Abstract
BackgroundThe coral skeleton consists of CaCO3 deposited upon an organic matrix primarily as aragonite. Currently galaxin, from Galaxea fascicularis, is the only soluble protein component of the organic matrix that has been characterized from a coral. Three genes related to galaxin were identified in the coral Acropora millepora.ResultsOne of the Acropora genes (Amgalaxin) encodes a clear galaxin ortholog, while the others (Amgalaxin-like 1 and Amgalaxin-like 2) encode larger and more divergent proteins. All three proteins are predicted to be extracellular and share common structural features, most notably the presence of repetitive motifs containing dicysteine residues. In situ hybridization reveals distinct, but partially overlapping, spatial expression of the genes in patterns consistent with distinct roles in calcification. Both of the Amgalaxin-like genes are expressed exclusively in the early stages of calcification, while Amgalaxin continues to be expressed in the adult, consistent with the situation in the coral Galaxea.ConclusionComparisons with molluscs suggest functional convergence in the two groups; lustrin A/pearlin proteins may be the mollusc counterparts of galaxin, whereas the galaxin-like proteins combine characteristics of two distinct proteins involved in mollusc calcification. Database searches indicate that, although sequences with high similarity to the galaxins are restricted to the Scleractinia, more divergent members of this protein family are present in other cnidarians and some other metazoans. We suggest that ancestral galaxins may have been secondarily recruited to roles in calcification in the Triassic, when the Scleractinia first appeared. Understanding the evolution of the broader galaxin family will require wider sampling and expression analysis in a range of cnidarians and other animals.
Highlights
The coral skeleton consists of CaCO3 deposited upon an organic matrix primarily as aragonite
Calcification – the deposition of calcium salts, usually to provide a form of skeletal support – is a widespread trait among animals, scleractinian corals are distinguished by the scale on which they carry out this process
Identification of three galaxin-related genes in Acropora Two unigenes identified during EST analyses of Acropora millepora gave strong matches with Gfgalaxin, the gene coding for a soluble protein identified as a major component of the exoskeleton of the coral Galaxea fascicularis [8,11]
Summary
The coral skeleton consists of CaCO3 deposited upon an organic matrix primarily as aragonite. In contrast to the internal skeletons of tetrapods and bony fish, which are based on calcium phosphate in the form of hydroxyapatite, many invertebrate skeletons are composed of CaCO3. This may take the form of either calcite or aragonite, which have distinct structural properties. Both forms are known in cnidarians and molluscs, with the massive exoskeletons of extant scleractinian corals consisting predominantly of the latter. Calcification by corals is known to be strongly enhanced by light (e.g. [1,2]) and is assumed to be driven indirectly by photosynthesis in their dinoflagellate symbionts, but very little is known of the mechanism of calcification or the nature of the organic matrix on which the CaCO3 of the skeleton is deposited
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