Variations in mitochondrial tRNA gene organization of reptiles as phylogenetic markers |
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Authors: | Kumazawa Y; Nishida M |
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Institution: | Department of Molecular and Cell Biology, University of California at Berkeley, USA. |
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Abstract: | Amplification and sequencing of mitochondrial DNA regions corresponding to
three major clusters of transfer RNA genes from a variety of species
representing major groups of birds and reptiles revealed some new
variations in tRNA gene organization. First, a gene rearrangement from
tRNA(His)-tRNA(Ser)(AGY)-tRNA(Leu)(CUN) to tRNA(Ser)(AGY)-
tRNA(His)tRNA(Leu)(CUN) occurs in all three crocodilians examined
(alligator, caiman, and crocodile). In addition an exceptionally long
spacer region between the genes for NADH dehydrogenase subunit 4 and
tRNA(Ser)(AGY) is found in caiman. Second, in congruence with a recent
finding by Seutin et al., a characteristic stem-and-loop structure for the
putative light-strand replication origin located between tRNA(Asn) and
tRNA(Cys) genes is absent for all the birds and crocodilians. This
stem-and-loop structure is absent in an additional species, the Texas blind
snake, whereas the stem-and-loop structure is present in other snakes,
lizards, turtles, mammals, and a frog. The disappearance of the
stem-and-loop structure in the blind snake most likely occurred
independently of that on the lineage leading to birds and crocodilians.
Finally, the blind snake has a novel type of tRNA gene arrangement in which
the tRNA(Gln) gene moved from one tRNA cluster to another. Sequence
substitution rates for the tRNA genes appeared to be somewhat higher in
crocodialians than in birds and mammals. As regards the controversial
phylogenetic relationship among the Aves, Crocodilia, and Mammalia, a
sister group relationship of birds and crocodilians relative to mammals, as
suggested from the common loss of the stem-and- loop structure, was
supported with statistical significance by molecular phylogenetic analyses
using the tRNA gene sequence data.
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