Tandem duplication via light-strand synthesis may provide a precursor for mitochondrial genomic rearrangement.

A tandem duplication of the mitochondrial tRNA(Thr) and tRNA(Pro) genes in the amphisbaenian reptile Bipes biporus is the first case reported of a tandem duplication restricted to a single pair of tRNA genes in a vertebrate mitochondrial genome. Such duplications have been predicted, however, as intermediate steps in the evolution of observed mitochondrial genomic rearrangements through errors in light-strand replication. The tandem duplication reported here is evolutionarily associated with displacement of the origin for light-strand replication from its typical location in vertebrate mitochondrial genomes and loss of the dihydrouridine stem from the tRNA(Cys) gene; these factors implicate light-strand replicational errors in the tandem duplication of genic regions. Pseudogene formation in tandemly duplicated sequences appears to be an intermediate step in genomic rearrangement. However, formation of pseudogenes in the Bipes mitochondrial genome occurs in a pattern that precludes subsequent genomic rearrangement. Functional constraints placed on cleavage of mitochondrial transcripts by tRNA genes also may prevent mitochondrial genomic rearrangement.