Fig. 4. Model of Eve repressor function in segmentation. Early Eve activity establishes en expression in the proper positions. Reduction of Eve concentration or activity in the syncytial blastoderm (top half of figure; a gradient of both wild-type and reduced eve activity is diagrammed) reduces repression of at least two key target genes, slp and prd. Concentration-dependent effects at the anterior edge of each Eve stripe include expanded prd and slp expression (lighter colored bars) (see also Fujioka et al., 1995; Kobayashi et al., 2001). Prd activates both en and wg, while slp represses en. Thus slp (and possibly other en repressors that are repressed by Eve) can effectively subdivide the prd domain into wg- and en-expressing cells (en and eve repress wg). The border between slp and en becomes the parasegment boundary, and the overall width of the parasegment is largely determined by the location of this border. A dotted line indicates the shifted position of the parasegment boundary when early eve activity is reduced. The net effect of reducing early eve activity is to reduce the width of the odd-numbered parasegments, and to sometimes expand the odd-numbered en stripes, since prd sometimes expands more than does slp. There may also be effects at the posterior border of each early eve stripe, but these appear to be relatively minor. For example, ftz stripe 4 expression does not appreciably expand or shift in the absence of eve stripe 4 (Fujioka et al., 1995), although there may be an anterior shift of some ftz stripes in the absence of eve (see also Hughes and Krause, 2001). PS, parasegments; 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.

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