Worm Breeder's Gazette 10(3): 151

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Update of unc-104: A Mutation Affecting Chemical Synapse Formation

David H. Hall and Ed Hedgecock

Figure 1

Ultrastructure of unc-104 Nerve 
As we previously reported (WBG 10(1), 1987) the rh43 allele of unc-
104 has virtually no directed chemical synapses in the ventral nerve 
cord, neither between neurons nor NMJs, although electrical synapses 
are still present.  We have now conducted an extensive survey of the 
nerve ring of two alleles, rh43 and e1265, and compared these to wild 
type adults.  Serial thin section series were collected covering the 
nerve ring and ventral ganglion.  Approximately every sixth section 
was sampled - photographed at high power and printed to look for 
evidence of chemical synapses.  Transverse sections of the wild type 
nerve ring show an average of 13 synapses (comparing data from 9 
animals), while the ventral ganglion neuropil averages about 6 
synapses per section in the anterior portion, and less than 2 synapses 
per section posterior to its split into two parallel bundles.  The 
e1265 allele, which is severely paralyzed, shows about a 50% deficit (
see table below) (6 animals sampled).  The rh43 allele is perhaps even 
more paralyzed than e1265, and shows an 80% deficit (6 animals sampled)
.  In each allele, the deficit is equally striking in NMJs as in 
synapses between neurons.  Contrary to our previous report, there are 
occasional synapses in rh43 (and many in e1265) which appear 
completely normal.  For the analysis of synapse numbers shown in the 
table, a less rigorous definition of a synaptic contact was used.  
Remarkably, the overall cytology of the nerve ring is not severely 
disrupted, as the number of processes entering the ring appears normal 
and the nerve cords are positioned appropriately.  The most striking 
deficit, aside from the lack of synapses, is the general failure of 
synaptic vesicles to leave the somata.  Many of the neuron somata in 
the ventral, lateral and anterior ganglia contain abnormally high 
numbers of vesicles, while most axon profiles in the ring are 
diminished in caliber and very deficient in synaptic vesicles.  Due to 
the reduced caliber of most processes, the unc-104 nerve ring is 
reduced in volume.  It remains to be seen whether the remaining 
synapses in unc-104 animals represent certain categories of contacts, 
or whether they are distributed randomly.  A deficiency in a 
structural protein (part of the presynaptic density, for instance) 
might affect synapses at random.  An axonal transport defect might 
spare synapses occurring proximal to the soma of the presynaptic 
neuron (e.g.  outputs from cells like AIY, CEP, URA, IL2).  A cell-
cell recognition defect might alter the local 'neighborhoods' of some 
processes, thus denying them opportunities to form normal contacts.  
Whichever the case, the ability of the nervous system to assemble 
itself so well in unc-104 is surprising.
[See Figure 1]
Revertants of unc-104unc-104 (e1265; LG II) was isolated by D.  
Riddle following nitrosoguanidine treatment.  e1265 strains grow 
slowly and are severely uncoordinated.  unc-104 (rh43) was obtained 
following ethylmethanesulfonate treatment.  rh43 strains grow more 
slowly and are perhaps more completely paralyzed than e1265 mutants.  
Both mutations are recessive to the wild-type allele.  We obtained 3 
partial revertants of unc-104 (e1265) (rh123, rh124, rh125} and 2 
revertants of unc-104 (rh43) {rh126, rh127} from among the progeny of 
about 400 EMS-treated hermaphrodites of each strain.  A third rh43 
revertant {rh128) arose spontaneously in our laboratory stock.  We 
have examined the rh123, rh126, rh127, and rh128 revertants in more 
detail.  All four mutations appear to be intragenic or closely-linked 
to unc-104.  The phenotypes vary somewhat between strains, e.g., unc-
104 (rh43, rh126) mutants have wild type movement at 15 C and 20 C but 
are mildly uncoordinated at 25 C while unc-104 (rh43, rh128) mutants 
are severely uncoordinated at 15 C but only moderately uncoordinated 
at 20 C and 25 C.  Heterozygotes of each of the four revertants and 
unc-104 (rh43) are intermediate in phenotype.  The residual UNC 
phenotypes of homozygous revertants are recessive to the wild type 
chromosome II.  In light of the behavioral and ultrastructural 
differences between the e1265 and rh43 strains, it is moot whether 
rh43 represents a complete loss of gene function.  Conceivably, unc-
104 is an essential gene.  Ultrastructural and molecular analysis (
Jeyaprakesh and Otsuka, pers.  comm.) of the intragenic revertants 
described here may help clarify the mechanism by which unc-104 affects 
synaptic structure, formation or function.

Figure 1