Worm Breeder's Gazette 10(3): 106

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Muscle Positioning and Attachment Mutants

E. Hedgecock

Figure 1

Figure 2

The 95 body muscles are attached to the dorsal and the ventral 
hypodermis in two wide, longitudinal bands, each roughly four cells 
abreast.  In unc-5, we observed 
that occasional cells are positioned obliquely on the lateral 
hypodermis, bridging dorsal and ventral muscle quadrants (Fig.  1A).  
A second, perhaps unrelated, defect is that muscle cells, which are 
normally rhomboid (Fig.  2A), often fork where they abut their 
neighbors (Fig.  2B).  Such split sarcomeres also have been observed 
in unc-104 mutants and may reflect an influence of muscle innervation 
or contraction on cell shape.
Mispositioned muscle cells can create permanent bends or kinks in 
body posture.  When many cells are mispositioned, individuals are 
locked into rigid postures, usually irregular circles ('cheerios').  
We have used this phenotype to make a small collection of muscle 
positioning (mup) mutants in which variable numbers of body muscle 
cells bridge between dorsal and ventral muscle quadrants (Figs.  1A, 
1B).
A second class of mutants, similar in gross phenotype, have 
abnormally weak muscle attachments (mua) to the body wall.  These 
cells tear away from the hypodermis during embryonic, early, or late 
larval stages depending on the mutant.  In unc-23, for example, 
muscles progressively detach from the head on backwards during late 
larval stages (Waterston et al., Dev.  Biol.  77, 271 (1980)).  The 
Drosophila mutant, lethal myospheroid, disrupts a  -integrin gene.  
Conceivably the various mua mutations disrupt nematode genes that 
provide mechanical strength to muscle-hypodermis or muscle-muscle 
attachments.  The force of muscle contraction probably increases as 
the myofilament volume (Q3) while the area of muscle-hypodermal 
contact increases as Q2.  In mua mutants, a 10-fold increase in body 
length, causing a proportionate increase in contractile 
force/attachment area, may tear weakly attached cells from the body 
wall at characteristic forces, and hence, larval sizes.
[See Figures 1 & 2]

Figure 1

Figure 2