Worm Breeder's Gazette 10(2): 52

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.

Phenotypes Resulting from Deficiencies of the LGII Cluster

Francine A. Storfer, Paul E. Mains and William B. Wood

We are interested in identifying and characterizing genes that are 
required zygotically for normal embryonic development of C.  elegans.  
As a rapid screen to locate such genes and get a preliminary idea of 
the consequences of their loss of function, we are looking at the 
phenotypes of embryos homozygous for known deficiencies on each of the 
linkage groups.  We have previously reported observations made on 
embryos deficient for the X chromosome (1).  The earliest defect 
displayed by a deficiency embryo should generally be characteristic of 
the null phenotype of the earliest required deleted gene (2).  We 
report here analysis of another set of such deficiencies.  One- and 
two-cell embryos were dissected out of hermaphrodites heterozygous for 
a deficiency and mounted on slides.  Embryos were observed until after 
gastrulation by Nomarski microscopy and time-lapse videorecording, 
incubated overnight at 20 C, and then scored for hatching or embryonic 
lethality.  Lineages were subsequently followed on the videorecording 
through gastrulation for a minimum of two of the embryos that 
subsequently failed to hatch from each of two slides, and the terminal 
phenotypes of all embryos that failed to hatch were noted.  
Additionally, the percentage of embryos dying on slides was compared 
to the percentage of embryos dying on plates, and was found to be 
approximately 25% in all cases, indicating that there was little or no 
non-specific death resulting from the mounting procedure.  The 
analysis was carried out with five deficiencies that uncover loci in 
the cluster of LGII: mnDf96, mnDf98, 
homozygous for mnDf98 or mnDf77 
hatch and arrest as L2 or L1 larvae, respectively.  Analysis of early 
cleavages for these embryos and anatomy of the resulting arrested 
larvae are still in progress.  (Six of the 25 embryos from mnDf98/+ 
hermaphrodites showed an apparent pattern abnormality between the 8-
cell and 30-cell stages, but it is not yet established whether this 
feature is correlated with L2 arrest.)  Embryos homozygous for mnDf96, 
re hatching.  The early cleavages 
and gastrulation of these embryos appeared to be normal.  Their 
terminal phenotypes were as follows.  Embryos homozygous for mnDf96 
displayed twitching and gut granules; two of the six embryos that 
arrested began morphogenesis and progressed to the comma stage.  This 
range of terminal phenotypes is similar to that resulting from let-22(
mn22), which defines a zygotic lethal locus uncovered by mnDf96.  Such 
a result is consistent with the observations of Wieschaus and 
coworkers that the earliest observable defect in a fly homozygous for 
a given deficiency is similar to that resulting from a mutation in the 
earliest acting gene in the region uncovered by the deficiency (2).
Embryos homozygous for mnDf88 arrested before the onset of 
morphogenesis and displayed gut granules.  Lethal alleles defining 
four loci that map to the region uncovered by this deficiency are all 
described as resulting in larval lethality (3).  Three possible 
explanations for the observed embryonic lethality of the deficiency 
are: 1) it reveals the null phenotype for one of the let's, which was 
previously identified only by a hypomorphic point mutation; 2) defects 
resulting from mutations in the four genes are additive, such that 
deleting all of them results in embryonic lethality, or 3) the 
deficiency reveals a new gene essential for embryogenesis.  Embryos 
homozygous for mnDf66 arrested at approximately 200 cells and 
displayed gut granules.  In approximately half the embryos, the 8 E 
cells formed a circle, rather than the normal linear array.  However, 
there was no correlation between the abnormal E-cell configuration and 
embryonic death, although the rounded E cell morphology did appear to 
be specific to progeny of the mnDf66 heterozygote used.
1.  See Abstracts.  (1987) C.  elegans meeting.
2.  Wieschaus, E., C.  Nusslein-Volhard, and G.  Jurgens.  (1984) 
Mutations affecting the pattern of the larval cuticle in Drosophila 
melanogaster.  III.  Zygotic loci on the X-chromosome and fourth 
chromosome.  Roux's Arch.  Dev.  Biol.  193: 296-307.
3.  Sigurdson, D. C., G. J.  Spanier, and R. K.  Herman.  (1984) 
Caenorhabditis cy mapping.  Genetics 10 & 331-