concentrated in the anterior hemisphere. The foci remain anterior as the spindle becomes asymmetric and the zygote undergoes first mitosis, at which time fibers align circumferentially around the zygote where the cleavage furrow will form. The distribution of MFs seen by fluorescence suggests that the anterior foci of MFs may be directly responsible for the contractions of the anterior membrane seen during pseudocleavage. Contraction of MFs at the anterior end of the zygote may push or squeeze P granules posterior. The anterior MF foci may also participate in movement of the egg pronucleus toward the sperm pronucleus and then the asymmetric movements of the mitotic spindle. We are currently testing several predictions of this model. Phalloidin and anti-actin antibodies have also been used to visualize MFs in the somatic gonad. The sheath cells that surround maturing oocytes are visibly contractile and contain an unusual array of MFs; the MFs run roughly longitudinally from the loop of the gonad to the spermatheca and are interdigitated with myosin thick filaments, but not in a sarcomeric configuration. These actin and myosin filaments probably interact to cause sheath cell contraction.