Sonneborn Tracy Morton

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(b.Baltimore, Maryland, 19 October 1905; d. Bloomington, Indiana, 26 January 1981); genetics, evolution, protozoology, education. For the original article on Sonneborn see DSB, vol. 18.

Sonneborn was an early and visionary leader in microbial genetics. He was widely regarded for the sophistication of his experimental designs, which he used to explore a range of largely non-Mendelian hereditary phenomena in the ciliated protozoan Paramecium aurelia. Sonneborn also stood out among geneticists as a highly successful teacher, writer, and public speaker. He was a well-known critic of radical reductionism at a time, during the middle decades of the twentieth century, when studies of the simplest phenomena in the simplest organisms such as viruses and bacteria dominated the experimental life sciences. Since the original DSB article was published, new light has been shed on the philosophical tenets that guided his research and his role in the reaction to the politicization of genetics under Lysenko in the Soviet Union.

Sonneborn’s experimental mastery with P. aurelia can be traced to his relentless and painstaking efforts throughout the 1930s to achieve experimental control of cytological events in the single-celled organism for the purpose of Mendelian analysis. As a research associate to his mentor Herbert Spencer Jennings at Johns Hopkins, he pursued the development of methods for inducing breeding among diverse types of P. aurelia, leading to his groundbreaking discovery of mating types in the organism in 1937. From roughly 1937 to 1947 Sonneborn worked out much of the basic genetics of the organism. Beginning in 1939 this work was carried out in Sonneborn’s laboratory at Indiana University, where he remained for the balance of his career.

Although an avid experimentalist, Sonneborn’s experimental style was characterized by a natural-history-like approach to the understanding of the research organism. In particular, he capitalized on the existence of two functionally distinct nuclei in Paramecium, the macronucleus and the micronucleus. Sonneborn demonstrated that the former was the nuclear locus of phenotypic expression and that the latter controlled the regeneration of the macronucleus. Furthermore, he aimed to gain extensive control over the onset of life cycle events associated with conjugation (reduction division and cross-fertilization followed by macronuclear regeneration) and autogamy (reduction division and self-fertilization also followed by macronuclear regeneration). Such control was achieved through the manipulation of environmental conditions such as food supply, which when changed trigger certain cytological responses in the organism. The control of conjugation enabled both Mendelian analysis and the control of Mendelian phenomena. Control of autogamy enabled the production of a homozygous macronucleus comparable to the production of homozygosity through inbreeding in higher organisms.

The wealth of experimental tools arising from his knowledge of the natural history of the organism yielded Sonneborn a high level of control over both nuclear and nonnuclear sources of hereditary phenomena; these circumstances led him to explore and emphasize the interaction between the two in his genetics research. It likewise made possible his 1955 critique of the biological species concept as championed by the evolutionary biologist Ernst Mayr at a symposium sponsored by the American Association for the Advancement of Science. Although a strong proponent of a population-genetic conception of species, Sonneborn criticized Mayr’s biological species concept for its failure to incorporate asexual and obligatory inbreeding organisms, which represent a considerable portion of the living world. Mayr defined species as groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. In an effort to foster the development of a fully comprehensive species concept, in 1957 Sonneborn published a massive survey of the protozoa spanning from obligatory outbreeding to obligatory inbreeding and asexual organisms, which rejected the fundamental break in nature between sexual and asexual organisms maintained by many leading biologists.

Sonneborn’s discovery of mating types and subsequent research helped to launch a new era of genetics during the middle decades of the twentieth century, in which microorganisms became the favored tools of genetics research. Microorganisms were embraced largely due to their physiological simplicity, rapid rates of reproduction, and the ease and exactitude with which their environmental conditions could be controlled. Unlike other researchers, however, Sonneborn never migrated to genetics research with even simpler microorganisms such as bacteria and viruses that became popular shortly later, nor did he incorporate into his research program the molecular biology techniques that developed in concert with the use of these organisms.

Sonneborn stood at a crossroads then as a pioneer of microbial genetics who nonetheless remained throughout his career a classical geneticist in the tradition of such programs as Drosophila and maize genetics. At the same time, he aimed to modify the largely reductionist, nucleocentric legacy of classical genetics, by employing classical genetics techniques to explore non-Mendelian phenomena in Paramecium. Unlike the fruit fly Drosophila, the bacterium E. coli, or phage virus, Paramecium never became a widely employed model for genetics research. It nonetheless was appreciated by an active network of researchers that circulated around Sonneborn’s laboratory. Like Jennings, Sonneborn considered the protozoa biologically important due to their dual status as organisms and complex single cells comparable to those found in higher organisms. Throughout his career he promoted their value as models for the study of cellular heredity.

Sonneborn’s focus on the role of the cytoplasm and on the interaction between genic and nongenic sources in inheritance was counter to the hierarchical model held by most geneticists circa 1950 of the nuclear genes as the sole locus of heredity. As a skilled writer and public speaker as well as a respected experimentalist, Sonneborn became the effective spokesperson during the 1940s and 1950s for research in cytoplasmic inheritance. In his popular article “Beyond the Gene” published in 1949, for example, Sonneborn argued for a revision of what he referred to as the “general gene theory” to include not only nuclear genes but cytoplasmic genes as well (p. 58). In “Partner of the Genes,” published a year later, he maintained that the cellular material outside the nucleus should no longer be considered a “silent partner” but rather an “active partner with the nucleus in the control of inheritance” (p. 30).

During the same period, Sonneborn played a lead role in a major defense of classical genetics, when reports began to surface both inside and outside the Soviet Union that his research supported Lysenkoism. In 1948, Trofim D. Lysenko won the official support of Joseph Stalin and the Communist Party to lead efforts to improve Soviet agricultural output through the use of techniques predicated on the inheritance of acquired characteristics. Soviet policy and propaganda associated with these events proclaimed the invalidity of classical gene theory and labeled it as reactionary and idealist. These events nourished Cold War fears among scientists in the United States and Europe related to the suppression of scientific freedom in the Soviet Union. Sonneborn joined forces with leading geneticists including Hermann J. Muller and Theodosius Dobzhansky in attacking Lysenkoism on both scientific and political grounds. In defense of his own work, Sonneborn maintained that his research was an extension of classical genetics rather than a rejection of it, as had been suggested by Lysenkoists.

At both the undergraduate and graduate level and in the laboratory, Sonneborn’s infectious enthusiasm for the study of life fueled the interest of many in the study of genetics. In informal weekly seminars held at his home, Sonneborn attracted the faithful participation of not only his own students, but many from the laboratories of other university biologists. Sonneborn’s Bloomington laboratory was considered an international mecca for “Paramecium workers,” including many advanced researchers, postdocs, and more than forty students who earned their PhDs under his guidance.

Many biologists feel that Sonneborn’s work anticipated, when others had not, the late-twentieth-century preoccupation with genetic systems and biological complexity brought about by the use of molecular biology tools and techniques beginning in the 1960s. Sonneborn represented his philosophy of biology not as the antithesis of reductionism, but as a centrist position similar to that held a generation earlier by Jennings, who was also both a critic of reductionism and a champion of the principle of emergent evolution. Sonneborn maintained that genetic understanding would not arise from the study of the simplest biological models alone, but from the investigation of evolutionary innovation from the simplest to the highest organisms. In addition to those honors listed in the original DSB article, Sonneborn was elected to the American Philosophical Society in 1952.



“Sex, Sex Inheritance and Sex Determination in Paramecium aurelia.” Proceedings of the National Academy of Sciences of the United States of America 23 (1937): 378–385.

“Recent Advances in the Genetics of Paramecium and Euplotes.” Advances in Genetics 1 (1947): 263–358.

“Beyond the Gene.” American Scientist 37 (1949): 33–59.

“Partner of the Genes.” Scientific American 183 (November 1950): 30–39.

“Heredity, Environment, and Politics.” Science, n.s., 111 (1950): 529–539.

“The Role of the Genes in Cytoplasmic Inheritance.” In Genetics in the 20th Century, edited by Leslie Clarence Dunn. New York: Macmillan, 1951.

“Breeding Systems, Reproductive Methods, and Species Problems in Protozoa.” In The Species Problem, edited by Ernst Mayr. Washington, DC: American Association for the Advancement of Science, 1957.

“Does Preformed Cell Structure Play an Essential Role in Cell Heredity?” In The Nature of Biological Diversity, edited by John M. Allen. New York: McGraw-Hill, 1963.

With Janine Beisson. “Cytoplasmic Inheritance of the Organization of the Cell Cortex in Paramecium aurelia.” Proceedings of the National Academy of Sciences of the United States of America 53 (1965): 275–282.

“The Evolutionary Integration of the Genetic Material into Genetic Systems.” In Heritage from Mendel, edited by R. Alexander Brink. Madison: University of Wisconsin Press, 1967.


Nanney, D. L. “Tracy M. Sonneborn (1905–1981).” Genetics 102 (1982): 1–7.

——. “T. M. Sonneborn: Reluctant Protozoologist.” Progress in Protozoology: Proceedings of VI International Congress of Protozoology, Special Congress Volume of Acta Protozoologica. Part I. Warsaw: Panstowwe Wydawn. Nauk., 1982.

Preer, John R. Jr. “Tracy Morton Sonneborn, October 19, 1905–January 26, 1981.” Biographical Memoirs, vol. 69. Washington, DC: National Academy of Sciences, 1996. Available at

Sapp, Jan. “Concepts of Organization: The Leverage of Ciliate Protozoa.” In A Conceptual History of Modern Embryology, edited by Scott F. Gilbert. Baltimore: Johns Hopkins University Press, 1991.

Schloegel, Judith Johns. “From Anomaly to Unification: Tracy Sonneborn and the Species Problem in Protozoa, 1954–1957.” Journal of the History of Biology 32 (1999): 93–132.

——. “Intimate Biology: Herbert Spencer Jennings, Tracy Sonneborn, and the Career of American Protozoan Genetics.” PhD diss., Indiana University, Bloomington, 2006.

Judith Johns Schloegel