Correlation between amelogenin-gene variation and structural diversity of tooth enamel in whales

为了鉴定和鉴定鲸鱼釉原蛋白基因的第5-6外显子，利用11种鲸鱼的基因组DNA进行了聚合酶链式反应，重点研究了须鲸牙齿特异基因的保守性及其与这些物种牙釉质结构变异表达的相关性。所有供试物种的扩增产物经序列分析均为釉原蛋白基因，证实了须鲸釉原蛋白基因的良好保守性。因此，这些基因在牙齿表达退化后的一段地质时期内在基因组DNA中是保守的。外显子6是鲸鱼编码70%釉原蛋白基因的主要部分，由363bp组成，它们之间有很高的同源性(95%)。在本研究的物种中，只有小须鲸具有外显子6的358个碱基，其中5个碱基缺失了外显子6的中心部分。这种缺失导致了该物种氨基酸成分的移码。本研究还证实了由外显子6中心部分的一个点突变所引起的终止密码子的存在。据报道，在釉质发生不完善的人的釉质中，Phcoenoides和Neopocaena的釉质表现出相当大的低钙化和无棱柱结构。目前的研究表明，这些釉原蛋白基因突变导致了鲸鱼正常釉质结构的变化。这项研究还揭示了内含子5的序列对于鲸鱼和其他哺乳动物的系统发育分析是一个强有力的指标。

In order to identify and characterize the exons 5-6 of the amelogenin gene in whales, PCR was performed by using the genomic DNA in 11 species, with special reference to the conservation of the dental specific gene in baleen whales and the correlation with the expression of variations in the tooth enamel structure of these species. PCR amplification obtained the products in all the species used, and these products were identified as amelogenin genes after the sequence analyses, which confirmed the well conservation of the amelogenin genes in baleen whales. Therefore, it is concluded that these genes have been conserved in genomic DNA for geological period after the degeneration of the expression of teeth. The alignment of the exon 6, a major portion of the gene encoding 70 % amelogenin protein, consisted of 363 bp in the whale and showed a high (95 %) homology each other. In the species used in this study, only the minke whale possessed the 358 bp exon 6 where 5 bp was deleted the central portin of the exon 6. This deletion caused the frameshift of the amino acid component in this species. The present study also demonstrated the presence of stop codons induced by one point mutations at the central portion of the exon 6 in the Phocoenoides, Neophacaena and Ziphius. It has been reported that the enamel of Phocoenoides and Neophocaena shows a considerable hypocalcification and prismless structure as found ; in the human enamel of amelogenesis imperfecta. The present study suggests that these mutations in amelogenin genes cause the variation of normal enamel structure in whales. This study also disclosed that of the intron 5 sequence is a powerful indicator for phylogenetic analyses of whales and other mammals.