Grasshopper hunchback expression reveals conserved and novel aspects of axis formation and segmentation

While the expression patterns of segment polarity genes such as engrailed h ave been shown to be similar in Drosophila melanogaster and Schistocerca am ericana (grasshopper), the expression patterns of pair-rule genes such as e ven-skipped are not conserved between these species. This might suggest tha t the factors upstream of pair-rule gene expression are not conserved acros s insect species. We find that, despite this, many aspects of the expressio n of the Drosophila gap gene hunchback are shared with its orthologs in the grasshoppers S. americana and L. migratoria. We have analyzed both mRNA and protein expression during development, and f ind that the grasshopper hunchback orthologs appear to have a conserved rol e in early axial patterning of the germ anlagen and in the specification of gnathal and thoracic primordia. In addition, distinct stepped expression l evels of hunchback in the gnathal/thoracic domains suggest that grasshopper hunchback may act in a concentration-dependent fashion (as in Drosophila), although morphogenetic activity is not set up by diffusion to form a smoot h gradient. Axial patterning functions appear to be performed entirely by zygotic hunch back, a fundamental difference from Drosophila in which maternal and zygoti c hunchback play redundant roles. In grasshoppers, maternal hunchback activ ity is provided uniformly to the embryo as protein and, we suggest, serves a distinct role in distinguishing embryonic from extra-embryonic cells alon g the anteroposterior axis from the outset of development a distinction mad e in Drosophila along the dorsoventral axis later in development. Later hunchback expression in the abdominal segments is conserved, as are p atterns in the nervous system, and in both Drosophila and grasshopper, hunc hback is expressed in a subset of extra-embryonic cells. Thus, while the ex pected domains of hunchback expression are conserved in Schistocerca, we ha ve found surprising and fundamental differences in axial patterning, and ha ve identified a previously unreported domain of expression in Drosophila th at suggests conservation of a function in extra-embryonic patterning.