Muscle actin genes and muscle cells in the appendicularian, Oikopleura longicauda: Phylogenetic relationships among muscle tissues in the Urochordates

Appendicularians (larvaceans) are planktonic tunicates. They possess a tail throughout their life, which marks a distinct difference between appendicu larians and the other tunicate groups, including salps, doliolids, pyrosome s, and ascidians. We isolated cDNA clones encoding muscle-type and cytoplas mic-type actin isoforms from the appendicularian, Oikopleura longicauda. Th e types of the actin isoforms were confirmed by amino acid identities in th e diagnostic residues compared to those of the other chordate muscle and cy toplasmic actins. Interestingly the appendicularian muscle actin isoform se quence has an intermediate feature between the ascidian tail (larval) muscl e actin isoform and the body-wall (adult) muscle actin isoform. Analysis of a genomic clone from a muscle actin gene revealed that it contains only on e intron in the coding region. It is located at a position that does not co rrespond to those of any introns reported in other deuterostome actin genes . Whole-mount in situ hybridization shows that the muscle actin transcript is detected specifically in the tail muscle cells of late tailbud-stage emb ryos. A signal is also found transiently in the region where the heart will form. In adults, the gene is expressed in tail muscle cells but not in the heart. Together with results of cytochemical and histochemical studies dem onstrating the structure of muscle tissue, the present study highlights cha racteristics of appendicularian muscle organization, which are compared wit h those of the larval and adult muscle of ascidians. (C) 2000 Wiley-Liss, I nc.