Worm Breeder's Gazette 9(2): 30

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 Candidate myo-3 Hypomorphic Allele

R.H. Waterston

Figure 1

The gene myo-3 which codes for the minor body wall myosin heavy 
chain myoA (Miller et al PNAS in press), has been localized to LGV 
near Sma-1 (Albertson, EMBO 4, 2493-2498).  Increased accumulation of 
myoA through tandem duplication of the gene suppresses unc-54(0) 
mutations, but hypomorphic or null alleles of the gene have not been 
identified.  One postulate has been that myo-3(0) mutations when 
homozygous are lethal.  To look for lethal mutations that might lie in 
the myo-3 gene, Kathy Kellerman, a rotation student at Washington 
University, and I screened over 700 mutagenized F1 clones of a sma-
1/daf-11 strain (see map below) for clones failing to produce expected 
numbers of normal Sma-1 progeny.  Of the several mutations identified, 
one, st378, resulted In the arrest of homozygous animals as paralyzed 
L1's.  The st378 mutation has several properties consistent with the 
hypothesis that it is a myo-3 hypomorphic allele as described below, 
but the relatively small numbers of purified homozygotes obtained so 
far have frustrated efforts to measure myoA levels directly.
The st378 mutation is closely linked to, and right of, Sma-1 as 
shown on the map below, based on 2 and 3 factor data.  Although the 
Sma-1 mutation has been separated from st378, a chromosome carrying 
only gt378 has not been obtained yet.  The Sma-1(e30) st378 
homozygotes proceed normally through embryonic cell divisions and 
resemble Sma-1 in morphogenesis, with the exception that the double 
mutant never changes its position achieved at the end of elongation.  
Only slight movements of the tip of the head and tail are visible.  
Hatching occurs only slightly later than the wild type sibs.  The 
pharynx pumps and bacterial ghosts are present in the L1 gut, yet the 
animal arrests in the L1 stage without ever unfolding.  ctDf1/e30 
st378 animals have a similar if not identical phenotype.
Electron microscopy of Sma-1(e30) st378 homozygotes show that the 
body wall myofilament lattice is abnormal.  Few normal thick filaments 
are present; in addition there are filaments larger than the normal 
thick filaments of irregular outline.
The st378 is essentially recessive in a wild type background, but is 
semi dominant in unc-56(e190) or unc-15(e73) backgrounds.  The unc-54(
e190); ) st378/++ animals for example are more 
severely paraiyzed than unc-54(e190) homozygotes, and the few animals 
of this genotype that reproduce have very small brood sizes.  sup-3 
mutations in trans counterbalance these effects; e.g.  unc-54(e190); 
07)/sma-1(e30) st378 animals resemble unc-54(e190) 
homozygotes, and are more paralyzed than unc-54(e190); 
07)t+ animals.
The ability in the nematode to identify a protein of interest, to 
recover its DNA and to localize it by cytogenetics, and to obtain 
subsequently mutations in the genomic copy of that gene is an approach 
of considerable potential.

Figure 1