In animal cells, the position of the spindle at the end of anaphase determines placement of the cleavage furrow, which forms at right angle to the spindle axis and equidistant from the spindle poles. In the one-cell stage C. elegans embryo (P0), accurate anaphase spindle positioning follows from two processes: first, a 90o rotation of the centrosomes that positions them along the antero-posterior (A-P) axis, and, second, a posterior displacement of the spindle during anaphase B. As a result, the cleavage furrow divides P0 perpendicular to the A-P axis into a larger AB blastomere and a smaller P1 blastomere. We searched for mutants in anaphase spindle positioning in P0 by screening an extensive collection of chromosome III maternal-effect embryonic lethal mutants by time-lapse DIC videomicroscopy. We found that one of the loci crucial for P0 anaphase spindle positioning is
zyg-8, which was previously identified among maternal-effect embryonic lethal ts mutations by Wood et al.. In 30% of
zyg-8 mutant embryos, rotation of the centrosomes is impaired, and the spindle sets up perpendicular to the A-P axis In the remaining 70% of
zyg-8 mutant embryos, the spindle sets up in the correct orientation but the posterior spindle pole is abruptly pulled towards the posterior during anaphase B. In both sets of
zyg-8 mutant embryos, placement of the cleavage furrow is aberrant. These phenotypes are observed in 5/7
zyg-8 alleles, and when a
zyg-8 mutation is put over deficiency, suggesting that they correspond to strong loss-of-function phenotypes. Why would the loss of
zyg-8 function result in excess posterior displacement during anaphase B? One formal possibility is that overall embryonic polarity would be affected. However, like in wild-type, PAR-3 protein is restricted to the anterior cortical domain and P-granules are segregated to the posterior in
zyg-8 mutant embryos, suggesting that overall polarity is, in fact, not altered. Another formal possibility is that astral microtubules are affected in some way. We found that astral microtubules in
zyg-8 mutant embryos appear shorter than in wild-type, often not reaching all the way to the cortex. Our working model is that
zyg-8 normally restricts the extent of posterior spindle displacement during anaphase B by ensuring proper anchorage of astral microtubules to the anterior cortex.