Specification of thoracic identity in the milkweed bug O. fasciatus. B.T. Rogers , S.W. Herke , N.V. Serio. Dept Biol Sci, Louisiana State Univ, Baton Rouge, LA 70803.

   Experiments in Drosophila and Tribolium suggest that there are two, well defined, distinct regulatory circuits controlling the specification of the thorax. Previous evidence suggests that, in the Drosophila embryo, thorax specification relies on the independent and cooperative activities of the HOX genes and tsh. The specification of thorax in Drosophila imaginal discs and the Tribolium embryo has been shown to be largely dependent on HOX function and is either independent of, or mediated by tsh. A third, more complex model suggests that thorax specification relies on the interplay between the HOX genes, Dll and the HOX cofactors Hth, exd, and tsh. We have used RNA-I to reduce the expression of orthologs of the HOX genes Scr, Antp, Ubx, abd-A and the single tsh ortholog in the milkweed bug O. fasciatus. Milkweed bugs are hemimetabolous and are placed more basally on the phylogenetic tree. They may more closely represent the ancestral state in insects than fruit flies. Our results suggest that in milkweed bugs, apart from the appendages, the HOX and tsh genes are not required for the specification of the thorax. Our evidence further supports the conclusion that there is no general thoracic specifier, but instead, thoracic identity is bestowed upon the appendages (legs) by a different set of factors than either sternites or tergites. We will present evidence for the potential roles of HOX,exd, Dll, tsh,and Hth in the specification of thoracic identity.