Origins of Skeletal Biomineralization: Molecular Evidence
Extended Abstract – Kyoto 2006

Kazuyoshi Endo
Earth Evolution Sciences, University of Tsukuba, Tsukuba, Japan

Number of Words: 321
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Running header text: Origins of Skeletal Biomineralization

Despite controversies over when and how on Earth the animals originated and various body plans diversified, it seems beyond dispute that the “Cambrian Explosion” is real, at least in terms of when animal skeletons were established.  Detailed mechanisms that underlie this abrupt and simultaneous appearance of controlled biomineralization in various animal lineages are still enigmatic, but it is generally agreed that this ability evolved independently a number of times.	
Although observations of osteogenic activities of molluscan shell powder point to a possibility that genetic machineries of skeletonization are shared between vertebrates and molluscs, at least in part, the notion of independent origins has been reinforced by molecular characterization of both the matrix proteins contained in those skeletons and the transcription factors and signal molecules involved in skeletonization.  For instance, BMP, Cbfa 1, Osx and Sox9 are employed in the genetic network to form vertebrate bones and cartilage, while echinoderms use Ets 1 and A1x1 to produce their embryonic spicules, and molluscs use Engrailed and BMP to differentiate their shell-secreting cells.  Matrix proteins entombed in vertebrate bones and teeth, molluscan shells, and skeletons of various other animals such as sponges, corals, shrimps and sea urchins, have certain features in common, e.g. predominance of acidic proteins and possession of repeated structures, but virtually no amino acid sequences are shared beyond phyla.
Deployment of different genetic switches and matrix proteins in those animals in an apparently well organized fashion suggests that production of each of these biominerals is a highly functional and complex process, which may not be induced by simple and single extrinsic factors, such as temperature, oxygen levels, and chemical composition of the ocean, although certain conditions may have been necessary for skeletons to evolve in the first place.  Independent origins of these complex machineries, on the other hand, tend to indicate the strength of adaptational pressures and the versatility of the genetic networks to produce hard skeletons in those ancestral animals.