DIFFERENTIAL GENE-EXPRESSION AND INTRACELLULAR MESSENGER-RNA LOCALIZATION OF AMPHIOXUS ACTIN ISOFORMS THROUGHOUT DEVELOPMENT - IMPLICATIONSFOR CONSERVED MECHANISMS OF CHORDATE DEVELOPMENT

The cephalochordate amphioxus is thought to share a common ancestor wi th vertebrates. To investigate the evolution of developmental mechanis ms in chordates, cDNA clones for two amphioxus actin genes, BfCA1 and BfCA1, were isolated. BfCA1 encodes a cytoplasmic actin and is express ed in a variety of tissues during embryogenesis, beginning in the dors olateral mesendoderm of the mid-gastrula. At the open neural plate sta ge, BfCA1 transcripts accumulate at the bases of the neuroectodermal c ells adjacent the presumptive notochord. The 3' untranslated region of BfCA1 contains a sequence that is similar to the ''zipcode'' sequence of chicken beta-cytoplasmic actin gene, which is thought to direct in tracellular mRNA localization. BfCA1 is also expressed in the notochor d through the early larval stage, in the pharynx and in the somites at the onset of muscle-cell differentiation. BfMA1 is a vertebrate-type muscle actin gene, although the deduced amino acid sequence is fairly divergent. Transcripts first appear in the early neurula in the somite s as they begin to differentiate into axial muscle cells and persist i nto the adult stage. In young adults, transcripts are localized in the Z-discs of the muscle cells. Smooth muscle cells around the gill slit s and striated muscle cells in the pterygeal muscle also express BfMA1 ; however, there is never any detectable expression in the notochord, which is a modified striated muscle. Together with the alkali myosin l ight chain gene AmphiMLC-alk, the sequence and muscle-specific express ion of BfMA1 implies a conserved mechanism of muscle cell differentiat ion between amphioxus and vertebrates. Evolution of the chordate actin gene family is discussed based on molecular phylogenetic analysis and expression patterns of amphioxus actin genes.