Pax group III genes and the evolution of insect pair-rule patterning

Pair-rule genes were identified and named for their role in segmentation in embryos of the long germ insect Drosophila. Among short germ insects these genes exhibit variable expression patterns during segmentation and thus ar e likely to play divergent roles in this process. Understanding the details of this variation should shed light on the evolution of the genetic hierar chy responsible for segmentation in Drosophila and other insects. We have i nvestigated the expression of homologs. of the Drosophila Pax group III gen es paired, gooseberry and gooseberry-neuro in short germ flour beetles and grasshoppers. During Drosophila embryogenesis, paired acts as one of severa l pair-rule genes that define the boundaries of future parasegments and seg ments, via the regulation of segment polarity genes such as gooseberry, whi ch in turn regulates gooseberry-neuro, a gene expressed later in the develo ping nervous system. Using a crossreactive antibody, we show that the embry onic expression of Pax group III genes in both the flour beetle Tribolium a nd the grasshopper Schistocerca is remarkably similar to the pattern in Dro sophila. We also show that two Pax group III genes, pairberry1 and pairberr y2, are responsible for the observed protein pattern in grasshopper embryos . Both pairberry1 and pairberry2 are expressed in coincident stripes of a o ne-segment periodicity, in a manner reminiscent of Drosophila gooseberry an d gooseberry-neuro. pairberry1, however, is also expressed in stripes of a two-segment periodicity before maturing into its segmental pattern. This ea rly expression of pairberry1 is reminiscent of Drosophila paired and repres ents the first evidence for pair-rule patterning in short germ grasshoppers or any hemimetabolous insect.