DEVELOPMENT OF TAIL MUSCLE ACETYLCHOLINESTERASE IN ASCIDIAN EMBRYOS LACKING MITOCHONDRIAL LOCALIZATION AND SEGREGATION

¹ The Wistar Institute of Anatomy and Biology, Philadelphia, Pennsylvania 19104 and Marine Biological Laboratory, Woods Hole, Massachusetts 02543

The ascidian Molgula arenata produces an anural larva in which development of the tail and other urodele features has been suppressed. The embryos nevertheless developed part of the histospecific tail muscle acetylcholinesterase; the presumptive myoblasts have obviously acquired the muscle differentiation program. When cleavage-stage embryos were prevented from undergoing further division by treatment with cytochalasin B, acetylcholinesterase evenutally developed in blastomeres of the muscle lineage. These anural embryos apparently segregate a cytoplasmic determinant concerned with acetylcholinesterase development into cells of the muscle lineage. In this species there was no localization and segregation of mitochondrial succinic dehydrogenase and cytochrome oxidase in the muscle lineage, as found in embryos of two urodele ascidians, Ciona intestinalis and Molgula occidentalis. The causal determinant of histospecific acetylcholinesterase expression is not, therefore, a differential localization of mitochondria nor is segregation of the muscle determinants linked directly to mitochondrial segregation.