Ultrastructural studies into nephridial development and embryonic coelom fate in arthropods III: reconstructionof ancestral states

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Serial arrangement of nephridia and embryonic coelomic cavities is traditionally considered as evidence for common ancestry of arthropods and annelids. Based on ultra-structural studies of mesoderm development in selected representatives of all high-ranking subgroups of the Arthropoda (Chelicerata, Myriapoda, Crustacea, Hexapoda), we reconsider this evidence in the light of the Ecdysozoa hypothesis, which assumes an unsegmented, nemathelminth-like ancestor for Panarthropoda. Our current insights render the traditional view ambiguous that embryonic coeloms form an ancestral feature of arthropods: while their existence in horseshoe crabs (Chelicerata: Xiphosura) and winged insects (Hexapoda:Pterygota) is confirmed, they proved to be absent in crustaceans as well as basal offshoots of the Chelicerata and Hexapoda. Results obtained thus far indicate that different constraints caused convergent, independent gain of transitory coelomic cavities within arthropods. In order to corroborate this hypothesis, we primarily need to extend our ultrastructural studies to myriapods and tardigrades, for which the relevant data is still missing. Additional inclusion of the Araneae, Remipedia, and Zygentoma aims to test the validity of our current extrapolations for arachnids, crustaceans, and insects. The outgroup comparison will be improved by studies on ovoviviparous velvet worms. This extended taxon sampling will finally permit a reconstruction of the ancestral state in arthropods based on combined cladistic analysis of the data acquired within the scope of the DMP-program.