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We determined the nucleotide sequences of cDNAs encoding precursors of vasotocin (VT) from two cyclostomes, the lamprey Lampetra japonica and the hagfish Eptatretus burgeri, for estimation of their phylogenetic relationships. Although only 47% similarity was found between the VT cDNAs, the predicted VT precursors of the lamprey and the hagfish were both composed of a signal peptide, VT, Gly-Lys-Arg and a neurophysin, as has been shown for precursors of vasopressin (VP) family hormones, including VP, VT and molluscan conopressin.

The central region of the lamprey neurophysin was very similar to those of previously characterized gnathostome neurophysins. Conspicuously, all the positions of 14 Cys residues were conserved in the lamprey neurophysin. The C-terminal region did not have a distinctive Leu-rich core segment, which is always found in the glycopeptide (copeptin) moiety of VP precursors. In contrast, the hagfish neurophysin showed at least two insertions and one deletion in the conserved central region including 14 Cys residues, but contained a potential N-linked glycosylation site and had a high proportion of Leu residues in the C-terminal region, like the neurophysin of another hagfish, Eptatretus stouti.

The evolutionary relationships of the precursors of VP family hormones among the lamprey, hagfish, gnathostomes and a mollusc were estimated by a maximum likelihood method. The phylogenetic tree with the highest bootstrap probability showed that the lamprey VT precursor is more closely related to the gnathostome VT and VP precursors than to the hagfish VT precursors.

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