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Fig. 3. Early eve stripes set parasegment spacing and activate en, while the late stripes ensure maintenance of en expression and repression of slp. (A,B,D,J) Expression patterns of wg (blue) and En (orange), visualized by in situ hybridization and monoclonal (4D9)  -En staining. (C,G,H) Expression pattern of Eve, visualized by polyclonal -Eve staining. (I) Expression pattern of en (blue) and Eve (orange), visualized by in situ hybridization and polyclonal -Eve staining. (K,L) Expression pattern of slp, visualized by in situ hybridization. (A) An eve null mutant rescued by two copies of the wild-type rescue transgene (from –6.4 kb to +9.2 kb); both wg and En are expressed normally, and parasegments are equally spaced. (B) An eve null mutant rescued by a single copy of the same wild-type rescue transgene; note that the odd-numbered parasegments are severely narrowed (all wg and En stripes are expressed, but there are few if any non-en/wg-expressing cells in odd-numbered parasegments). (C) Early Eve stripe expression from a transgene that lacks the early stripe 4+6 enhancer ( 46). (D) The embryonic phenotype of an eve null mutant rescued by 46; note that parasegments 7 and 11 (marked by bars) are severely narrowed, although en stripes 7 and 11 are expressed, at least at early stages (by this stage in this embryo, stripe 11 has almost faded). (E) Wild-type adult fly with normal segmentation. (F) The adult phenotype of an eve null mutant rescued by 46; note that there are two fewer abdominal segments. (G) Normal Eve expression, in Df(2R)eve rescued by two copies of the wild-type rescue transgene. On the right is a magnified view of the boxed region; note that the anterior (left) edge is sharply defined, with the anterior-most cell usually expressing the highest level. (H) Eve expression from two copies of a transgene that lacks the late element (late), in a Df(2R)eve background; note that there is residual expression from early stripes, but that the high level expression at the anterior edge of each early stripe is missing. On the right is a magnified view of the boxed region; note that the anterior edge is less sharply defined than in G, that the stripe appears broader, and that the anterior-most cell row is not usually the highest expressing. The embryo in H is actually over-stained relative to that in G, as suggested by the fact that the stripes appear narrower in G, owing to a lack of detection of the low level expression in the posterior of each stripe. (I) The late rescued phenotype early in gastrulation: odd-numbered en stripes are activated normally (marked by dots); note the regular parasegment spacing (except for parasegment 3, which is slightly narrower due to weaker than normal expression of early Eve stripe 3 in this line). (J) The late rescued phenotype during germ band extension: odd-numbered en stripes are either narrowed or lost (marked by dots), and some wg stripes are expanded posteriorly. (K) Expression pattern of slp (indistinguishable from wild type) in an eve null mutant rescued by the wild-type rescue transgene. (L) Expression pattern of slp in a late-rescued eve null; note that in even-numbered parasegments, slp is expanded posteriorly (into the regions of odd-numbered en stripes, marked by dots).
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indicates repression of target genes. Late Eve expression is required to maintain en expression. The absence of late eve activity (bottom half of figure) results in the expansion of slp expression, and the concomitant loss of en expression, beginning with the anterior of each en stripe. The continued presence of Eve just posterior to slp is thus necessary to prevent ‘encroachment’ of slp into the en stripe, and disruption of the incipient parasegment boundary.