Genic Control of Development

GENIC CONTROL OF DEVELOPMENT ERNST CASPARI, Ph.D.* I. Relation ofGenetics and Embryology When genetics was born, at the beginning of this century, it was regarded as closely related to embryology. Some of the foremost workers in the field, particularly from the zoological side—Bateson and Morgan, for instance—-had previously worked on embryological problems. It was apparently expected that the study of heredity would yield information that would lead to a deeper understanding ofthe processes ofdevelopment. Actually, genetics and embryology went in opposite directions. Genetics became an atomistic science which interprets biological phenomena in terms of the behavior of ultimate units, the genes. It therefore must ask questions about the physical and chemical nature ofthese units. The nature ofthe gene has been the core of genetics, and work on this problem has led to the great triumphs ofthelastdecadethat haverevolutionized our concepts ofbiology. Embryology, on the other hand, was most profoundly influenced by the discovery ofthe phenomenon ofinduction by Spemarm and his collaborators and has, during the last fifty years, been primarily concerned with the analysis ofthe mutual interaction oforgan primordia and cell complexes, and more recently with the behavior ofcells in cellcomplexes. The analysis has led to concepts like the "morphogenetic field" ofWeiss and the "prepattern" ofStern, which refer to properties of cell complexes and cannot be described in terms offundamental entities. Genes are responsible for morphological differences between organisms. Since in higher organismsmorphologicaldifferencesaretheresult ofdifferences in developmental processes, these processes must be under genetic control. The genes, on the other hand, are known to be active constituents * Wesleyan University, Middletown, Connecticut; present address: Department of Biology, University ofRochester, Rochester, New York. 26 Ernst Caspari · Genie Control ofDevelopment Perspectives in Biology and Medicine · Autumn i960 ofthe individual cells. The question ofthe genie control ofdevelopment may be formulated by asking which ofthe cell properties that are under genie control are instrumental in the supracellular processes thatconstitute the development ofa multicellular organism. It must be admitted that we do not know so much about this problem as we do about many other aspects of genetics. We know at present a good deal about the nature of genes, and more material is added to our knowledge every year. We have a theory about genie action in the cell which is in good agreement with our picture ofthe structure ofthe genie material and supported by a formidable amount of experimental information , particularly from microorganisms. Finally, the behavior ofgenes in populations is well understood; it forms the basis ofthe modern theory of evolution. Developmental genetics stands intermediate between these levels ofinvestigation. A fuller understanding ofits problems would give us a consistent representation ofthe organismic world in all its aspects, in which the problem of identical reproduction would be central, and in which the other biological problems could be understood as problems of the properties, effects, and history ofthe materials responsible for identical reproduction, the genes. II. "Phenogenetic" Approach The problem ofthe genie control ofdevelopment was first clearly formulated byValentin Haecker in 1918 (1) and designated by a specialname, "phenogenetics." He also supplied a method of attacking the problem: the mutant phenotype was to be followed back in development to the point where it becomes indistinguishable from the normal. At this point, it is assumed, normal and mutant phenotype first differ from each other, and all later differences are to be regarded as secondary consequences of this first abnormality. This method, which may be called "phenogenetic" in the narrow sense, has been extensively used in a number of organisms, particularly the mouse, the chicken, and the fly Drosophila. The extensive work ofGrüneberg (2) with a large number ofmutants in the mouse may be taken as an example. From the adult phenotype, the mutant characters are traced back in development step by step. Ifseveral mutant characters are dependent on one gene, each one ofthem is traced back and finally linked up with an earlier appearing deviation from the normal development. In this way, a 27 pedigree ofphenotypic effects ofone gene may be obtained, the earliest observed deviation from normal development being supposed the cause ofthe later appearing ones. As a generalization, it is postulated that the primary action ofa gene in development is unitary, insofar as...