Gene Dmel\eve

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).