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Fig. 1. Defining a minimal version of so7 important for ocellus development. (A) Genomic map of the last intron of the so locus between exon6 (E6) and exon7 (E7) (black boxes). The physical mapping is indicated as: A, AseI; S, SspI; E, EcoRI; K, KpnI. An enlargement of the region deleted in so¹ shows relative positions of the enhancer fragments so7, so10 and so9. so10 contains EY/TOY- and TOY-specific binding sites (black boxes and black triangles, respectively). So9 harbours the SO-binding site. The deletion map illustrates the constructs that were tested for the expression pattern they mediate (X indicates introduced mutations). Nr, number. Twenty-seven bp fused to so10 (number 14) are sufficient to resemble the so7-mediated pattern including ocellus-expression (B, referred to as +++ expression in ocellus and +++ expression in compound eye). All constructs including so10 but missing the 27 bp (represented by number 21) just resemble the so10-pattern and show no expression in the ocellar region (indistinguishable from D). Constructs that are devoid of functional EY and TOY sites (number 17, 18, 19 and 21) but include the 27 bp of number 21, show an expression pattern identical to so9 (C, illustrated by the pattern mediated by number 21). Fragment number 21 is sufficient to recapitulate the expression pattern of so9; it is referred to as soAE in the article. Construct number 20 (no so10 and no number 21) does not mediate any expression at all (–). (B-E) Arrows indicate the ocellar region. (B) Expression pattern mediated by number 14, similar to wt so expression pattern and the so7-mediated pattern. (C) Number 21-lacZ: expressing only along the posterior margin of the eye disc similar to so9-lacZ. (D) so10-lacZ: resembling wt-expression despite the ocellar signal. (E) So7mut: so7 where the SO-binding site is mutated mediates only weak expression to the ocellar region (fragment number 22). (F) Vertex region of a wt fly showing three normal ocelli. (G) so driven by so10-soAE (number 14) rescues ocellus development of so¹ mutant flies. The lateral ocelli appear almost normal (arrowhead). The size of the anterior ocellus is reduced (empty arrowhead).

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