Two-headed mutants of the lamprey, a basal vertebrate.

BACKGROUND: This is the first report of two-headed (bicephaly) lamprey twins. Although lampreys sit at a crucial phylogenetic position, there are only a few reports on their teratology and developmental abnormalities. RESULTS: Two-headed mutants were obtained by artificial fertilization in the laboratory as spontaneous occurrences. All mutants were derived from single fertilizations using single male and female gametes, suggestive of a genetic background. The anterio-posterior position of the axonal bifurcation and symmetricity varied in each mutant. Other malformations were coincidently observed, including pericardial edema, yolk sac edema and axial bending. Asymmetrical (lateral- branched) mutants displayed more severe abnormalities in the cranial nerves than symmetrical mutants. CONCLUSION: Two-headed mutants of the lamprey are described. These mutants have similar malformations to dorsal blastopore lip-transplanted lamprey embryos, suggesting that they could be generated by a disorder in head-organizing activity.