V(H)GENE ORGANIZATION IN A RELICT SPECIES, THE COELACANTH LATIMERIA-CHALUMNAE - EVOLUTIONARY IMPLICATIONS

The living coelacanth Latimeria chalumnae is a relict species whose higher-level phylogenetic relationships have not been resolved clearly by traditional systematic approaches. Previous studies show that major differences in immunoglobulin gene structure and organization typify different phylogenetic lineages. To date, mammalian-, avian-, and elasmobranch-type gene organizations have been identified in representatives of these different phylads. A fourth form or organization is found in Latimeria, which possesses immunoglobutin heavy-chain variable region (V(H)) elements separated by almost-equal-to 190 nucleotides from diversity (D) elements. Adjacency of V(H) and D elements is characteristic of the elasmobranch ''clustered'' arrangement, although many other features of coelacanth V(H) gene organization and structure are more similar to those of bony fishes and tetrapods. These observations strongly support a phylogenetic hypothesis in which Latimeria occupies a sister-group relationship with teleosts and tetrapods.