THE MAIN FEATURES OF THE CRANIATE MITOCHONDRIAL-DNA BETWEEN THE ND1 AND THE COI GENES WERE ESTABLISHED IN THE COMMON ANCESTOR WITH THE LANCELET

We have cloned the mitochondrial DNA fragment extending from tRNA-Leu to the cytochrome oxidase subunit 1 (CO1) genes of Branchiostoma lance olatum, Myxine glutinosa, Lampetra fluviatilis, and Scyliorhinus canic ulus and have determined their respective gene sequences and organizat ion. In all four species, this region contains the ND1 and ND2 genes a nd the genes coding eight tRNAs, namely, tRNA-Ile, -Gln, -Met, -Trp, - Ala, -Asn, -Cys, and -Tyr. The gene order is the same in the hagfish, lamprey and dogfish. In the lancelet, the location of the tRNA genes i s slightly different. The mitochondrial code of Myxine, Lampetra, and Scyliorhinus is identical to that of vertebrates. The code used by the lancelet is the same with the exception of AGA (a stop codon in verte brates), which codes for glycine in the lancelet. From the comparison of the four maps with already published ones for other species, we pro pose that the main features of the craniate mtDNA between the ND1 and CO1 genes were established in the common ancestor to cephalochordates and vertebrates more than 400 MYA. The origin of replication of the li ght-strand (Ori-L), usually located between the tRNA-Asn and tRNA-Cys genes in vertebrates, was not found in the lancelet, hagfish, or lampr ey (Lampetra). In contrast, it was found in the dogfish. Thus the posi tion of Ori-L was established for the first time in the common ancesto r to the Chondrichthyes and Osteichthyes and remained present in all l ater-emerging vertebrates.