Worm Breeder's Gazette 10(3): 112

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.

pal-1 Specifies the Fate of V6 Selectively Blocking or Overriding Signals from T

David Waring and Cynthia Kenyon

We are trying to understand the mechanisms that specify V cell 
lineages in the C.  elegans male.  In studying the gene pal-1 we have 
found that V cell fates are specified, at least in part, by a 
mechanism that involves local cell interactions.
In the newly hatched male, six V cells and one T cell are aligned 
along each side of the animal.  V1-V4 each generate identical lineages 
and each produce two seam cells that will produce alae.  V6 undergoes 
additional cell divisions that lead to the production of five ray 
sensilla.  V5 produces one ray one seam cell and also the postdeirid.  
The T cell, which undergoes a lineage rather different from the V 
cells, produces three rays.  Sulston and White(1) first demonstrated 
that these cells are able to respond to environmental cues.  When V6 
is ablated, V5 gradually moves back into the position of V6 and 
generates a lineage similar to or identical to the wild type V6 
lineage.  When V5 and V6 are ablated, V4 moves back and produces rays. 
These ablations have no effect on T.  Likewise, the ablation of T has 
no effect on the V rays.
As we reported at the C.  elegans meeting last year, mutations in 
pal-1 cause V6 to generate a lineage identical to those of V1-V4, and 
so to produce seam cells instead of rays.  V5 and T are unaffected in 
these animals.  Further evidence that V6 is transformed into a V1-V4 
like cell by pal-1 mutations is provided by the pal-1; 
utant.  lin-22 mutations cause V1-V4 but not 
V6 to adopt a V5 like fate(2), and so to produce postdeirids in both 
sexes and rays in males.  In pal-1; 6 also 
produces a postdeirid and rays, as V1-V4 do in a lin-22 background.  (
V6 produces a postdeirid in pal-1; odites.)
When V6 is ablated in pal-1 animals, V5 produces additional rays, as 
it does when V6 is ablated in the wild type.  Similarly, when V5 and 
V6 are ablated in pal-1 animals, V4 can produce rays.  We were puzzled 
by these results.  If V6 is transformed to a V4 like cell, and V4 
makes rays when it moves into the position of V6 following the 
ablations, why does V6 not make rays? We were led (some of us kicking 
and screaming) to the idea that a cell produces extra rays in response 
to the ablations because it senses the absence of its posterior 
neighbor not simply because it moves into a region containing 
different positional information.  We predicted that if we ablated the 
posterior neighbor of V6 in a pal-1 animal, V6 would undergo a wild 
type lineage.  Therefore we ablated T.  We had reason to think that 
ablation of T would have no effect on V6.  First, in the wild type, V 
cells are unaffected by the ablation of T and vice-versa.  Second, T 
rays are unaffected by mutations in mab-5 and mab-3, two mutations 
that affect all the V rays.  Thus it seemed that T might be quite 
different from the V cells and that there might be no interaction 
between them.  To our surprise and delight, when T was ablated in pal-
1 animals, V6 produced rays, (13/13 animals) and in most cases behaved 
as it would in the wild type, producing five rays (8/13).  This has 
been seen with both pal-1 alleles, e2091 and mu13.  V6 does not move 
into the position of T following these ablations.  This suggests that 
the inhibition of ray formation that T asserts on V6 in the pal-1 
mutant is due to direct cell signals, and not simply a consequence of 
a change in V cell position.  It appears that the function of pal-1 is 
to inhibit or override these signals.
The production of rays by V6 following T ablation in pal-1 animals 
closely resembles the production of rays by V5 in response to the 
ablation of V6.  We suggest that V6 inhibits ray formation by V5 in 
the wild type by the same mechanism that T inhibits ray formation by 
V6 in a pal-1 mutant.  In other words, it seems that each V and T cell 
sends signals to its anterior neighbors which inhibit the formation of 
rays (or promote the formation of seam cells), and that these signals 
are selectively inhibited or overridden in V6 by the action of pal-1.  
We predict that there is an analogous activity that overrides these 
signals in V5 to allow the production of the V5 ray.  We do not know 
whether or not our pal-1 alleles are null alleles, thus it is possible 
that residual pal-1 activity in our pal-1 mutants could provide such 
an function.
We believe that these cell signals and pal-1 
olling the activity of the genes mab-5 and 
lin-22.  The inhibition of V6 rays in the pal-1 mutant is dependent on 
lin-22 activity (V6 produces rays in the pal-1; 
utant).  Thus pal-1 behaves as a negative 
regulator of lin-22.  When T is ablated in mab-5 animals V6 does not 
make rays.  The same is true in a pal-1;  mab-
5 activity is therefore required for the production of rays even in 
the absence of signals from T.  This suggests that the function of pal-
1 is to promote the activity of mab-5 in V6.  pal-1 could act by 
interfering with the transmission or reception of the signal from T, 
or by simply overriding the signal by more directly controlling the 
activity of mab-5 and/or lin-22 .
Although local cell interactions appear to specify V cell fates, 
there is also evidence of a separate control system that biases 
anterior V cells toward the production of seam cells and posterior V 
cells toward the production of rays.  For example, Sulston and White 
found that the further anteriorly a cell is located, the less likely 
it is to produce rays following the ablation of its posterior 
neighbors.  In fact, their results suggest that if local cell 
interactions were blocked completely, a variable pattern of rays and 
seam cells would develop, in which the density of rays would increase 
gradually toward the posterior of the animal.  It may be the case that 
the function of local cell signalling mechanism is to convert a 
variable, coarse pattern of seam cells and rays into a highly 
reproducible pattern with sharp boundaries between cell types.