Developmental Expression Patterns of Alpha 2 Macroglobulin in Amphioxus: An Insight into Liver Evolution

It has been considered that hepatic caecum of amphioxus is precursor of vertebrate liver. Alpha 2 macroglobulin (A2m) ancient native immune factors are mainly synthesized in liver in vertebrate and is essential for liver formation in fish. In this study, the developmental expression patterns of A2m in amphioxus Branchiostoma japonicum were examined to seek some embryonic functionally evidence for the view about liver origin. It was demonstrated that A2m expression is mainly specific in primitive gut at early larvae stages, and then is in both whole gut and hepatic caecum in 25day larvae and adult amphioxus. These suggested that the entoderm-derived gut and hepatic caecum in amphioxus are not fully functionally separated as liver and gut in vertebrate, and may represents an ancestral archetype of differentiation of liver and gut in vertebrate in evolution. In addition, the detection of abundant A2m transcript in amphioxus embryos at 2-4 cell stages revealed that A2m is a maternal molecule, which was further verified by Northern blot assay of A2m on unfertilized eggs.