opa is expressed in a broad domain encompassing the entire segmented region of the embryo during stages 5-9. At stage 5, expression extends from 16%-78% egg length in ventrally located cells fated to become mesoderm and in dorsally located ectodermal cells. During gastrulation, expression becomes periodic, showing 14 stripes. By late stage 9, expression is no longer seen in the ectoderm or the mesoderm with the exception of a small number of cells in the neurogenic region, the tip of the germ band, and the head. Late in stage 11, opa expression initiates in the visceral mesoderm in seven discrete clusters of cells, arranged in two groups. In stage 12, expression becomes continuous in each region yielding at stage 13, two distinct domains of expression in the visceral mesoderm. The anterior domain corresponds to parasegment 5 and the posterior domain to parasegments 9-12. During stage 15, the 2nd midgut constriction forms between the opa expression domains. At stage 16, the first midgut constriction forms at the posterior boundary of the anterior opa expression domain and the third midgut constriction forms within the anterior margin of the posterior opa expression domain.

opa transcripts are detected in embryonic, larval, and pupal stages with peak levels between 2 and 12 hours of embryogenesis. opa transcripts are first detected in early stage 5 embryos in a 10 cell wide stripe at about 80% egg length. Expression extends posteriorly and becomes uniform over the region from 20% to 80% egg length with the posterior border being more sharply defined than the anterior border. Expression continues in this domain through gastrulation. During germ band extension, a pattern of 14 stripes develops over a low background level of expression.