Dorsoventral pattern formation
Final Report Abstract
Dorsoventral axis formation in insects has been most thoroughly studied in Drosophila melanogaster. Here, the Toll signaling pathway plays a crucial role in transmitting dorsoventral spatial cues from the eggshell to the embryo. Asymmetric Toll signaling in the early embryo leads to the formation of a stable nuclear gradient of the transcription factor Dorsal/NF-κB, which regulates the expression of target genes in a concentration-dependent manner and thereby establishes a sequence of different cell fates along the dorsoventral axis. Thus, the specification of the dorsoventral axis by Toll signaling provides a classical example for a morphogen mechanism. How has this mechanism evolved within insects? Dorsoventral patterning in the red flour beetle Tribolium castaneum provides an interesting satellite system for comparison. Here, Toll signaling is dynamic and leads to a temporally changing Dorsal/NF-κB distribution. Within the iBeetle project we could identify genes required for Toll signal activation, which have no direct counterpart in Drosophila. One of them codes for a secreted serine protease, which apparently is involved in activating the Toll ligand. However, expression of this protease itself is activated by Toll signaling constituting a positive feedback loop. The knockdown of this protease does not abolish, but rather slows down Dorsal/NF-κB gradient formation. The resulting phenotypes provide strong evidence that Toll signaling in Tribolium instructs cell fates in a temporal sequence rather than through the spatially static concentration-dependent mechanism found in Drosophila.
