Nonconventional Genetic Systems

NONCONVENTIONAL GENETIC SYSTEMS JOHN R. PREER, JR.* In the 1930s and 1940s, after the major discoveries of the Morgan school of genetics and before Watson and Crick's DNA molecule, biologists were engaged in trying to assess the significance of the chromosome theory of inheritance. In particular, the cytoplasm seemed to play almost no role in this developing field of genetics. However, there was a group that believed that the cytoplasm actually played a major role in heredity, but that this role had simply not yet been discovered. This group was led by T. M. Sonneborn and consisted, among others, of E. Caspari, S. Spiegelman, B. Ephrussi, P. Michaelis, V. Jollos, and even a few cell physiologists and developmental biologists. Sonneborn wrote a paper whose title was "Beyond the Gene," and in another paper he spoke of the cytoplasm as "partner of the genes." One of the favorite examples was to hypothesize a "bag" containing all of the genes but none of the cytoplasm of the cell and to assert the belief that in no way could such a bag ever develop into a cell. I believe that most workers would still agree with this proposition today, even if the bag also contained all the mitochondrial and chloroplast DNA as well. Even Michael Crichton, in his highly imaginative Jurassic Park [1], found it desirable to introduce his resurrected dinosaur DNA into crocodile ova depleted of their DNA in order to produce real dinosaurs! For a short period around 1945, Sonneborn and Spiegelman argued that associated with every gene there was a "plasmagene," a cytoplasmic particle with genetic properties. The plasmagene theory, however, was soon abandoned by Sonneborn and Spiegelman on the basis of mounting evidence against it, much of the evidence obtained in their own laboratories. Many of the examples discussed in those days have since turned out to be due to extrachromosomal DNA found in the cytoplasm in mitochondria , chloroplasts, and in a variety of intracellular symbionts that Author's research supported by National Institutes of Health grant GM 31745 to Barry Polisky. *Department of Biology, Indiana University, Bloomington, Indiana 47405.© 1993 by The University of Chicago. All rights reserved. 0031-5982/93/3603-0812101.00 Perspectives in Biology and Medicine, 36, 3 ¦ Spring 1993 395 have become closely integrated into their hosts. There remain, however, substantial numbers of additional phenomena that seem to operate "beyond the gene." This paper will examine these cases in light of our present knowledge, and assess their possible mechanisms and significance . The Gene One of the greatest achievements of science was the discovery and characterization of the gene, which constitutes the major genetic system of all organisms. Mendel established the rules of transmission of genes from one generation to the next, and later workers showed that these rules are explained by the fact that genes are located on chromosomes. When it finally became clear that the gene was nucleic acid, it was seen that base pairing provides the mechanism for the replication of genes. Base sequence of DNA determines RNA sequence, which in turn determines protein structure. Base sequence, therefore, provides the means for gene action. Change in the linear array ofbases constitutes mutation. Thus, what we inherit is the information that determines DNA, RNA, and protein structure. This conventional view of the gene is so clear, so intellectually satisfying , and so pervasive that today we often just assume that heredity and nucleic acids are synonymous. However, there is no reason that our view of heredity should be bound by such a rigid concept. When all the cases of heredity that are due to nucleic acid genes are removed, what is left? It is the purpose of this paper to examine this question and to assess the significance of cases of nonconventional mechanisms of heredity. A Definition ofHeredity Heredity is the transmission of characters from one generation to the next. However, before the discovery of the role of DNA in inheritance the concept of heredity itself was often a bit fuzzy. In the early years of genetics after the turn of the century, Herbert Spencer Jennings, while struggling with the genetics of Protozoa, found it necessary to define what he meant by heredity...