Worm Breeder's Gazette 8(1): 26

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.

Egg-Laying Defective Mutants with Abnormal HSN Function

C. Desai, B. Horvitz

The two 'hermaphrodite-specific neurons' (HSNs) of C.  elegans are 
crucial for wild-type egg-laying.  The unique pharmacological 
characteristics of hermaphrodites lacking the HSNs have provided us 
with an opportunity to study the genetics of the development and 
functioning of this single class of neuron: whereas wild-type 
hermaphrodites are stimulated to lay eggs by both exogenously added 
imipramine and serotonin, worms in which the HSNs have been eliminated 
(either by laser microbeam ablation or by mutation, as in unc-86 or 
egl-1 animals) are responsive only to serotonin.  Such HSN worms 
retain eggs but eventually release most or all of their progeny.  In 
the dissecting microscope HSN animals are obviously swollen and are 
whitish due to the presence in the uterus of late stage eggs and, 
sometimes, hatched larvae.
The progeny of 40,000 F1's from EMS-mutagenized parents were 
screened for animals with this swollen phenotype.  Egl candidates were 
picked and their progeny tested with serotonin and imipramine.  Forty-
five mutants sensitive to serotonin and resistant to imipramine were 
isolated.  These mutations define at least 16 genes, including five 
that were previously known: tra-2 II, unc-86 III, egl-5 III, egl-1 V 
and egl-10 V.  New loci are defined by (n1086)  n995,n996) II; (n997) 
II; (n998,n1079,n1080,n1087) II; (n999) n1077) V (because these 
mutations are dominant they are only tentatively assigned to the same 
locus); (n1081,n1082) V; (n1108) V; (n1075,n1076) V; and (n1107) X.  
Mutants defective in eight of these loci have normal HSN cell bodies 
as observed with Nomarski optics.  Mutants defective in three other 
loci have variably missing and often abnormal HSNs, while mutants 
defective in the remaining five loci completely lack HSN cell bodies.
The HSNs in mutants defective in two of these last five loci --egl-1 
and egl(n1069) appear to lack HSNs because these cells undergo 
programmed cell death.  All seven mutations defining these two genes 
regain both wild-type egg-laying behavior and HSN cell bodies in 
double mutants with ced-3, which prevents programmed cell deaths.  
Mutations in both loci are dominant and of variable expressivity.  
These two genes map within 1 m. u. of each other on linkage group V, 
but are separated by him-5.  All alleles of egl-1 are heat-sensitive, 
whereas all alleles of egl(n1069) are cold-sensitive.  Mutations in 
these two genes interact, as the double heterozygote is more Egl than 
expected from the penetrance of the single heterozygotes alone.
Genes that control HSN cell deaths may be involved in the expression 
of sexual phenotype.  For example, the HSNs in egl-1 and egl(n1069) 
may undergo the normally male specific programmed cell death in the 
hermaphrodite.  In addition, alleles of the general sex determination 
loci may result in a partial sexual transformation that leads to the 
deaths of the HSNs in hermaphrodites; both her-1 and tra-2 alleles of 
this class have been previously identified (Trent, Tsung and Horvitz, 
Genetics 104, 619, 1983).  We have recently isolated a new tra-2 
allele, n1106, that is also of this class.  n1106 results in a very 
low fraction of intersex animals, a high fraction of Egl 
hermaphrodites and a few wild-type hermaphrodites.  Most n1106 animals 
lack both HSN cell bodies, and most of the rest lack one.  As expected,
the HSNs are restored and the egg-laying defect of tra-2(n1106) is 
largely suppressed by ced-3, whereas other phenotypic indications of 
maleness are not.