Cytoplasmic Action In Development

A commentary on CYTOPLASMIC ORGANIZATION SYSTEMS. Edited by George M. Malacinski. McGraw-Hill Publishing Company, New York. $59.95. xxiv + 482 p.; ill.; index. ISBN: 0-07-039749-X. 1990. T HE ONLY way to get from genotype to phenotype is via development. This is an enormous span, covering such diverse events as the activation of gene transcription in the early embryo, the formation of tooth enamel, and the maturation of the B lymphocytes. Not surprisingly, then, there are some areas of developmental biology whose problems are predominantly genotypic (e.g., "How might transcription factors interact with chromatin to activate specific genes in certain cells?"), and other areas whose problems are predominantly phenotypic (e. g., "How might cell surfaces interact to form tissues and organs?"). Genotypic questions have provided most of the subject matter for developmental genetics, while phenotypic questions have provided the subjects for embryology, both descriptive and experimental. In recent years, the advances made by the genotypic side of developmental biology have been so spectacular that it sometimes is feared that the phenotypic perspectives might become lost. Although the distinction between phenotypic and genotypic developmental biology is relatively new, the fear that developmental biology might be "taken over" by genetics is nearly as old as the separation of genetics from embryology in the 1920s. When experimental embryology separated itself from evolutionary problems at the turn of the century, its domain included the studies of inheritance, development, regeneration, and senescence. One of the largest questions for this newly organized field was: Which compartment of the zygote-the nucleus or the cytoplasmdirected heredity and development? In a series of interactions that lasted over two decades, E. B. Wilson and T. H. Morgan chased the evidence into the nucleus, although Morgan had thought at first that cytoplasmic factors determined all phenotypes. The path that led from experimental embryology into genetics was the X chromosome. Morgan and Wilson disagreed as to whether this nuclear entity actually controlled sex determination or whether it was a consequence of earlier, cytoplasmic, sex determining mechanisms. Eventually, Morgan and his coworkers correlated several inherited factors, as well as sex determination, to the X chromosome. In this way, the embryologist Morgan inadvertently created a new genetic science (Gilbert, 1978, 1987). Soon after 1911, genetics arose as a separate discipline, complete with its own techniques, favored organisms, rules of evidence, journals, and vocabulary. The remarkable success of genetics in the 1920s and 1 930s caused it to become the preeminent way to study inheritance, and it redefined the other disciplines in genetic terms. The study of inheritance became genetics, which Morgan defined as the discipline concerned with the transmission of nuclear genes (Morgan, 1926). Morgan's exclusion of cytoplasm and development from the realm of inheritance was soon viewed as dogma (Sapp, 1987). Embryology was redefined as the study of changes in gene expression over time (Morgan, 1934),