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 mutations.