Worm Breeder's Gazette 11(4): 81

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Interactions Among the par Mutations

Ken Kemphues, Niansheng Cheng, Colleen Kirby, Ian Korf, Diane Morton, Mike Roos and Betty Suh

Figure 1

In an effort to understand the relationships among the par genes we 
have been analyzing double mutant combinations.  The phenotypes of 
mutants in the five par genes are similar but readily distinguishable 
from one another.  Strong loss-of-function mutations in all genes 
affect cleavage spindle position and orientation, cleavage rate, P 
granule localization, and development of the intestine but do so in a 
gene-specific manner.  We have interpreted these phenotypes to 
indicate that the par genes encode components of a maternally provided 
system for establishing cleavage asymmetry and cytoplasmic 
localization in the early embryo.  
The results from the five doubles we have examined thus far indicate 
that the genes are not in a simple dependent pathway but they do 
interact.  There are no clear epistatic relationships, although 
individual aspects of the phenotypes may show epistasis (see below); 
the cellular phenotypes of the strong mutants are simply additive, not 
synergistic.  However, we often see synergism with respect to 
viability in double mutant combinations of weak alleles.  The most 
striking example is the double homozygote par-2(it5ts); 
7ts).  it5 gives greater than 70% viable progeny at 
15 C and it57 gives nearly 100% viable progeny at 15 C.  it5;it57 
doubles give almost complete lethality at 15 C.  We also see dominant 
enhancement of weak alleles of one par by one copy of a strong allele 
at a second par gene.
Two particularly interesting double mutant combinations are par-2 
par-3 and par-3;par-5.  par-3 affects the orientation of the second 
cleavage spindles in a way that is opposite to par-2 and par-5.[See 
Figure 1]
We were surprised to find that par-3 is epistatic to par-2 and par-5 
for this particular phenotype.  This indicates that none of the three 
genes are required for the rotation of the P1 spindle.  Rather, par-3+ 
acts to prevent spindle rotation and par-2+(5+) acts to insure that 
par-3+ acts only in the AB.  This could result either from 
localization of par-3+ activity to the AB or by action of par-2+(5+) 
in the P1.
We think it likely that par-2+(5+) acts in the P1 to inhibit or 
modify the action of par-3+, because we have obtained evidence that 
par-3+ activity is present in P1.  In the wild type, P1 spindle 
rotation occurs reproducibly in embryos that are slightly flattened (
squashed) by pressure from the coverslip.  Our initial observations of 
par-3 mutants were done on unsquashed embryos.  When we observe par-3 
mutant embryos under squashed conditions, the orientations of AB and 
P1 spindles are much more variable than under unsquashed conditions.  
We conclude, therefore, that although the rotation of the P1 spindle 
does not absolutely require par-3+ the strength of the rotation or the 
stability of the new spindle position is dependent upon par-3+ 
function.  Perhaps par-3+ encodes an element of the cytoskeleton that 
normally stabilizes spindle position in the early embryo.  par-2+ then 
acts in the P1 to modify the par-3+ activity allowing the spindle to 
rotate.  Another aspect of the par phenotype may relate to this point. 
In the wild type, the asters of the first cleavage spindle show 
different behavior in P1 and AB.  In the P1 cell the aster becomes 
disc shaped while in the AB the aster remains spherical.  In par-1 and 
par-4, the P1 aster becomes disc shaped.  In par-3, both AB and P1 
asters become disc shaped, although not to the same extent as in wild 
type.  In par-2 and par-5 both asters remain spherical.  In the par-2 
he asters are disc shaped (par-3;par-5 has 
not yet been checked).  Thus the change in aster shape at the end of 
the first division seems to correlate with spindle rotation.

Figure 1