The unique skeleton of siliceous sponges (Porifera; Hexactinellida and Demospongiae) that evolved first from the Urmetazoa during the Proterozoic: a review

摘要

Sponges (phylum Porifera) had been considered as an enigmatic phylum, priorto the analysis of their genetic repertoire/tool kit. Already with theisolation of the first adhesion molecule, galectin, it became clear that thesequences of sponge cell surface receptors and of molecules forming theintracellular signal transduction pathways triggered by them, share highsimilarity with those identified in other metazoan phyla. These studiesdemonstrated that all metazoan phyla, including Porifera, originate from onecommon ancestor, the Urmetazoa. The sponges evolved prior to theEdiacaran-Cambrian boundary (542 million years ago [myr]) during two major"snowball earth events", the Sturtian glaciation (710 to 680 myr) and theVaranger-Marinoan ice ages (605 to 585 myr). During this period the oceanwas richer in silica due to the silicate weathering. The oldest spongefossils (Hexactinellida) have been described from Australia, China andMongolia and are thought to have existed coeval with the diverse Ediacarafauna. Only little younger are the fossils discovered in the Sansha sectionin Hunan (Early Cambrian; China). It has been proposed that only the spongespossessed the genetic repertoire to cope with the adverse conditions, e.g.temperature-protection molecules or proteins protecting them againstultraviolet radiation.The skeletal elements of the Hexactinellida (model organismsMonorhaphis chuni and Monorhaphis intermedia orHyalonema sieboldi) and Demospongiae (models Suberitesdomuncula and Geodia cydonium), the spicules, are formedenzymatically by the anabolic enzyme silicatein and the catabolic enzymesilicase. Both, the spicules of Hexactinellida and of Demospongiae, comprisea central axial canal and an axial filament which harbors the silicatein.After intracellular formation of the first lamella around the channel andthe subsequent extracellular apposition of further lamellae the spicules arecompleted in a net formed of collagen fibers.The data summarized here substantiate that with the finding of silicatein anew aera in the field of bio/inorganic chemistry started. For the first timestrategies could be formulated and experimentally proven that allow theformation/synthesis of inorganic structures by organic molecules. Thesefindings are not only of importance for the further understanding of basicpathways in the body plan formation of sponges but also of eminentimportance for applied/commercial processes in a sustainable use ofbiomolecules for novel bio/inorganic materials.