Basal Gnathostomes Provide Unique Insights into the Evolution of Vitamin B12 Binders

delfa afni;

The uptake and transport of vitamin B12 (cobalamin; Cbl) in mammals involves a refined system with three evolutionarily related transporters: transcobalamin 1 (Tcn1), transcobalamin 2 (Tcn2), and the gastric intrinsic factor (Gif). Teleosts have a single documented binder with intermediate features to the human counter parts. Consequently, it has been proposed that the expansion of Cbl binders occurred after the separation of Actinopterygians. Here, we demonstrate that the diversification of this gene family took place earlier in gnathostome ancestry. Our data indicates the presence of single copy orthologsof the Sarcopterygii/Tetrapodaduplicates Tcn1 and Gif, and Tcn2, inChondrichthyes. In addition, a highly divergent Cbl binder was found in the Elasmobranchii. We unveila complex scenario forged by genome,tandem duplications and lineage-specific geneloss. Our findings suggest that fromanancestral transporter, exhibiting large spectrum and high affinity binding, highly specific Cbl transporters emerged through gene duplication and mutations at the binding pocket