Characterization and expression of amphioxus ApoD gene encoding an archetype of vertebrate ApoD proteins

Here we report a homologue of the apolipoprotein D gene (AmphiApoD) in amphioxus, Branchiostoma belcheri tsingtauense, the first such finding in a basal chordate cephalochordate. The main features of the protein predicted from AmphiApoD are characteristic of the apolipoprotein D. Phylogenetic analysis places AmphiApoD at the base of the phylogenetic tree, suggesting that AmphiApoD is the archetype of the vertebrate ApoD genes. Both whole mount in situ hybridization and Northern blotting and RT-PCR as well as in situ hybridization histochemistry reveal that AmphiApoD is expressed in tissues derived from mesoderm and endoderm including notochord and hind-gut, which contrasts with the strong expression patterns of ApoD genes in the ectodermal derivatives in mammals and birds. The expression profiles of the ApoD gene may have been changed to be expressed in the endo-mesodermal derivatives in amphioxus after the vertebrate and cephalochordate lineages diverged; alternatively, the ApoD gene may first have been expressed in the endo-mesoderm during embryogenesis in the last common ancestor of all chordates, and subsequently came to be expressed in the ectodermal derivatives of vertebrates including mammals and birds.