EVOLUTIONARY INFERENCES FROM RESTRICTION MAPS OF MITOCHONDRIAL DNA FROM NINE TAXA OF XENOPUS FROGS

Restriction endonuclease cleavage maps were prepared by the double digestion method for mitochondrial DNAs (mtDNAs) purified from Xenopus borealis, X. clivii, X. fraseri, X. muelleri, X. ruwenzoriensis, X. vestitus, X. laevis victorianus, X. l. laevis, and a variant of X. laevis designated X. laevis “davis.” An average of 21 cleavage sites per genome were mapped with 11 restriction endonucleases. Among the four invariant sites found are three conserved not only among the Xenopus mtDNAs tested but also among nearly all vertebrate mtDNAs examined to date. Two of these are Sac II sites in the 12S and 16S ribosomal RNA genes, and one is a Hpa I site in the gene for asparagine transfer RNA. These three sites permit the alignment and comparison of mtDNAs from different vertebrate classes. Although most of the differences observed among the Xenopus maps are attributable to point mutations causing gain or loss of restriction sites, the maps also differ by three large length mutations in or near the displacement loop.