The complete nucleotide sequence of the mitochondrial DNA of the agnathan Lampetra fluviatilis: Bearings on the phylogeny of cyclostomes

There are two competing theories about the interrelationships of craniates: the cyclostome theory assumes that lampreys and hagfishes are a clads, the cyclostomes, whose sister group is the jawed vertebrates (gnathostomes); t he vertebrate theory assumes that lampreys and gnathostomes are a clade, th e vertebrates, whose sister group is hagfishes. The vertebrate theory is be st supported by a number of unique anatomical and physiological characters. Molecular sequence data from 18S and 28S rRNA genes rather support the cyc lostome theory, but mtDNA sequence of Myxine glutinosa rather supports the vertebrate theory. Additional molecular data are thus needed to elucidate t his three-taxon problem. We determined the complete nucleotide sequence of the mtDNA of the lamprey Lampetra fluviatilis. The mtDNA of L. fluviatilis possesses the same genomic organization as Petromyzon marinus, which valida tes this gene order as a synapomorphy of lampreys. The mtDNA sequence of L. fluviatilis was used in combination with relevant mtDNA sequences for an a pproach to the hagfish/lamprey relationships using the ma?;imum-parsimony, neighbor-joining, and maximum-likelihood methods. Although trees compatible with our present knowledge of the phylogeny of craniates can be reconstruc ted by using the three methods, the data collected do not support the verte brate or the cyclostome hypothesis. The present data set does not allow the resolution of this three-taxon problem, and new kinds of data, such as nuc lear DNA sequences, need to be collected.