EVOLUTION OF THE WANCY REGION IN AMNIOTE MITOCHONDRIAL-DNA

In most vertebrate mitochondrial genomes, the site for initiation of l ight-strand replication, O-L, is found within a cluster of five transf er RNA (tRNA) genes (tRNA(Trp), tRNA(Ala), tRNA(Asn), and tRNA(Tyr)). This region and part of the adjacent cytochrome c oxydase subunit I (C OI) gene were sequenced for two crocodilian, two turtle, and one snake species and for Sphenodon punctatus; part of the adjacent nicotinamid e adenine dinucleotide dehydrogenase subunit 2 (ND2) gene was also seq uenced for the crocodilian and turtle species. All had the typical ver tebrate gene order. The turtles and the snake have a lengthy noncoding sequence between the tRNA(Asn) and tRNA(Cys) genes that we assumed to be homolgous to the mammalian O-L. The crocodilians and Sphenodon lac k such a sequence, a condition they share with birds. Most proposed ph ylogenies for the amniotes require that O-L at this position was lost at least twice during their diversification or was evolved independent ly more than once. Within the five tRNA genes, frequencies of substitu tions are much higher in loops than in stems. Many loops vary dramatic ally in size among the species; in the most extreme case, the D-arm of the Sphenodon tRNA(Cys) is a ''D-arm replacement'' loop of seven nucl eotides. Frequency of transitions in stems is relatively uniform acros s tRNAs, but frequency of transversions varies greatly. Mismatches in stems are infrequent, and their relative frequency in a specific tRNA is unrelated to the frequency of substitution in the corresponding gen e. Several features of mammalian mitochondrial tRNAs are conserved in WANCY tRNAs throughout amniotes. The inferred initiation codon for COI is GTG in crocodilians, turtles, and the snake, a condition they shar e with fishes, certain amphibians, and birds. TTG appears to be the in itiation codon for COI in Sphenodon; if correct, this would be a novel initiation codon for vertebrate mitochondrial DNA. Phylogenetic analy ses of the inferred amino acid sequences of ND2 and COI support the si ster-group relationship of birds and crocidilians and suggest that mam mals are an early derived lineage within the amniotes.