Segmentation and dorsoventral patterning in Tribolium

Embryos of the red flour beetle, Tribolium castaneum, develop from a short germ rudiment, composed of terminal, head and thoracic primordia covered by extra-embryonic membranes. In contrast, for Drosophila melanogaster, the entire body plan is contained within a long germ band and the extra-embryonic membranes are reduced to a patch of cells on the dorsal surface. While the maternal and zygotic gene systems that direct embryogenesis in Drosophila are the best understood developmental pathways to date, the extent to which they are employed in other insects is just being realized. Homologs of maternal coordinate genes have yet to be found outside the Diptera. However, gap (hunchback, Kruppel), anterior gap (orthodenticle) and pair-rule gene homologs (hairy, runt, fushi tarazu, even-skipped) are expressed in blastoderm embryos. Segment-polarity gene (engrailed, wingless) are expressed at parasegmental borders, and homeotic gene homologs regulate regional identity along the anterior-posterior axis. In addition, homologs of dorsoventral (D-V) patterning genes (decapentapelegic, zerknullt, twist, snail) are expressed early in development. This, homologs of the Drosophila segmentation gene pathway, anterior head development, and D-V patterning system appear to function in similar regulatory networks in Tribolium. Although the genetic mechanisms involved appear to predate the radiation of holometabolous insects, differences in expression patterns reflect evolutionary adaptations to different morphological modes of development. The tractability of genetic analysis in Tribolium offers the potential to study the functional significance of these changes.