Homozygous lethal; embryos homozygous for a null
allele or a deficiency fail to undergo segmentation and their
ventral surfaces are covered by a lawn of short denticles
pointed toward the midline; denticles in the anterior region
show thoracic characteristics suggesting that segmental identities persist in the absence of segmentation. All derivatives of gnathal segments are missing, such as maxillary sense
organs, cirri, mouth hooks, labial sense organs and the mandibular parts of the cephalopharyngeal skeleton; the labrum,
antennal sense organs, and a rudimentary skeleton are the only
remains of the larval head. Posteriorally, anal plates and
some sensory organs persist as do remnants of spiracles,
filzkorper, and tufts. Homozygotes and hemizygotes for hypomorphic alleles display pair-rule segmentation defects. Denticle bands and adjacent naked cuticle of the prothoracic,
metathoracic and even-numbered abdominal segments removed;
some naked cuticle of the adjacent segment removed as well
(i.e., the odd numbered parasegments are removed). Combinations of alleles with Df(2R)eve raised at different temperatures can achieve an array of phenotypes between these
extremes. Expression of eve is first detected at the eleventh
nuclear division following fertilization; at this stage, eve
protein is uniformly distributed among the nuclei, both at the
periphery and deep within the egg; by the thirteenth nuclear
division, the anterior one-third of the embryo is devoid of
detectable protein; over the next 20 minutes, the antibody
staining in the posterior two-thirds of the embryo becomes
concentrated in seven transverse stripes four or five cells
wide separated by stripes three to four cells wide with lower
levels of staining. By the time of germ-band elongation, the
seven stripes have become narrowed and sharply defined and
seven new weakly expressing stripes, one to two cells wide,
appear between the major stripes; during germ-band elongation
all stripes gradually disappear. As eve stripes become more
sharply defined so too do ftz stripes, no longer overlapping
eve stripes, but forming a complementary pattern. At the same
time, a group of expressing cells appears at the posterior end
of the germ band; these cells form a ring around the anal
plate during germ-band shortening. Also during germ-band
shortening, a specific subset of sixteen neurons in each hemisegment of the CNS expresses eve product as does a row of
cell clusters on either side of the dorsal midline; lateral to
these clusters are curious rings of weakly staining cells; the
dorsal cells do not appear to be neuronal (Frasch, Hoey, Rushlow, Doyle, and Levine, 1987, EMBO J. 6: 749-59; Frasch and
Levine, 1987, Genes Dev. 1: 981-95). In homozygous eve1
embryos switched to restrictive temperature during neurogenesis, four specifically studied eve-expressing neurons in
each hemisegment are found to persist; two of them develop
normally, but two send axonal processes to abnormal destinations [Doe, Smouse, and Goodman, 1988, Nature (London)
333: 376-78]. Frasch and Levine observe that segmentation-gene-mutations generally have reciprocal effects on the
expression of eve and ftz, leading them to postulate that
their promoters respond reciprocally to the same positional
cues. eve concluded to be an early pair-rule gene, since its
expression is modified by gap-gene mutations, but not by most
other pair-rule gene mutations nor by segment-polarity gene
mutations. Three pair-rule genes do influence eve expression:
in either eve or h genotypes, eve expression is reduced and in
run embryos eve is overexpressed (Frasch and Levine, 1987).
In eve embryos, en stripes do not appear (Macdonald, Ingham,
and Struhl, 1986, Cell 47: 721-34). Ubx protein is detected
at high level in odd-numbered parasegments from 7 through 13
rather than in every parasegment from 6 through 12 (Martinez-Arias, and White, 1988, Development 102: 325-38). ftz
stripes are disrupted in regularity of position, size, and
timing (Carroll and Scott, 1986, Cell 45: 113-26).
