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Identification and characterization of amelogenin genes in monotremes, reptiles, and amphibians - PubMed

  • ️Thu Jan 01 1998

Identification and characterization of amelogenin genes in monotremes, reptiles, and amphibians

S Toyosawa et al. Proc Natl Acad Sci U S A. 1998.

Abstract

Two features make the tooth an excellent model in the study of evolutionary innovations: the relative simplicity of its structure and the fact that the major tooth-forming genes have been identified in eutherian mammals. To understand the nature of the innovation at the molecular level, it is necessary to identify the homologs of tooth-forming genes in other vertebrates. As a first step toward this goal, homologs of the eutherian amelogenin gene have been cloned and characterized in selected species of monotremes (platypus and echidna), reptiles (caiman), and amphibians (African clawed toad). Comparisons of the homologs reveal that the amelogenin gene evolves quickly in the repeat region, in which numerous insertions and deletions have obliterated any similarity among the genes, and slowly in other regions. The gene organization, the distribution of hydrophobic and hydrophilic segments in the encoded protein, and several other features have been conserved throughout the evolution of the tetrapod amelogenin gene. Clones corresponding to one locus only were found in caiman, whereas the clawed toad possesses at least two amelogenin-encoding loci.

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Figures

Figure 1
Figure 1

Structural organization of the human, caiman, and African clawed toad amelogenin genes. Exons, indicated by boxes, are numbered 1–7, and the numbers below exons indicate the length in base pairs. The variable length of human X and Y exons is indicated by two numbers separated by a slash. Exons known to be alternatively spliced are indicated by hatched boxes. The first exon–intron boundary in the toad amelogenin genes has not been identified.

Figure 2
Figure 2

Amino acid alignment of amelogenin sequences. A simple-majority consensus sequence is shown at the top. Identity with the consensus is indicated by a dash (-); an asterisk (∗) indicates an alignment gap, and a dot (⋅) indicates an unknown sequence. The common names of the species are shown together with accession codes for sequences obtained from databases.

Figure 3
Figure 3

Hydrophilicity plot of amelogenins from selected species prepared by using the method of Kyte and Doolittle (25). The plots share the following characteristics: the hydrophobic leader peptide of about 20 amino acid residues is followed by a short hydrophilic segment (10 residues), another short hydrophobic segment (10 residues), and a larger hydrophilic segment (about 20 residues). The conserved α-helical segment lies on the border of the hydrophobic and hydrophilic domains (approximately corresponding to residues 35–45). This shared hydrophobicity signature is followed by a more variable internal segment (residues 60–110) of irregularly alternating hydrophilicity and hydrophobicity. The repeat region (residues 110–190) and the C-terminal region are hydrophilic. Although the primary sequence of amelogenin varies greatly between species, the hydropathy pattern is well conserved.

Figure 4
Figure 4

Plot of nonsynonymous substitution percent (Ka%) against amino acid residue position (numbering follows that of Fig. 2) in selected pairwise comparisons of human (H), cattle (Ct), rat (R), caiman (C), and toad-2 (T) amelogenin sequences. An initial sliding window of 30 codons was used to estimate Ka by the method of Li et al. (32), and a further sliding window of 30 codons was used to smooth the values obtained. Small alignment gaps (<8 residues) were ignored in the plot; larger gaps are indicated by vertical lines.

Figure 5
Figure 5

Neighbor-joining tree of sequences shown in Fig. 2. Distances were estimated from the proportion of differences in pairwise comparisons following exclusion of gaps. The common names of the species are indicated together with accession codes for sequences obtained from the databases. Numbers on the nodes indicate the percent recovery of that node in 500 bootstrap replications.

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