DAVIS, G.K.*; JARAMILLO, C.A.; PATEL, N.H.: Pax group III genes and the evolution of insect pair-rule patterning

We have investigated the expression of homologues of the Drosophila Pax group III (PgIII) genes paired (prd), gooseberry (gsb) and gooseberry-neuro (gsb-n) in short germ insects. During Drosophila embryogenesis, prd acts as one of several pair-rule genes to define the boundaries of future segments via the regulation of segment polarity genes, including gsb. gsb, in turn, regulates gsb-n, a gene expressed later in the developing nervous system. In order to ascertain the role these genes play in the segmentation of short germ insects, we have taken a two-pronged approach. First, we have developed a polyclonal antibody which crossreacts not only to the products of the three fly genes but also to the homologous proteins of other insects. Using this antibody, we show that the embryonic expression of presumptive PgIII orthologues in the flour beetle Tribolium closely mimics the pattern in Drosophila. Second, we have isolated cDNAs of two PgIII orthologues from embryos of the grasshopper Schistocerca. Using both the cDNAs and the polyclonal antibody, we show that both genes are expressed in ectodermal stripes whose timing and appearance support a role for these genes in both defining and patterning segments. After the appearance of stripes of the segment polarity gene engrailed (en), both are expressed in coincident stripes of a one-segment periodicity. In a manner reminiscent of gsb and gsb-n, each of these stripes is restricted to the neuroectoderm and lies just anterior to its corresponding en stripe. Prior to the appearance of en, however, one of these genes is expressed in stripes of a two-segment periodicity before maturing into its segmental pattern. This early expression is more reminiscent of prd and provides for the first time evidence that pair-rule patterning is a facet of grasshopper segmentation.