Characterization and evolutionary studies of carcharhine shark immunoglobulin heavy chain genes

Abstract

Sharks, the most primitive vertebrates that have been found to possess bona fide immunoglobulins, are known as "living fossils". This dissertation describes my studies on the characterization of carcharhine shark Ig heavy chain genes. Shark V sc H genes have been cloned from both cDNA and genomic DNA, and the sequence data reveals that these carcharhine shark Ig V scHS(Vmu) contain essentially all V scH canonical structures and are associated with a four domain IgM-like constant region. Shark Vmus are shown to be highly diversified and can be divided into six families based on the sequences of this work. Another class of V scHS (Vo) associated with six C-region domains has been found in the carcharhine shark. This molecule represents a new isotype of heavy chain (IgW). The higher homology of the Vo with its own constant regions may indicate that Vo is more primitive. Phylogenetic analysis reveals that the V scHS of the horned shark and the skate can also be divided into two major classes (Vo and Vmu). The divergence of these two major V scH classes must have occurred prior to the divergence of the sharks and the skates, which is about 350 million years ago. The V scHS of all other vertebrates are found located between these two major shark/skate classes on phylogenetic trees. Here, I suggest that Vo is the primordial V sc H-region based on my phylogenetic analysis. The cDNA encoding a whole IgM heavy chain was cloned and sequenced. This heavy chain has four C-region domains which are homologues to the IgM C-region domains of representative vertebrate classes. Phylogenetic analysis shows that the divergence of IgM C-region domains from the primordial C-domain occurred rapidly prior to the divergence of the sharks and other vertebrates. Afterwards, C-region domains have evolved independently of each other with a constant rate. Here it is shown that IgM C-region domains can be used in the studies of vertebrate phylogeny. Although it is not clear at present how the hypothesized precursor Ig gene diverged into separate V- and C-domain genes, it is clear from this work that this divergence occurred rapidly during the hypothetical "big bang".