Large, rapidly evolving intergenic spacers in the mitochondrial DNA of the salamander family Ambystomatidae (Amphibia: Caudata)

We report the presence, in the mitochondrial DNA (mtDNA) of all of thesexual species of the salamander family Ambystomatidae, of a shared 240- bpintergenic spacer between tRNAThr and tRNAPro. We place the intergenicspacer in context by presenting the sequence of 1,746 bp of mtDNA fromAmbystoma tigrinum tigrinum, describe the nucleotide composition of theintergenic spacer in all of the species of Ambystomatidae, and compare itto other coding and noncoding regions of Ambystoma and several othervertebrate mtDNAs. The nucleotide substitution rate of the intergenicspacer is approximately three times faster than the substitution rate ofthe control region, as shown by comparisons among six Ambystomamacrodactylum sequences and eight members of the Ambystoma tigrinumcomplex. We also found additional inserts within the intergenic spacers offive species that varied from 87-444 bp in length. The presence of theintergenic spacer in all sexual species of Ambystomatidae suggests that itarose at least 20 MYA and has been a stable component of the ambystomatidmtDNA ever since. As such, it represents one of the few examples of a largeand persistent intergenic spacer in the mtDNA of any vertebrate clade.