Axial Bifurcation in Serpents by Bert Cunningham

Developmental malformations in lampreys, which belong to a basal group of vertebrates (cyclostomes), may similarly provide new insights into the early evolution of vertebrates. In fact, axial bifurcation in the caudal fin of the twin-tail goldfish is caused by ventralization during early embryonic development, which is in turn due to a mutation in a chordin gene that may have occurred during domestication. More recently, in the field of evolutionary developmental biology, researchers have focused more on malformations, as these can represent developmental constraint, variability, and evolvability. As described in their works, conjoined twins, referred to as “axial duplicity” or “double monsters” in Bateson, also represent striking examples of such anomalies. For example, Étienne Geoffroy Saint-Hilaire and his colleagues, including his son Isidore, described many developmental anomalies and sought to explain the mechanisms underlying their production. Malformed mutants, or so-called “monsters”, have attracted the attention of morphologists since the inception of the discipline. 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. Two-headed mutants of the lamprey are described. Asymmetrical (lateral- branched) mutants displayed more severe abnormalities in the cranial nerves than symmetrical mutants. Other malformations were coincidently observed, including pericardial edema, yolk sac edema and axial bending. The anterio-posterior position of the axonal bifurcation and symmetricity varied in each mutant. All mutants were derived from single fertilizations using single male and female gametes, suggestive of a genetic background. Two-headed mutants were obtained by artificial fertilization in the laboratory as spontaneous occurrences. Although lampreys sit at a crucial phylogenetic position, there are only a few reports on their teratology and developmental abnormalities. This is the first report of two-headed (bicephaly) lamprey twins.

Axial Bifurcation in Serpents by Bert Cunningham