Abstract
The genetic bases of skeletogenesis are expected to shed light on the origins of metazoan biomineralization. Here we review aspects of genetic machineries of invertebrate skeletogenesis, including regulatory genes involved in biomineralization as well, and with an enumerative reference to the genes encoding skeletal matrix proteins. The complete primary structure has been determined for a total of 77 skeletal matrix proteins in invertebrates representing five animal phyla. Presence of repeated sequences and prevalence of acidic proteins stand as common features among those proteins. Similarities are interpreted as convergence because these proteins are not similar at the primary structure level. C-type lectin-like domains are shared by the calcium carbonate skeletal matrix proteins of molluscs and deuterostomes. However, the important sites for carbohydrate binding are not conserved between these two groups. Several arthropod skeletal matrix proteins have the Rebers-Riddiford consensus sequence which is characteristic of noncalcified cuticular proteins of arthropods, indicating that these skeletal matrix proteins were recruited from the non-calcified cuticular proteins after arthropods diverged from other metazoan groups. Dermatopontin, a molluscan shell matrix protein, is also inferred to represent a cooption for biomineralization after molluscs diverged from other metazoan groups based on the molecular phylogemetic analysis. Those findings support the premise that the genetic machineries of biomineralization evolved independently many times after the divergence of metazoan phyla, and that some common genes that served for other functions have been coopted for biomineralization in various lineages.
