The Engrailed-expressing secondary head spots in the embryonic crayfish brain: examples for a group of homologous neurons in Crustacea and Hexapoda?
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Hexapoda have been traditionally seen as the closest relatives of the Myriapoda (Tracheata hypothesis) but molecular studies have challenged this hypothesis and rather have suggested a close relationship of hexapods and crustaceans (Tetraconata hypothesis). In this new debate, data on the structure and development of the arthropod nervous system contribute important new data (“neurophylogeny”). Neurophylogenetic studies have already provided several examples for individually identifiably neurons in the ventral nerve cord that are homologous between insects and crustaceans. In the present report, we have analysed the emergence of Engrailed-expressing cells in the embryonic brain of a parthenogenetic crayfish, the marbled crayfish (Marmorkrebs), and have compared our findings to the pattern previously reported from insects. Our data suggest that a group of six Engrailed-expressing neurons in the optic anlagen, the so-called secondary head spot cells can be homologised between crayfish and the grasshopper. In the grasshopper, these cells are supposed to be involved in establishing the primary axon scaffold of the brain. Our data provide the first example for a cluster of brain neurons that can be homologised between insects and crustaceans and show that even at the level of certain cell groups, brain structures are evolutionary conserved in these two groups.
KeywordsArthropod Neurophylogeny Evolution Engrailed Tetraconata
We gratefully acknowledge the comments of one of the anonymous reviewers that greatly improved the manuscript. The 4D9 anti-En/invected monoclonal antibody developed by Corey Goodman (University of California, Berkeley, Department of Molecular and Cell Biology, 519 Life Sciences Addition, Berkeley, CA 94720, USA) was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the NICHD and maintained by The University of Iowa, Department of Biological Sciences, Iowa City, IA 52242, USA. S.S. is the recipient of an EC fellowship within the MOLMORPH network under the 6th Framework Programme Marie Curie Host Fellowships for Early Stage Research Training (EST; contract number MEST-CT-2005-020542). The experimental part of this study was supported by DFG grant HA 2540/6 and a Heisenberg fellowship of the DFG to S.H.
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