Worm Breeder's Gazette 14(2): 65 (February 1, 1996)

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.

Intercellular interactions and 2' vulval lineage polarity.

Wendy S. Katz2, Thomas R. Clandinin1, Paul W. Sternberg1

1 Division of Biology and Howard Hughes Medical Institute, California Institute of Technology, Pasadena CA 91125.
2 Department of Biochemistry, University of Kentucky Medical Center, 800 Rose Street, Lexington, KY 40536-0084.

The vulva is formed from a symmetric 1' lineage flanked by two asymmetric
2' lineages.  These asymmetries normally have invariant orientation with
respect to the center of the vulva.  The 4 vulval precursor cell (VPC)
descendants distal to the anchor cell adhere to the ventral body wall,
while the 3 or 4 VPC descendants proximal to the anchor cell detach from
the ventral body wall and move to the dorsal region of the ventral cord. 
We are performing ablation experiments to investigate the role of gonad
and of neighboring VPCs or their descendants in orienting this asymmetry. 
If a signal is required to establish as well as orient asymmetry, we would
expect loss of asymmetry following ablation of a signal source.  If a
signal is required to orient an existing asymmetry, ablation might cause
the asymmetries to become uniformly or randomly oriented.
We previously demonstrated that asymmetry is retained after gonad
ablation, with uniform posterior orientation (WBG 11(4):98).  These
experiments were performed in lin-12(n137gf) mutants, in which all 6 VPCs
adopt 2' fates, independent of the gonad.  Because we were concerned that
the gain of function mutation might aberrantly affect 2' lineage
orientation, we have developed two alternate strategies for inducing 2'
fates in gonad-ablated animals.  The first strategy uses the EGF domain of
LIN-3, the vulval inductive signal, under control of a heat shock
promoter.  Upon mild heat shock of gonad ablated animals, this construct
induces some 2' fates with no 1' fates[1].  The second strategy uses
let-23(sa62), a gain of function mutation in the receptor for inductive
signal.  After gonad ablation,  sa62/+ heterozygotes display 3' and 2' but
no 1' fates[2]. In both of these backgrounds, after ablation of the gonad
we observe mostly posteriorly-oriented lineages under conditions that
induce 2' but no 1' fates:
genotype       #posteriorly-oriented        #anteriorly-oriented
sa62/+                15/15                             0/15
hsLIN-3EGF, mild hs   19/20                   1/20

These results support the conclusions of the lin-12(n137) experiments:
first, that neither the gonad nor a neighboring 1' VPC or its descendants
are required to establish the asymmetry of 2' lineages, and second, that
the 2' lineages either have a posteriorly-oriented default state, or that
a weak orienting signal is made in the posterior of the animal and is
normally overriden by the orienting signal from the gonad.
lin-12(gf) mutants lack an anchor cell, the source of inductive signal,
suggesting that LIN-3 is not the orienting signal from the gonad.  To test
this genetically, we examined lin-12(n137); lin-3(n378/n1059), the
strongest viable lin-3 loss of function genotype.  16/16 of these double
mutants displayed anteriorly-oriented lineages, supporting the hypothesis
that the orientation signal is distinct from the inductive signal.  Thus
ablation of the gonad but not removal of lin-3 prevents anteriorly
oriented lineages.  We are currently examining 2' lineage orientations in
animals mutant in other known signalling pathways, and investigating the
role of VPC-VPC interactions in vulval lineage orientation.

[1]Katz, W.S,  R.J. Hill, T.R. Clandinin and P. W. Sternberg (1995).
Different levels of the C. elegans growth factor LIN-3 promote distinct
vulval precursor fates.  Cell 82: 297-307.
[2]Katz, W. S., G. Lesa, D. Yannoukakos, T.R. Clandinin and P. W.
Sternberg (in press). A point mutation in the extracellular domain
activates LET-23, the C. elegans EGF Receptor homolog.  Mol. Cell. Biol.