Fig. 5.

Evolution of segmentation gene systems in onychophorans and arthropods. In the long-germ insect Drosophila, segmentation is under control of the segmentation gene cascade (reviewed in Pankratz and Jäckle 1990). The gap gene system likely evolved in the lineage leading to Drosophila (see supplementary text for further information). In onychophorans, neither the gap gene system (see supplementary text for further information) nor the downstream acting pair-rule gene (PRG) system is conserved. Dynamic expression of Dl/N signalling genes is not present either. It is thus unclear how exactly the conserved patterns of the segment polarity genes (SPGs) are established in onychophorans. Possibly, SPGs are under direct control of the conserved posterior patterning system (cad, Wnt). Black ovals represent embryos. Gene expression is in light blue. Red X indicates the loss of function during segmentation. Red X with decreased opacity indicates a somewhat unclear situation (in Arthropoda) and an incomplete data set (in Onychophora). Blue circular arrow indicates the dynamic gene expression in the segment addition zone. cad caudal, Dl/N Delta/Notch signalling genes, GAP gap genes, H head, PRG pair-rule genes, SPG segment polarity genes, T trunk