DUPLICATION OF AN AMPHIOXUS MYOGENIC BHLH GENE IS INDEPENDENT OF VERTEBRATE MYOGENIC BHLH GENE DUPLICATION

Gene duplication is thought to be a major genetic change that may have permitted the evolution of vertebrates from invertebrates. The myogen ic genes encode basic helix-loop-helix (bHLH) transcriptional factors essential for the formation of skeletal muscle. The invertebrate genom e contains only a single myogenic bHLH gene, whereas the vertebrate ge nome contains four (MyoD, Myf-5, myogenin and MRF4). Since the tunicat e genome contains a single myogenic bHLH gene, its duplication might h ave occurred some time during chordate evolution. To determine whether the duplication of the myogenic bHLH gene occurred prior to, or after the divergence of vertebrates from the cephalochordate lineage, we am plified target fragments from the amphioxus, Branchiostoma floridae, b y means of PCR. Sequence analysis and genomic Southern analysis reveal ed that the amphioxus genome contains two myogenic bHLH genes (BMD1 an d BMD2). A comparison of the amino acid sequences in the bHLH domain b etween BMD1, BMD2 and four vertebrate myogenic bHLH gene products, how ever, showed that neither BMD1 nor BMD2 resembled any of the four gene s. These results suggested that the duplication of amphioxus myogenic bHLH gene occurred independently of that leading to the four myogenic bHLH genes in vertebrates.