Brachyury (T) gene expression and notochord development in Oikopleura longicauda (Appendicularia, Urochordata)

Appendicularia (Larvacea) is a subgroup of Urochordata (Tunicata) comprised of holoplanktonic organisms that retain their tailed architecture througho ut their life history, while other tunicates, including ascidians and dolio lids, resorb the tail after metamorphosis. In order to investigate the char acteristics of the appendicularian unresorbed notochord, we isolated a part ial ge nomic clone and a full-length cDNA sequence homologous to the mouse Brachyury (T) gene from the appendicularian Oikopleura longicauda. Brachyur y is known to be predominantly expressed in the notochord cells and plays a n important role in their differentiation in other chordates. While phyloge netic analysis robustly supports the orthology of the isolated Brachyury ge ne, the exon-intron organization found in the genomic clone was distinct fr om that well-conserved among other T-box genes. In addition to a detailed o bservation of notochord development in living specimens, whole-mount double in situ hybridization was carried out using a Brachyury probe along with a muscle actin probe. The Brachyury transcripts were found in the notochord of the tailbud embryos and persisted into later stages. The present study h ighlights characteristics of notochord development in the appendicularian. Furthermore, these results provide basic knowledge for comprehensive unders tanding of the cellular- and molecular-based mechanisms needed to build the characteristic cytoarchitecture of notochord that varies among tunicate sp ecies.