Expression of engrailed-family genes in the jumping bristletail and discussion on the primitive pattern of insect segmentation
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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.
KeywordsSegmentation Short-germ Engrailed Archaeognatha Apterygote insects In situ hybridization
We wish to express our sincerest thanks to Dr. Shigeo Hayashi and Dr. Nao Niwa of RIKEN CDB for their valuable support and advice for the experimentation and critical reading of the manuscript. Thanks are also due to Prof. Tetsuo Hashimoto of the University of Tsukuba for his helpful advice, constant encouragement, and critical reading of the manuscript. We also thank Dr. Kaoru Sekiya for her helpful advice and constant encouragement. This work was supported by a Grant-in-Aid for Japan Society for the Promotion of Science (JSPS) Fellows Grant Number 20·6763 to YN, and Grants-in-Aid for Scientific Research from JSPS (Scientific Research (B) 17370030 and (C) 21570089) to RM. This work is a contribution from the Sugadaira Montane Research Center, University of Tsukuba.
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