We are interested in understanding how polarity is established in the early C. elegans embryo.
nop-1(
it142) was shown by Rose et al. (Dev. Biol.1995) to be a viable mutant that lacks a pseudocleavage furrow and has reduced cortical and central cytoplasmic flows. Since most of the partitioning (par) genes are required for cytoplasmic flows (R.J. Cheeks and B. Goldstein, in preparation), we wondered whether the reduced flows in
nop-1 mutant embryos might be a sign that
it142 is a hypomorphic allele of a gene in the par pathway. We examined the phenotype of
nop-1(
it142) embryos and found that a proportion of them had no cytoplasmic flows, and these embryos failed to hatch. Interestingly, most of these defective embryos either had a Par phenotype that resembled that produced by
par-2 or
par-5 loss of function, or failed to complete first cytokinesis. The penetrance of these two phenotypes was greatly enhanced (to 95%) in embryos in which
nop-1 function was reduced by having
it142 in trans to a deficiency. These results indicate that
nop-1 plays an essential role in early embryonic events. Furthermore, we found that the Par phenotype of
nop-1 mutant embryos was synergistically enhanced by either a homozygous mutation in
par-4 or a heterozygous mutation in
par-5. Disruption of
par-1,
par-3 or
par-6 had no effect on the penetrance of this phenotype. Interestingly, the pseudocleavage defect of
nop-1 embryos was suppressed by a homozygous mutation in
par-5. Therefore,
nop-1 genetically interacts with some components of the par pathway for the establishment of embryonic polarity. We set out to clone
nop-1 as a step toward further defining its role in early embryogenesis.
nop-1(
it142) was previously mapped by Lesilee Rose on chromosome III, between
dpy-17 and
unc-32. Preliminary attempts identified cosmid B0336 as partially rescuing the weak flow phenotype of
nop-1 embryos. However, this is likely to be due to a gain-of-function effect: using several mutations and deficiencies in the area, we refined the mapping of
nop-1 and positioned it between
sma-4 and
kin-18, a region that excludes B0336. While this genetic region contains five genes with an early embryonic phenotype (Gonczy et al., Nature 2000), RNAi to these genes fails to phenocopy the Nop phenotype. We are currently refining the mapping of
nop-1 using SNPs in the area, sequencing potential candidates and assaying for enhancement of the Nop phenotype by weak RNAi feeding; progress will be reported.