Worm Breeder's Gazette 10(3): 151
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.
Ultrastructure of unc-104 Nerve
Ring
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.