PHYLOGENETIC EMERGENCE OF IMMUNOGLOBULIN CLASSES DISTINCT FROM IgM

SUMMARY The low molecular weight immunoglobulin from the marine toad ( Bufo marinus ) was found to differ markedly from classical mammalian IgG immunoglobulin. Higher intact molecular weight of 168,000 ± 4000 was the result of larger heavy chains (61,400 ± 2300) than mammalian γ chains (50,000 daltons). The light chains were of normal molecular weight (23,000). Polyacrylamide gel electrophoresis in SDS buffers was shown to be a simple and reliable way of determining the type of heavy chain present in immunoglobulins from different species, despite the fact that the mobility of the chains in this system does not accurately reflect their molecular weight. Using this technique it was shown that the heavy chain from the LMW Ig present in Xenopus laevis , an anuran amphibian, and the chicken, a representative of the birds, showed similar properties to the toad LMW Ig heavy chain. In addition, the LMW Ig heavy chain from the possum ( Trichosurus vulpecula ), a marsupial, showed properties which were characteristic of classical γ heavy chains. We conclude that the LMW Ig present in anuran amphibians does not belong to the IgG class as had been previously thought. This Ig class appears to have been retained in the reptiles and birds. The appearance of IgG appears to be a relatively recent event, recurring at a time after the divergence of the mammalian line from the stem reptiles.