Shifting roles of pair-rule gene orthologs: segmental expression and function in the milkweed bug .

The discovery of pair-rule genes (PRGs) in revealed the existence of an underlying two-segment-wide prepattern directing embryogenesis. The milkweed bug , a hemimetabolous insect, is a more representative arthropod: most of its segments form sequentially after gastrulation. Here we report the expression and function of orthologs of the complete set of nine PRGs in Seven -PRG-orthologs are expressed in stripes in the primordia of every segment, rather than every-other segment, is PR-like, and several are also expressed in the segment addition zone. RNAi-mediated knockdown of , and impacted all segments, with no indication of PR-like register. We confirm that Of-E75A is expressed in PR-like stripes, although it is not PR in , demonstrating the existence of an underlying PR-like prepattern in These findings reveal that a switch occurred in regulatory circuits leading to segment formation: while several holometabolous insects are "-like," utilizing PRG-orthologs for PR-patterning, most -PRGs are expressed segmentally in , a more basally-branching insect. Thus, an evolutionarily stable phenotype - segment formation - is directed by alternate regulatory pathways in diverse species.