Worm Breeder's Gazette 11(1): 49

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.

Essential Genes Affecting Muscle

Benjamin D. Williams and Robert H. Waterston

Lethal alleles have been recovered previously for a small subset of 
genes that affect muscle in C.  elegans.  A characteristic phenotype 
is associated with many of these mutations: embryos arrest elongation 
at the '2-fold' stage and subsequently die.  Examples include myo-3(
st378), a mutation in the gene that encodes the minor isoform of 
myosin heavy chain in body wall muscle (Waterston, EMBO in press); deb-
1(st555), a mutation in the gene that encodes vinculin, a component of 
the dense body structures of body wall muscle cells (Barstead and 
Waterston, unpubl.); and unc-45(st601), a mutation affecting an as yet 
unidentified gene product that may be involved in thick filament 
assembly (Venolia and Waterston, submitted).  Although the cause of 
elongation arrest in these mutants is not understood, it appears to be 
associated with a catastrophic failure in body wall muscle cell 
function.  The characteristic twitching and rolling motions normally 
observed in wild-type embryos at times shortly before the 2-fold stage 
are nearly absent in the myo-3,   
Correlated with this functional defect is a gross disorganization of 
the myofilament lattice in the body wall muscle cells.
Here we report the preliminary results of a genome-wide screen for 
'2-fold arrest' mutants.  The rationale for this screen is that it may 
allow us to identify new 'muscle affecting' genes.  The most obvious 
alternative approach, isolation of additional visible mutants with 
abnormal muscle function and structure, seems less preferable since 
the visible class is near saturation.
Following EMS mutagenesis, 1700 F1 clones were screened and 20 were 
identified that segregate 1/4 '2-fold arrest' progeny.  To date, 16 
have been analyzed.  Time-lapse video observations show that the 
mutants exhibit a range of motility phenotypes in the period shortly 
before they cease to elongate.  Two of the mutants (st540 and st550) 
show nearly normal vigorous twitching and rolling motions, suggesting 
that the primary defect in these animals may not be in the muscle 
cells.  Mapping and complementation analysis show that st540 is an 
allele of emb-9, which may encode a basement membrane specific 
collagen (Guo, X, and J.  Kramer, 1989 CSH Meeting, abstract 16).
At the other end of the spectrum are the mutants which show greatly 
diminished movement and, in some cases, only very minor twitching.  
These pat (paralyzed, arrested at two-fold) mutants include 5 with 
lesions in previously defined 'muscle affecting' genes.  st536 is an 
allele of lev-11, a gene previously identified through the mutation 
x12, which confers levamisole resistance and also causes an incessant 
twitching of the body wall muscles.  st385 and st386 are additional 
lethal alleles of the deb-1 and myo-3 genes, respectively.  Finally, 
st546 and st549 are alleles of unc-52, a gene originally defined by 
visible mutations that cause the myofilament lattice to grow slowly 
and become disorganized.
Other pat mutants appear to identify new 'muscle affecting' loci.  
st538 and st543 are allelic and map to the cluster on III between dpy-
19 and unc-32: /28)st538(4/28)unc-32.  They define 
the locus pat-2.  Preliminary 2-factor data place st551 within several 
map units of unc-45.  Since this mutation complements unc-45(e286), it 
may also define a new locus.  st541 maps to the cluster on I in the 
interval between dpy-5 and unc-13: 47)st541(37/47)
unc-13.  Since st541 complements the previously defined 'muscle 
affecting' genes in this region (unc-87 and unc-94), it also 
identifies a new locus that affects muscle.  Mapping and 
complementation analysis are in progress for the remaining pat