The Expression of the Transthyretin Gene in Liver Evolved during the Radiation of Diprotodont Marsupials in Australia

Thyroid hormone-binding proteins in blood plasma were identified in 28 different marsupial species by their capacity to bind radioactive thyroxine. All species contained albumin. Transthyretin was not found in the blood from any of 12 polyprotodont marsupial species, but was abundant in the blood from all of 16 diprotodont marsupial species investigated. Transthyretin mRNA was absent from the liver of the stripe-faced dunnart, a polyprotodont marsupial, but abundant in the liver of the diprotodont grey kangaroo. Diprotodont marsupials probably evolved in Australia from polyprotodont marsupials after their transantarctic migration from South America, about 40 million years ago. It is suggested that hepatic transthyretin expression evolved in marsupials during the radiation of herbivorous, diprotodont species in Australia. The earlier appearance of transthyretin gene expression in the choroid plexus of the stem reptiles, about 300 million years ago, contrasts with hepatic transthyretin synthesis, a relatively late evolutionary event, occurring independently in at least three lineages.