Kawasaki Kazuhiko, Amemiya Chris T
J Exp Zool B Mol Dev Evol. 2014 Sep;322(6):390-402. doi: 10.1002/jez.b.22546.
The coelacanth is the basal-most extant sarcopterygian that has teeth and tooth-like structures, comprising bone, dentin, and enamel or enameloid. Formation of these tissues involves many members of the secretory calcium-binding protein (SCPP) family. In tetrapods, acidic-residue-rich SCPPs are used in mineralization of bone and dentin, whereas Pro/Gln-rich SCPPs participate in enamel formation. Teleosts also employ many SCPPs for tissue mineralization. Nevertheless, the repertoire of SCPPs is largely different in teleosts and tetrapods; hence, filling this gap would be critical to elucidate early evolution of mineralized tissues in osteichthyans. In the present study, we searched for SCPP genes in the coelacanth genome and identified 11, of which two have clear orthologs in both tetrapods and teleosts, seven only in tetrapods, and two in neither of them. Given the divergence times of these vertebrate lineages, our discovery of this many SCPP genes shared between the coelacanth and tetrapods, but not with teleosts, suggests a complicated evolutionary scheme of SCPP genes in early osteichthyans. Our investigation also revealed both conserved and derived characteristics of SCPPs in the coelacanth and other vertebrates. Notably, acidic SCPPs independently evolved various acidic repeats in different lineages, while maintaining high acidity, presumably important for interactions with calcium. Furthermore, the three Pro/Gln-rich SCPP genes, required for mineralizing enamel matrix and confirmed only in tetrapods, were all identified in the coelacanth, strongly suggesting that enamel is equivalent in the coelacanth and tetrapods. This finding corroborates the previous proposition that true enamel evolved much earlier than the origin of tetrapods.
腔棘鱼是现存最原始的具有牙齿和类牙结构的肉鳍鱼类，这些结构由骨骼、牙本质和釉质或类釉质组成。这些组织的形成涉及分泌型钙结合蛋白（SCPP）家族的许多成员。在四足动物中，富含酸性残基的SCPP用于骨骼和牙本质的矿化，而富含脯氨酸/谷氨酰胺的SCPP参与釉质形成。硬骨鱼也利用许多SCPP进行组织矿化。然而，硬骨鱼和四足动物的SCPP库有很大不同；因此，填补这一空白对于阐明硬骨鱼类矿化组织的早期进化至关重要。在本研究中，我们在腔棘鱼基因组中搜索SCPP基因，鉴定出11个，其中两个在四足动物和硬骨鱼中都有明确的直系同源基因，七个仅在四足动物中有，另外两个在两者中都没有。考虑到这些脊椎动物谱系的分化时间，我们发现腔棘鱼和四足动物之间共享许多SCPP基因，而与硬骨鱼没有，这表明早期硬骨鱼类中SCPP基因的进化方案很复杂。我们的研究还揭示了腔棘鱼和其他脊椎动物中SCPP的保守和衍生特征。值得注意的是，酸性SCPP在不同谱系中独立进化出各种酸性重复序列，同时保持高酸度，这可能对与钙的相互作用很重要。此外，在腔棘鱼中也鉴定出了三个参与釉质基质矿化且仅在四足动物中得到证实的富含脯氨酸/谷氨酰胺的SCPP基因，这强烈表明腔棘鱼和四足动物的釉质是等效的。这一发现证实了之前的观点，即真正的釉质进化比四足动物的起源要早得多。
