ELABORATION OF BODY PLAN AND PHASE-CHANGE DURING DEVELOPMENT OF ACETABULARIA - HOW IS THE COMPLEX ARCHITECTURE OF A GIANT UNICELL BUILT

While uninucleate and unicellular, Acetabularia acetabulum establishes and maintains functionally and morphologically distinct body regions and executes phase changes like those in vascular plants. Centimeters tall at maturity, this species has allowed unusual experimental approa ches. Amputations revealed fates of nucleate and enucleate portions fr om both wild type and mutants. Historically, graft chimeras between nu cleate and enucleate portions suggested that morphological instruction s were supplied by the nucleus but resided in the cytoplasm and could be expressed interspecifically. Recently, graft chimeras enabled rescu e of mutants arrested in vegetative phase. Since the 1930s, when Aceta bularia ia provided the first evidence for the existence of mRNAs, a d ogma has arisen that it uses long-lived mRNAs to effect morphogenesis. While the evidence favors translational control, the postulated mRNAs have not been identified, and the mechanism of morphogenesis remains unknown. Amenable to biochemistry, physiology, and both classical and molecular genetics, Acetabularia may contribute yet new insights into plant development and morphogenesis.