Immunoglobulins genes in Neoceratodus forsteri and Protopterus annectens explain the origin of the immunoglobulins of the animals that passed ashore.

Sarcopterygian fishes are a taxon of bony fishes. They include lungfish and coelacanths (six species of lungfish and two species of coelacanths). Evolutionary adaptations arose with these fish, such as the appearance of lungs and paired lobed fins that help them move over the bottom of the sea. In the Devonian period, they came ashore, and tetrapods (amphibians, reptiles, and mammals) arose from them. Within immunology, they can teach us about the emergence of immunoglobulins D, A/X, and Y already present in amphibians. We have studied the genes of the immunoglobulins in the fish Sarcopterygii Neoceratodus forsteri and Protopterus annectens. In the first fish, we find that several loci for the constant chains of immunoglobulins are distributed across 4 chromosomes. We have found four genes for IgM, a gene for IgW and a gene for IgN. In the second, we find one locus with genes for IgN and IgM and another with one gene for IgW. With these sequences, together with those obtained in other publications, we have been able to study the possible evolution and emergence of immunoglobulin classes. We conclude that there are two evolutionary lineages, one focused on IgM and very conservative, and the other focused on IgW, which allows high variability. In the case of the animals that went to land, their IgD is formed only by domains whose origin is in the W lineage. IgA/X and IgY are unique since they arose from the recombination between the two evolutionary lineagess (M and W). In both IgA/X and IgY, the CH1 and CH2 domains come from domains whose origin is the W lineage, while their CH3 and CH4 derive from the M lineage.