Nonconventional genetic systems.

In the 1930s and 1940s, after the major discoveries of the Morgan school of genetics and before Watson and Crick's DNA molecule, biolo- gists were engaged in trying to assess the significance ofthe chromosome theory ofinheritance. 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 mito- chondria, chloroplasts, and in a variety of intracellular symbionts that Author's research supported by National Institutes of Health grant GM 31745 to Barry Polisky.