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.

Visualization of Microfilaments in C. elegans Oocytes, Zygotes, and Gonadal Tissue

S. Strome

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.