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Sonneborn, Tracy Morton


(b. Baltimore, Maryland, 19 October 1905; d. Bloomington, Indiana, 26 January 1981)

genetics, protozoology.

Tracy Sonneborn first received scientific recognition for his 1937 discovery of mating types in the ciliate Paramecium. This discovery made controlled matings in this organism possible and extended genetic analysis to microorganisms ten years prior to the discovery of genetic recombination in bacteria by Joshua Lederberg and Edward L. Tatum. Most of Sonneborn’s work dealt with the interaction of nuclear Mendelian factors with various types of non-Mendelian hereditary mechanisms. The richness of non-Mendelian phenomena in ciliates is probably a consequence of the fact that the organism inherits the entire parental body when a new life cycle starts at fertilization.

Sonneborn was raised in Baltimore and did his undergraduate work at Johns Hopkins, graduating in 1926. As an undergraduate he came into contact with H. S. Jennings, who pioneered the study of heredity in protozoa. Jennings’ courses and his way of thinking made a great impression on Sonneborn, and in 1926 he started graduate work with Jennings. His doctoral research dealt with the development of the turbellarian Stenostomum and the vegetative inheritance of an abnormal doublet biotype that was stably propagated over many cycles of vegetative fission. He received the Ph.D. from Johns Hopkins in 1928. During his postdoctoral research with Jennings, he studied the production and inheritance of a doublet phenotype in the ciliate Colpidium. In both of these cases Sonneborn suspected that the abnormal traits were inherited cytoplasmically, independently of the nuclear Mendelian genes. He was unable to prove this, because genetic breeding analysis was not possible with these organisms at the time.

On 6 June 1929 Sonnborn married Ruth Meyers. They had two sons, Lee and David. As academic positions were difficult to obtain following the economic collapse of 1929, Sonneborn took a position as Jennings’ research associate in 1930 and began studying factors controlling conjugation in Paramecium, in the hope of being able to reliably carry out Mendelian breeding analyses. It was not until 1937, however, that the key factor controlling conjugation, mating types, was discovered. The central observation was that while conjugation occurred rarely among cells in clonal cultures derived from a single progenitor cell, it occurred readily in mixtures of cells pooled from several different clonal cultures. Pairwise mixture of cultures led directly to the concept of mating type: cells of one clone could mate only with certain other clones, but not among themselves.

During the next ten years Sonneborn worked out the basic genetics of Paramecium. This included elucidation of the nature of conjugation (cross-fertilization) and autogamy (self-fertilization) and the roles of these two processes in the life history of the organism, as well as procedures to control the transmission of cytoplasmic factors between cells during mating. Sonnebom also developed a method for transferring part of a mature macronucleus into a young cell, a procedure that was critically important in his analysis of epigenetic determination in macronuclei. His early investigations of mating also revealed that the morphological species Paramecium aurelia was in fact a complex of several genetically isolated sibling species, each with its own characteristic breeding strategy. In later years he created proper Linnean binomials for these biological species.

During this period Sonnebom discovered a variety of non-Mendelian hereditary phenomena that provided the basis of most of his subsequent work, almost all of which took place at Indiana University in Bloomington, where he had taken a position in 1939. His analysis revealed that these phenomena were of several different types.

These types included a hereditary symbiosis that manifested itself in the killer trait, whereby cells of one strain were able to kill cells of other strains. The inheritance of this trait proved to be remarkable, for it relied on both a normal nuclear Mendelian factor and a self-replicating, genelike cytoplasmic particle that later analysis revealed to be a symbiotic bacterium. In absence of the nuclear gene, the symbionts were unable to grow. In absence of the symbionts, the killer trait could not be produced, even in the presence of the required nuclear factor.

A second category of non-Mendelian phenomena proved to involve epigenetic differentiation of macronuclei in response to cytoplasmic or environmental factors. Sonneborn demonstrated that both mating type and cell surface antigen type were epigenetically determined in response to cytoplasmic factors inherited at fertilization with the parental cytoplasm. Through an ingenious series of experiments he was able to demonstrate the cytoplasmic nature of the determinative factors and to show that they affected only new macronuclei produced after fertilization; they had no effect on old macronuclei present in the same cytoplasm. Later, in the 1970’s, Sonneborn discovered and analyzed in detail another phenomenon, trichocyst nondischarge, which showed a similar pattern of determination and inheritance. These studies are important because they provide paradigmatic examples of epigenetic differentiation and indicate that the basic machinery of epigenesis occurs in unicells as well as multicellular eucaryotes.

With the development of suitable genetic procedures, Sonneborn returned in the 1960’s to the problem of the inheritance of abnormal patterns of cell organization, and demonstrated a third type of non-Mendelian phenomenon. He conclusively proved that the doublet phenotype in Paramecium was not controlled by Mendelian factors, but was a result of the cytoplasmic inheritance of the doublet organization. In a brilliant series of experiments with Janine Beisson he demonstrated through grafting that supernumerary longitudinal rows of basal bodies and flagella were perpetuated because the new structures produced prior to cell division were assembled in a precise geometric relationship to preexisting structures within a single longitudinal row of basal bodies.

Sonneborn received many honors during the course of his career. He was elected to the National Academy of Sciences in 1946, to the American Academy of Arts and Sciences in 1949, and to foreign membership in the Royal Society in 1964. In 1959 he received the Kimber Award for Genetics from the National Academy of Sciences, and in 1965 the Mendel Medal of the Czechoslovak Academy of Sciences. Sonneborn was awarded honorary degrees by Johns Hopkins University (1957), Northwestern University (1975), the University of Geneva (1975), Indiana University (1978), and the University of Munster (1979). He served as president of the American Society of Naturalists (1949), the Genetics Society of America (1949), the American Society of Zoologists (1956), and the American Institute of Biological Sciences (1961).


I. Original Works. A list of Sonneborn’s scientific papers is included in G. H. Beale’s biographical memoir (see below). A list of his major papers through 1975 was published in Genetical Research, 27 (1976), 349–351. Complete bibliographies and unpublished material, including an autobiographical sketch, are in the Lilly Library. Bloomington, Indiana.

II. Secondary Literature. G. H. Beale, “Tracy Morton Sonneborn,” in Biographical Memoirs of Fellows of the Royal Society, 28 (1982), 537–574; D. L. Nanney “T. M. Sonneborn; An Interpretation,’ in Annual Review of Genetics, 15 (1981), 1–9; “The Ciliates and the Cytoplasm,” in Journal of Heredity, 74 (1983), 163–170; “Heredity Without Genes: Ciliate Explorations of Clonal Heredity,” in Trends in Genetics, 1 (1985), 295–298; J. Harwood “The Erratic Career of Cytoplasmic Inheritance,” in Trends in Genetics, 1 (1985), 298–300; Jan Sapp, “Inside the Cell: Genetic Methodology and the Case of the Cytoplasm,” in J. A. Schuster and R. R. Yeo, eds., The Politics and Rhetoric of Scientific Method (Dordrecht, Netherlands, 1986), 167–202, and Beyond the Gene: Cytoplasmic Inheritance and the Struggle for Authority in Genetics (New York and Oxford, 1987).

James D. Berger

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