POSTERIOR END MARK, A NOVEL MATERNAL GENE ENCODING A LOCALIZED FACTORIN THE ASCIDIAN EMBRYO

Ascidian embryogenesis is regarded as a typical 'mosaic' type. Recent studies have provided convincing evidence that components of the poste rior-vegetal cytoplasm of fertilized eggs are responsible for establis hment of the anteroposterior axis of the embryo. We report here isolat ion and characterization of a novel maternal gene, posterior end mark (pem). After fertilization, the pem transcript is concentrated in the posterior-vegetal cytoplasm of the egg and later marks the posterior e nd of developing ascidian embryos. Despite its conspicuous localizatio n pattern, the predicted PEM protein shows no significant homology to known proteins. Overexpression of this gene by microinjection of synth esized pern mRNA into fertilized eggs results in development of tadpol e larvae with deficiency of the anteriormost adhesive organ, dorsal br ain and sensory pigment-cells. Lineage tracing analysis revealed that the anterior epidermis and dorsal neuronal cells were translocated pos teriorly into the tail region, suggesting that this gene plays a role in establishment of anterior and dorsal patterning of the embryo. The ascidian tadpole is regarded as a prototype of vertebrates, implying a similar function of pem in vertebrate embryogenesis.