Worm Breeder's Gazette 8(3): 51

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.

Ups and Downs of the Lateral Microtubule Neurons

E. Hedgecock

Figure 1

The ALM neurons are located on the sides of the animal somewhat 
anterior to the gonad.  They are embryonic sisters of the BDU neurons 
located nearer the head.  In the wild type, the ALM/BDU mother cell 
divides fairly late (430 min) and the ALM daughter migrates posterior 
to its final position.  The ALM extends a process anteriorly along the 
edge of the dorsal body muscles and the cell body rests against the 
muscle edge in newly hatched larvae.  In the homeotic mutant lin-32(
e1926), the ALM cell bodies are displaced anterior of their normal 
position.  In a small fraction of the neurons, the cell bodies are 
located against the ventral muscle edge and extend a process 
anteriorly along it.  Some of these ventral processes switch again to 
the dorsal muscle edge as they approach the nerve ring but most remain 
ventral.  The ALM neurons in early L1 larvae are usually positioned 
laterally and near BDU.  We speculate that they differentiate later 
than normal.  Cynthia Kenyon (previous Newsletter) has found that the 
Q/V5 mother cell divides late in this mutant.  Conceivably the ALM/BDU 
division is similarly delayed.  In any event, the ALM neurons appear 
able to grow along the ventral muscle edge as a second choice to their 
normal path.
The PLM neurons are located in the tail and normally extend axons 
anteriorly along the ventral muscle edges.  The PLM cell bodies are 
displaced anteriorly in lin-32(e1926) mutants perhaps reflecting 
delayed maturation.  No misplaced PLM axons were observed.  M.  
Chalfie (personal communication) has found a new lin-32 allele that 
apparently blocks the birth or differentiation of the PLM neurons 
The PLM axons in unc (rh34) mutants sometimes extend along the 
dorsal muscle edge. These displaced processes appear to extend forward 
for their normal distance and stop somewhat before the ALM cell body.  
In a few animals, we have followed the displaced processes back into 
the tail where they have ended in exceptionally large soma.  The rh34 
mutant has not been mapped but its phenotype, blocked cytokinesis in 
certain postembryonic lineages, closely resembles the published 
description of unc-59 mutants (Sulston and Horvitz, 1981).  We 
speculate that certain late embryonic cell divisions also fail in rh34 
including the PLM/ALN division.  Interestingly, the ALN axon normally 
arows along the dorsal muscle edge.  We speculate that when the 
division fails, a hybrid cell is formed with the distinctive process 
specializations of the PLM neuron but the dorsal preference of the ALN 
In lin-30 (e1908; LGIII) mutants, cytokinesis fails in many late 
embryonic and postembryonic divisions including the ALM/BDU division.  
In rare animals where this division occurs.  the ALM and BDU neurons 
are normal.  We have not yet looked at the axons in the blocked cells 
but such cells remains lateral at the normal BDU position.
[See Figure 1]

Figure 1