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for V4 in
cwn-1 egl-20
). Similarly, V6 polarity is disrupted in quadruple Wnt
mutants (
lin-44
,
cwn-1
,
cwn-2 egl-20
), but not in any triple combinations.
Therefore, these cells are properly polarized in the presence of one Wnt
(
cwn-1
,
cwn-2
,or
egl-20
for V1-V4, and
lin-44
,
cwn-1
,
cwn-2
,
or egl-20
for
V6). In contrast, V5 polarity is affected in
egl-20
single mutant (
Whangbo
et al., 2000
), although the phenotype is modified by further mutations of
cwn-1
and
cwn-2
. In any combinations of Wnt mutations including
quintuple Wnt mutants that have mutations in all five Wnt genes present
in
C. elegans
genome, seam cells are mostly polarized in normal or reverse
orientation, while loss of polarity is less frequent. The observation is
consistent with apparently Wnt-independent POP-1 asymmetry in
sequentially isolated AB
4
daughters (
Fig. 3.5
C). In contrast, in triple Wnt
receptor mutants (
lin-17
/Fzd
mom-5
/Fzd,
cam-1
/Ror), all seam cells lose
their polarity, resulting in symmetric division and loss of seam cells
(
Fig. 3.6
). Therefore, Wnts are required for proper orientation of
polarity, while their receptors function in the polarization process
(in addition to orientation) even in the absence of Wnts.
Among these four Wnts, three (
lin-44
,
cwn-
1, and
egl-20
) are expressed in
the posterior side of seam cells, while
cwn-2
is expressed anteriorly strongest
around the pharynx (
Harterink et al., 2011; Whangbo & Kenyon, 1999;
Yamamoto et al., 2011
). Therefore, seam cells except V5 are polarized in
the normal orientation by a single Wnt gene that is expressed either
anteriorly or posteriorly. When anteriorly (CWN-2) and posteriorly
(CWN-1) expressed Wnts are ectopically expressed in the posterior and
anterior regions, respectively, polarity orientation defects of the triple
Wnt mutant can be rescued (
Fig. 3.6
)
Yamamoto et al., 2011
),
suggesting that Wnts may not function as cues for polarity orientation.
Similarly, EGL-20 functions are suggested to be permissive, as the
expression of EGL-20 in the pharynx using the
myo-2
promoter rescues
polarity reversal of the V5 cell in
egl-20
mutants, although it was
suggested that the
myo-2
promoter may drive EGL-20 expression in
posterior regions close or posterior to V5, arguing against the validity of
this experiment.
In contrast to these experiments that appear to suggest permissive func-
tions of Wnts, LIN-44 instructively orients polarity of the T cell (a posterior
seam cell), as described above. Similarly, it was suggested that EGL-20
instructs polarity orientation of the P7.p cell that positions anterior to the
EGL-20-expressing cell, as its ectopic expression in the anteriorly positioned
anchor cell causes polarity reversal in some genetic backgrounds (see below)
