Worm Breeder's Gazette 13(5): 70 (February 1, 1995)

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.

A screen for essential genes required for asymmetric cell divisions

Helen M. Chamberlin and David L. Baillie

IMBB, Simon Fraser University, Burnaby, BC, V5A 1S6

Many C. elegans genes that are required for specific postembryonic
developmental decisions have a lethal null phenotype.  For example, many
lin-3/let-23 pathway genes and lag pathway genes are essential.  To
identify additional essential genes involved in development, we have begun
a systematic screen of existing lethal mutants for those exhibiting
postembryonic lineage defects.  We have focused on the first asymmetric
cell division of several postembryonic blast cells (lateral and ventral
epidermis, somatic gonad, and male B and Y).  These cells all divide early
(L1 larval stage), and morphological differences between the two daughter
cells are apparent immediately following cell division.  Thus we can
screen all lethal mutants that arrest after L1 for the disruption of these
cell lineages.  To date, we have screened 167 mutations representing 149
loci on LG III(left) and LG V(left) for asymmetric cell division defects. 
We have identified seven candidates; four are weakly (~50%) penetrant for
a Lin phenotype, whereas three are strongly penetrant, including let-765
III, discussed below.

let-765 is identified by a single mutation, s2575 (Stewart et al., WBG
13(4) 95).  In s2575 homozygous males, the first division of the B cell is
disrupted.  In wild-type animals cytokinesis is unequal and the B.a
daughter is larger than the B.p daughter, but in let-765 mutants the cells
are of equal size.  Some other asymmetric cell divisions are not affected
by the let-765 mutation, as the initial Z1/Z4, Pn, and Vn cell divisions
are normal.  s2575 homozygous animals arrest in L2 stage, with no further
cell division in the B lineage.

Multipoint mapping has placed let-765 between sma-4 and unc-36, but to the
left of sma-3.  let-765 is uncovered by sDf127, and s2575/sDf127 animals
arrest after hatching, but prior to postembryonic cell division.  Thus,
s2575 may not represent a null mutation in let-765.  To better understand
the function of let-765, we are currently trying to rescue the gene by
testing cosmids in the region (Collins et al., WBG 13(4) 94).  Once we
obtain rescue, we will be able to use the cosmid transgene as a mini-
duplication to recover additional alleles of let-765.