Worm Breeder's Gazette 11(4): 83

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Notch-Head Revisited

Andrew Chisholm

Notch-head worms have been known since Sydney Brenner isolated vab-1(
e2) in October 1967, but the exact nature and cause of this bizarre 
phenotype is as yet unknown.  Notched worms have deranged head 
morphology: in all mutants giving this phenotype the disruption is 
very variable, ranging from a slight displacement of the buccal cavity 
to what appear to be almost two-headed worms.  Morphology is almost 
always worse ventrally.  Highly penetrant Notch mutations have been 
found for the three loci vab-1 II, vab-2 IV, and vab-3 X; The 
phenotypes of vab-1 and vab-2 mutants are very similar, while vab-3 
mutants exhibit amusing pleiotropies.  In the hope that the Notch 
mutants will provide clues to the embryonic morphogenesis of the head, 
I have taken a closer look at them, starting with vab-3.The two 
previously described alleles of vab-3(e648, e1062) appear intermediate 
in strength, based on penetrance.  e41, e1022 and e1178 were found 
lurking in the e list; e1178 is the strongest allele known (60% of a 
brood die as larvae, apparently because the worms cannot take in food).
e2429 is a weak allele found in an outcross of an unrelated mutant.
By chance, I noticed that the gonad in vab-3 hermaphrodites is 
abnormal, as a result of a distal tip cell migration defect.  This Mig 
is similar to that described by Hedgecock for lin-20(e1796).  vab-3 
and e1796 map to the same region on XR, and indeed e1796 fails to 
complement stronger vab-3 alleles for the Mig phenotype, although it 
complements them for Vab.  All the above alleles are assigned to vab-3 
by similarity of phenotype and failure of e1796 to complement for Mig.  
The Mig in strong alleles is fully penetrant, but does not seem more 
severe than in e1796.  Thus, e1796 appears to be a hypomorphic allele 
of vab-3.The other phenotype described for e1796 is that the lineages 
of postembryonic head blast cells are abnormal, in some cases 
transformed to those of posterior homologues (Hedgecock, TINS 1985).  
A preliminary check of e2429 and e648 showed that similar lineage 
alterations occur in these mutants.  In wild-type and e1796, the alae 
fade out anteriorly between the pharyngeal bulbs, but in strong vab-3 
mutants they continue up to the tip (if there is one), or may pass 
through the notch to link up with the alae on the other side of the 
worm; this may suggest a stronger transformation of H0 and H1 terminal 
The head defects in vab-3 worms arise during embryonic development.  
To learn more about their cause, I have started looking at the 
embryonic lineages of vab-3(e1178) using John White's 4-D microscope.  
In one embryo the lineages of the H cells were normal.
vab-3 males cannot mate, as a result of abnormal internal blast cell 
lineages.  Helen Chamberlin (p.c.) has studied the male tail lineages 
in e648 and another putative vab-3 allele, sy66.  In these males and 
one e2429 male lineaged, the B and Y lineages were abnormal: the 
defects could be interpreted as partial transformations of the fates 
of B.al/r to B.p and Y.pl/r to Y.a.  e1796 male tails appear wild-type 
internally, although they have a 'Sixless' ray defect (Baird and 
Emmons, WBG 11.2, p116).  Since none of the stronger alleles has any 
fan defect, this may be due to a second mutation in the strain (mab-18 
maps nearby on X).
The pleiotropies of vab-3 mutants await convincing explanation.  The 
Notch may be a result of abnormal embryonic cell migrations; perhaps 
only mutations weakly affecting this process will be viable, hence the