Segments, lineage boundaries and the domains of expression of homeotic genes

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Abstract
It is now possible to monitor directly the pattern of activity of homeotic and segmentation genes in theDrosophilaembryo. Precisely bounded domains of expression are established in the blastoderm, at the time when cells became committed to specific segment identities. Some patterns of expression appear in their final form; others evolve rapidly during formation of the blastoderm. Transcripts of the homeotic geneUltrabithoraxaccumulate at high levels in a single parasegment of the blastoderm, and in a block of seven parasegments of the extended germ band. The boundaries of theseUbxdomains appear to lie precisely at presumptive A -P compartment boundaries. During formation of the germ band, the abundance ofUltrabithoraxtranscripts shows a transient segment ‘pair-rule’ modulation. I suggest that this reflects an interaction between theUltrabithoraxgene and a segment pair-rule function, which may serve to establish the precise correlation between lineage boundaries and the domains ofUltrabithoraxexpression.