Expression of engrailed-family genes in the jumping bristletail and discussion on the primitive pattern of insect segmentation.

It has been shown that segmentation in the short-germ insects proceeds by a two-step mechanism. The anterior region is simultaneously segmented in a manner similar to that in Drosophila, which is apparently unique to insects, and the rest of the posterior region is segmented sequentially by a mechanism involving a segmentation clock, which is derived from the common ancestor of arthropods. In order to propose the evolutionary scenario of insect segmentation, we examined segmentation in the jumping bristletail, the basalmost extant insect. Using probes for engrailed-family genes for in situ hybridization, we found no sign of simultaneous segmentation in the anterior region of the jumping bristletail embryos. All segments except the anteriormost segment are formed sequentially. This condition shown in the jumping bristletail embryos may represent the primitive pattern of insect segmentation. The intercalating formation of the intercalary segment is assumed to be a synapomorphic trait shared among all insects after the branching of the jumping bristletail.