Worm Breeder's Gazette 14(3): 18 (June 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.

Preliminary characterization of a maternal-effect mutant defective in gastrulation initiation.

Jennifer Knight, Bill Wood

MCD Biology, University of Colorado, Boulder, CO 80309-0347.

        The genetic control of gastrulation is not well understood in C.
elegans. We have isolated a temperature-sensitive mutation, ct226ts,
which results in failure to initiate gastrulation.  The mutation,
tentatively defining a gene which we propose to call gad-1 (gastrulation
defective), exhibits a strict maternal effect.  At 16C, homozygous worms
grow slightly slower than wild-type, are variably defective in
egg-laying and gonad placement, and lay 24% dead eggs.  At 25C, 100% of
embryos laid by a homozygous hermaphrodite arrest with a clear
gastrulation defect: the E blastomeres do not migrate into the embryo,
and the embryo does not undergo gastrulation.  Analysis of ct226 mutant
embryos with the 4D microscope has shown that the 2E4 division is
premature and in the a-p rather than the d-v direction.  The 4E8
division is also early and in an aberrant plane, and all 8 E cells
remain on the ventral surface of the embryo.  Gastrulation initiation
appears to be completely blocked; there is no delayed migration of the
gut precursors, nor do the mesoderm or germ-line precursors migrate into
the interior of the embryo.
        Despite lack of morphogenesis, terminally arrested defective
embryos have differentiated (though misplaced) muscle and hypodermal
cells, as assayed with monoclonal antibodies MHCA (5.6.1) and MH27,
respectively.  Gut granules are also present, although clearly
mislocalized in a cluster at the very posterior of the embryo.
        We have mapped ct226 to the center of LG V, between unc-83 and
dpy-11.  This region is uncovered by the deficiency nDf18, but not by
the deficiency sDf26.  Embryos laid by ct226/nDf18 hermaphrodites at 16C
are inviable about 90% of the time, a substantially higher penetrance
than for embryos from ct226 homozygotes, suggesting that ct226 is a
hypomorphic allele.  The Gad phenotypes of the inviable embryos from
ct226/nDf18 and ct226/ct226 hermaphrodites are similar.  None of the
lethals in this immediate region, (let-439, let-473, and let-337) fails
to complement ct226. Thanks in part to deficiency endpoint mapping of
nDf18 by C. Malone and A. McGee in the Han lab, we have narrowed the
location of ct226 on the physical map to a small region between the left
endpoint of nDf18  and dpy-11.  We are currently injecting appropriate
cosmids in an attempt to rescue ct226 homozygous mutant animals.
        Previously described mutations in at least seven other genes
cause gastrulation defects; five of these show maternal effects (emb-5,
emb-13, emb-16, emb-23, and emb-31) and two do not [zen-1  (Ferguson et.
al WBG 13(5):66) and end-1(Zhu et. al 1995 Worm Mtg. abstract 33)].  It
is noteworthy that the non-maternal-effect mutants isolated to date have
delayed E cell migration, but are able to complete gastrulation in most
cases, while all the maternal-effect mutants arrest with mislocalized
gut granules and only partial or no gastrulation (1, reviewed in 2).
emb-16(g19ts)  and ct226 display the most severe phenotype, with very
early Ea and Ep divisions and no E cell migration.  Interestingly, these
two characteristics are also shared by embryos in which embryonic
transcription has been blocked by ama-1 antisense RNA (Powell-Coffman,
Knight and Wood WBG 14(1):70).  We intend to combine our
characterization of ct226 with a search for embryonically transcribed
genes required for gastrulation in the hopes of understanding the
functional relationship between maternally and embryonically expressed
genes in the control of gastrulation.

1.  Denich, K.T.R., Schierenberg, E., Isnenghi, E., and Cassada, R.
(1984). Rouxs Arch. Dev. Biol. 193, 164-179.
2.  Bucher, E.A. and Seydoux, G. (1994).  Seminars Dev. Biol. 5,