The shibire locus is characterized by its
temperature-sensitive alleles, which are reversibly paralyzed
by exposure to 29, but are essentially normal at 22 (Grigliatti et al.). Exposure of developing animals to the restrictive temperature for pulses of one to several hours leads to a
plethora of developmental defects, which are specific for the
stage treated (Poodry, Hall, and Suzuki, 1973, Dev. Biol.
66: 442-56) (see following table). Short exposures to restrictive temperatures at the time of delamination of the neuroblasts from the neurogenic ectoderm leads to excess neurogenesis at the expense of epidermogenesis, as seen in the neurogenic mutants (Poodry, 1990, Dev. Biol., in press). Differentiation of myoblasts and neuroblasts is inhibited in shi1
embryonic cells in vitro at 30 (Buzin, Dewhurst, and Seecof,
1978, Dev. Biol. 66: 442-56). Embryonic neurons cultured at
30 show reduced adhesion to the substrate, retardation of
growth cone formation and suppressed neuron formation and
elongation; reversed by shift to permissive temperature (Kim
and Wu, 1987, J. Neurosci. 7: 3245-55). Lethal embryos
disorganized by the restrictive temperature can be cultured in
vivo as tumorous masses (Poodry). Eye-antenna discs can also
be cultured as tumorous masses for several transfer generations (Williams, 1981, DIS 56: 158-61). Primary in vivo culture of cut leg imaginal discs leads to an exceptionally high
rate of transdetermination (Poodry).
The temperature-sensitive alleles differ in the severity of
their paralysis, recovery period, the restrictive temperature
for developmental effects, and in their viability as hemizygotes. They are all hypomorphs, being recessive and having a
more extreme expression in combination with a deficiency than
when homozygous. A wild-type paternal gene can rescue an egg
from a homozygous mother only after 10 hr of development
(Swanson and Poodry, 1976, Dev. Biol. 48: 205-11). Of the
developmental effects tested, all are autonomous in mosaics
generated by somatic recombination or in gynandromorphs (Poodry). The developmental effects on bristles is not enhanced
or suppressed by the presence of temperature-sensitive alleles
of N; shi is epistatic to N (Lujan, 1981, DIS 56: 86).
Physiological studies of shi have revealed the loss of transients in electroretinograms (Kelley and Suzuki, 1974, Proc.
Nat. Acad. Sci. USA 71: 4906-09) and failure of neuromuscular
transmission at the restrictive temperature (Ikeda, Ozawa, and
Hagiwara, 1976, Nature 259: 489-91; Siddiqi and Benzer, 1976,
Proc Nat. Acad. Sci. USA 73: 3253-57), though axonal conduction and muscle membrane excitability are unimpaired (Ikeda et
al.). Exposure of shi1 adults to 29 causes the depletion of
synaptic vesicles from the neuromuscular synapse and their
replacement with large cisternae (Poodry and Edgar, 1979, J.
Cell Biol. 81: 520-27; Koenig, Saito, and Ikeda, 1983, J.
Cell Biol. 96: 1517-22). Accumulation of acetyl choline is
reduced at the restrictive temperature, not because of reduced
synthesis but because of an abnormally rapid rate of release
from the cell, which is not reduced by inhibiting
tetrodotoxin-sensitive nerve activity (Wu, Merneking, and
Barker, 1983, J. Neurochem. 40: 1386-96). Endocytosis is
reversibly blocked in the nerve terminus (Kosaka and Ikeda,
1983, Neurobiol. 14: 207-25; Masur, Kim, and Wu, 1990, J.
Neurosci.) and may limit the ability of nerves to regenerate
synaptic vesicles. Neuromuscular transmission temperature is
sensitive in mosaics in which the neuron but not the muscle is
mutant, but not in the converse situation (Koenig and Ikeda,
1983, J. Neurobiol. 14: 411-19). During recovery from exposure to 30 shits1 muscles display a multimodal distribution of
miniature excitatory junction potential amplitudes never seen
in wild type (Ikeda and Koenig, 1987, J. Physiol. 406: 215-23). Further, as the temperature is increased the amplitude
of evoked excitatory junction potentials decreases; the
numbers of vesicles per synapse displays a correlated decrease
(Koenig, Kosaka, and Ikeda, 1989, J. Neurosci. 9: 1937-42).
Endocytosis is also blocked in the garland cells (Kosaka and
Ikeda, 1983, J. Cell Biol. 97: 499-507). Vesiculation of cell
membranes results in fusion of blastoderm cells (Swanson and
Poodry, 1981, Dev. Biol. 84: 465-70) and vesiculation of surface membranes accompanies secretion of protein epicuticle