5. Evolutionary Relationships among GLUT Members
Figure 1 presents a timetree, which was constructed with Molecular Evolutionary Genetics Analysis Version 6.0 (MEGA6) software [77], for GLUT members from human, mouse, chicken, turkey (Melga) and X. tropicalis. The tree was calibrated with GLUT12, assuming the time of separation between mammals and birds was 300 million years. The human GLUT11-A was included in the phylogenetic analysis. This analysis was conducted using the Neighbor–Joining bootstrap method with 50 replicates. Topological branching point divergence times were calculated with maximum likelihood based on the Jones–Taylor–Thornton matrix-based method and are based on units of the number of amino acid substitutions per site.
GLUT amino acid sequences were downloaded from UniProt or NCBI and derived from evidence at the transcript level, protein level or homology. Dataset for the timetree contained 63 amino acid sequences with a total of 426 positions included in the final dataset. Positions with fewer than 95% site coverage were eliminated. Less than 5% alignment gaps, missing data and ambiguous bases were allowed at any position. The timetree is drawn to scale with the relative number of substitutions per site.
According to this timetree, Class III GLUTs separated from Class I and Class II approximately 2000 million years ago. Class I and Class II GLUTs separated about 1700 million years ago, around the time when multicellular life began. GLUTs 1 and 3 separated approximately 800 million years ago. Evidently, GLUT13/HMIT orthologs could not be resolved well with this tree construction method, which can be seen from the distance between mammals and birds being not reflecting the species tree. GLUT8 and GLUT10 orthologs had the least constraints among the GLUT family members.
Based on the phylogenetic analysis as well as the result of experiments conducted in our laboratory, we have recognized that accession number gg5L_X1_XP_426528.4, which is annotated as GLUT member 5-like isoform X1 in chickens, is a gene product that is separate from the true GLUT5 member of chicken and other species and has more similarity to the GLUT9. Several GLUT11-like members were also found in chickens, each of them being the product of a discrete gene (Figure 1).