BMP signaling in the dorsal-ventral patterning system of the milkweed bug Oncopeltus fasciatus

BMP signaling plays an essential role in dorsal-ventral (DV) axis patterning throughout the animal kingdom. However, in insects Toll signaling also has a remarkable influence on DV patterning and, in higher branching lineages, fulfills some functions, which depend on BMP signaling in more basally branching lineages. Thus, the DV patterning system of the highly derived fruit fly Drosophila melanogaster is extremely dependent on Toll, which determines also the pattern of dorsal cell fates by specifying the polarity of BMP signaling. However, the wasp Nasonia vitripennis, which belongs to the most basally branching holometabolous lineage, the hymenopterans, uses Toll signaling only as mesoderm inductor. In this study for the first time the DV patterning system of a hemimetabolous insect, the milkweed bug Oncopeltus fasciatus was analyzed. O. fasciatus is a short germ insect, i.e. its posterior segments successively arise from a posterior growth zone, after the onset of gastrulation. In contrast, the anterior segments are synchronously established during the blastoderm stage. A different emergence of anterior and posterior segments might also be reflected in the DV patterning system. To understand the DV patterning system of O. fasciatus candidate genes were knocked down via parental RNA interference (pRNAi) and the resulting phenotypes were investigated for morphological as well as molecular deviations from wild type embryos. Nuclear staining techniques, in situ hybridization (ISH) and antibody staining were performed for this purpose. BMP signaling was found to be able to completely repress mesodermal fates and to be required to restrict it to the ventral side. Furthermore the repression of the BMP inhibitor short gastrulation (sog) seemed also to be mediated by BMP signaling. The lack of DV polarity upon depletion of the extracellular BMP inhibitors Sog and Twisted gastrulation provided further evidence for the high impact of BMP signaling on the O. fasciatus DV patterning system. The absence of DV asymmetry upon depletion of Toll was indicated to be due to a loss of later but not initial expression of sog. These results led to the proposal of a highly self-regulating BMP-dependent DV patterning system for O. fasciatus, which is only polarized by Toll signaling. In addition, differences between the early blastoderm and the later germ band DV pattern in knockdown embryos suggested that the transition of the two dimensional blastoderm DV patterning system into the three dimensional growth zone DV patterning system requires the establishment of two opposing signaling center located close to the posterior pole at the onset of gastrulation