Worm Breeder's Gazette 11(5): 96

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.

Embryonic Behaviors and Molting in unc-104 Mutants

Ed Hedgecock and David Hall

Figure 1

unc-104 mutations cause a generalized loss of chemical synapses, 
including neuromuscular junctions, and disrupt neural regulation of 
behaviors including locomotion, feeding, and defecation (1).  unc-104 
is a kinesin-related protein and, by inference, functions as a 
microtubule-based motor (2).  Based on behavioral and anatomical 
phenotypes, we suggest this motor is neuron-specific and used for 
anterograde transport of vesicles to presynaptic active zones but not 
for axonal growth.  We examined embryonic and molting behaviors in 
sublethal null rh142 or severe viable rh43 mutants, respectively.  
FLIPPING - Body muscle contractions are required during embryonic 
elongation, possibly to ensure that myofilaments align along the 
longitudinal axis of the hypodermis (3,4).  These contractions begin 
at 430 minutes, shortly after the onset of embryonic elongation (5).  
Although movements are small and localized at first, by 460 minutes (2-
fold) all dorsal or ventral muscles contract synchronously and 
vigorously.  These contractions alternate regularly between dorsal and 
ventral muscles (up to 5 per minute) and continue throughout 
elongation.  Because the embryo is folded within its rigid eggshell, 
each longitudinal contraction creates torque which flips the embryo 
180  about its anteroposterior axis.  At the onset of cuticle 
synthesis, flipping declines abruptly and the larval locomotor pattern 
(spatially & temporally alternating contractions) emerges.  In rh142 
embryos, early body muscle movements closely resemble wild type and 
rates of flipping are comparable throughout elongation (see Figure).  
Flipping declines normally but the larval pattern never emerges.  
Instead animals often assume coiled postures for minutes at a time.  
These embryonic contractions may spread via gap junctions between arms 
of neighboring body muscles (6).  The contraction rate could be 
controlled autonomously by muscle cells.  Alternatively, any of three 
mesoglia (hmc, hmc homolog, GLR), which form gap junctions with body 
muscles in the head, could act as pacemakers (7).
[See Figure 
HATCHING - About 30 minutes before hatch, pharyngeal muscles begin 
contracting at 10-15 cycles per minute; each pumping cycle comprises 
simultaneous contraction of the corpus and terminal bulb, followed by 
contraction of the isthmus (5).  The intestine, initially collapsed, 
quickly becomes distended with ingested fluid.  This extraembryonic 
fluid appears to recirculate through the alimentary tract by a cycle 
of ingestion, and presumably release through the rectum, that 
continues until hatch.  Eggshells invariably rupture within 20-30 
minutes after the onset of pumping.  In most rh142 embryos, the 
pharynx never contracts or quivers only briefly, and the intestine 
remained undistended.  In some embryos, one or more complete pumping 
cycles occur and the intestine becomes slightly distended.  Generally, 
hatch is delayed by 30-120 minutes and some 10% of mutant larvae fail 
ever to rupture their eggshell.  The purpose of embryonic pumping is 
unknown.  The extraembryonic fluid could acquire enzymes necessary for 
digesting the eggshell while recirculating through the pharynx and 
intestine.  Possible sources for these enzymes are the pharyngeal gl 
glands and the intestine itself (5, 8).  Hatch-defective mutants of 
the hch-1 gene may identify a protease for digesting the outermost 
shell (10).  
MOLTING - Pharyngeal gl glands are exocrine cells with axon-like 
processes connecting cell bodies in the terminal bulb to active zones 
of exocytosis at the anterior (g1P) or posterior (g1AL/g1AR) limit of 
the corpus (9).  These cells are continuously active during feeding 
but have a novel cycle of secretion during molts (8).  During the 
lethargus, large secretory granules accumulate in the gland cell 
bodies.  About 1 hr before ecdysis, granules begin entering the g1P 
process but not the shorter g1P processes.  Milling irregularly, they 
gradually fill the proximal segment (isthmus) but do not enter the 
distal segment (corpus).  By 30 minutes before ecdysis, the proximal 
segment of the g1P process and both g1A cell bodies are densely packed 
with granules.  Abruptly, granules in all three cells begin rapid, 
steady anterograde translocation (ca.  1 micron/second) to the active 
zones where they are released within a few minutes.  Possibly these 
secretions help loosen and weaken the old cuticle where it must tear 
apart at ecdysis (8).  In rh43 larvae, lethargus is generally longer 
than normal and the timing of granule synthesis and transport is 
poorly regulated.  However, each molt concludes with a rapid, steady 
anterograde translocation of secretory granules.  These mutants 
suggest that distinct vesicle populations, with separate molecular 
motors, are used for axonal growth, synaptogenesis, and exocrine 
functions.  In particular, secretory granules are likely translocated 
by an anterograde motor distinct from unc-104.

Figure 1