Worm Breeder's Gazette 15(2): 27 (February 1, 1998)

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.

Explaining pal-1 embryonic phenotypes

Lois Edgar, Bill Wood

MCD Biology, University of Colorado, Boulder, CO 80309

        Deletion mutations of the caudal homolog pal-1 cause severe
posterior defects in embryonic morphogenesis.  We have continued
analysis of embryonic pal-1  expression by lineage analysis of a
pal-1::GFP transgenic line which shows an expression pattern very close
to that of antibody staining (Craig Hunter, thanks again for the
antibodies), to extend and correct our earlier lacZ reporter data.  In
the GFP line, all cells of the early C and D lineages express PAL-1::GFP
from the 4C and 4D stage until early morphogenesis; this corresponds to
the antibody staining seen in  staged embryos.  The MS descendants M and
mu int R and their dying sisters (rather than Z1 and Z4 as we earlier
thought) express the reporter.  Ventral AB descendants plpappp, plppppp,
and prppppp, and their daughters express strongly; other ABpxpppxx
neighboring cells stain weakly with antibodies and occasionally express
GFP.  These cells gastrulate to interior positions and generate a mixed
bag of mesectodermal cell types: neurons, muscles, and hypodermis of the
rectal and tail regions.  Their pal-1 expression, except in the rectal
cells, is gone before hatching.  Transient antibody staining is
additionally seen in the posterior P and some V cells as they move to
enclose the body; this has not been seen with the GFP reporter. 
Finally, the two int-5 intestinal cells express the GFP reporter from
early morphogenesis through hatching.  Overall, pal-1  is expressed
transiently in the posterior in a number of cell lineages during the
period of morphogenesis.
        4D lineage analysis of pal-1(ct224) mutant embryos reveals only
minor lineage defects, but gross defects in cell positions and movements
of PAL-1-expressing cells.  About 80% of ct224  homozygous embryos hatch
with a Nob terminal  phenotype; the remainder fail in hypodermal
enclosure.  We previously reported mispositioning of the dorsal
hypodermal C cells as early as the 8C-cell stage, resulting in an
ill-defined or skewed dorsal midline; and a failure of the contralateral
nuclear migration and elongation in these cells during early
morphogenesis.  We have now analysed two ventrally oriented embryos, one
enclosed and one unenclosed.  In the C muscle lineage, some cells failed
to make the terminal division and appeared to differentiate early,
albeit as muscle.  C muscle was spatially disorganized, with some Cpp
cells not moving to the normal right-side positions.  Severe ventral
defects were observed in the process of late gastrulation (just
pre-lima, ~350 cells).  Normally, the mesectodermal contralateral
homologs Abplpappp/prpappp and plppppp/prppppp initially border a
'ventral  cleft' [Sulston et al.(1983), Dev. Biol. 100:64-119].  At
about 3.5 hr post-fertilization these cells move from lateral surface
positions to meet at the ventral midline; at the same time the ABpxppppp
cells contact the most posterior of the C hypodermals Caapv and Cpapv. 
The cells appear to adhere on contact and the ventral cleft rapidly
closes from posterior to anterior as these cells divide.  Subsequently
the lateral hypodermal row of P cells  moves ventrally to effect
hypodermal enclosure [Williams-Masson et al.(1997), Dev.
Biol.124:2889-2901].  All of these cells except ABprpappp express pal-1 
during this period.  In the unenclosed ct224 embryo, these AB:ventral
midline and C:posterior contacts never occurred and the ventral cleft
remained open; hypodermal enclosure also failed.  In the Nob ct224
embryo the ventral cleft closed late, after the bordering cells divided,
and the posterior AB:C contact was transient.  Hypodermal enclosure was
delayed at least 2 hr, and no posterior elongation occurred.
        In summary, patterning defects suggesting failures in cell-cell
recognition or some other positional cues have been observed in most of
the cell lineages expressing pal-1.  Together these defects appear
sufficient  to explain the observed embryonic phenotypes.