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TRACY MORTON SONNEBORN October I 9. I 905-fanuary 26, I 98 ' BY JOHN R. FREER, JR. WITH PIPETTES, CULTURE VESSELS, a low- and a high-power microscope, en c! a few collections of water from nearby poncis en c! streams, Tracy Sonneborn worker! with species of the Paramecium aurelia group en c! learner! more about the basic biology of protozoa than anyone else ever has. He cliscoverec! mating types in Paramecium, thereby advancing biological studies on the protozoa by a quantum leap. He clemonstratec! simple Menclelism en c! establisher! the be- havior of genes, nuclei, en c! cytoplasm in the complex pro- cesses of the life cycle. He shower! that the uniparental nuclear reorganization that occurs perioclically in many para- mecia is the sexual process of autogamy, not the asexual process of endomixis as originally thought. He cliscoverec! macronuclear regeneration en c! cytoplas- mic exchange, both invaluable for genetic analysis. He clem- onstratec! caryonicial inheritance, showing that incliviclual ciliate macronuclei, although descended asexually from iden- tical micronuclei, can acquire different genetic properties during their development. He showed that the phenotype of Paramecium is cleterminec! by the macronucleus, not the micronucleus. He acivancec! our unclerstancling of the states of immaturity, maturity, and senescence in the life cycle of the ciliatec! protozoa, showing that aging can be reverser! 269
270 B I O G RA P H I C A L EMOIRS by autogamy as well as conjugation. His analysis of species in lower organisms proclucec! novel evolutionary concepts. Primarily he will be rememberer! for his studies on non- Menclelian inheritance. When he began his work, the role of the cytoplasm in heredity was entirely unknown. He shower! that the various cases of non-Menclelian inheritance conic! be ciassifiec! into distinct groups, most involving interac- tions between nuclear genes en c! the cytoplasm. His early studies on the cytoplasmic factor "kappa" established the first case of cytoplasmic inheritance in animals, en c! subse- quent work by him en c! his students shower! that intracellu- lar symbioses en c! cell organelles have become inextricably combiner! cluring evolution. His studies on surface proteins shower! that complex systems of interacting elements in pro- tein synthesis can create stable states of gene expression clepenclent on factors present in the cytoplasm. In a most elegant series of experiments on the ciliate cortex, he en c! his collaborators shower! that the form en c! arrangement of preexisting structures determine the form en c! arrangement of new structures. Finally, studying mating type en c! an unusual trichocyst mutant, he uncoverec! the first examples of a strange non- Menclelian phenomenon in which the macronucleus of cili- ated protozoa determines the cytoplasm, and the cytoplasm in turn determines newly forming macronuclei, thereby pass- ing genetic information from the old disintegrating macro- nucleus to the newly forming macronuclei. PERSONAL HISTORY i Tracy Morton Sonneborn was born on October 19, 1905' n Baltimore, MarylancI. His mother was Daisy Bamberger, en c! his father, Lee, was a businessman. Both encourages! him in his education. Others having an important influ
TRACY MORTON SONNEBORN 271 once in his early life were his uncle, Jacob Bamberger, en c! his cousin, Louis Bamberger. It was Louis Bamberger who establishec! the Institute for Advance c! Stucly at Princeton. As a teenager, Tracy became interested in the humanities en c! religion en c! at one point seriously consiclerec! becom- ing a rabbi. However, his beliefs soon changed, and, after attending Baltimore Polytechnic High School for two years en c! Baltimore City College High School for two more years, he enterer! Johns Hopkins University with the intention of studying literature. His interests changer! to science when he took an introductory course in biology taught by E. A. Ancirews. He receiver! the B.A. degree from Hopkins in 1925. He then began graduate work on the flatworm, Stenostomum, uncler the supervision of Herbert S. Jennings, director of the Zoological Laboratories at Johns Hopkins. Jennings was a remarkable scholar, one of the pioneers of biology. He publisher! extensively en c! was renownec! as a scientist, phi- Tosopher, en c! educator. Jennings hac! a broac! view of biol- ogy. He worker! on lower organisms en c! was concernec! with the most funciamental aspects of behavior, inheritance, development, population biology, and evolution. Jennings hac! a profounc! influence on Tracy's clevelopment as a sci- entist. Tracy's passion for thoroughness en c! cletail en c! his . . . broac! view of biology were like that of his teacher. He re- ceivec! the Ph.D at Johns Hopkins in ~ 928. At that time, he receiver! a National Research Council fellowship and spent 1928 and 1929 with Jennings at Hopkins working on the ciliate, Colpidium. In 1929 he marries! Ruth Meyers, it was a happy union that lasted until his death fifty-two years later. At the enc! of Tracy's fellowship in 1930 his attempts to obtain a faculty position failecI, but he was offerer! a position as a research assistant at Hopkins with Jennings, who hac! just obtainer! a research grant from the i,
272 B I O G RA P H I C A L EMOIRS Rockefeller Foundation for work on Paramecium. During the perioc! 1930 to 1939, Tracy hell! the positions of research associate en c! then associate at Johns Hopkins. After seven years of basic studies on the life cycle of Paramecium, he made his discovery of mating types in 1937, which immedi- ately won him fame as an investigator. In 1939 Fernanclus Payne persuaclec! him to accept a position at Indiana Uni- versity as an associate professor. There he stayer! for the rest of his life, becoming professor in 1943, clistinguishec! service professor in 1953, en c! clistinguishec! professor emeri- tus in 1976. His first son, Lee, was born in 1929 in Baltimore en c! became a mathematician. His seconc! son, David, was born in 1934, also in Baltimore and, like his father, became a biologist. Tracy's family life was remarkable. His wife, Ruth, was eclucatec! as a social worker en c! might have hac! a clis- tinguishec! career of her own. Instead, she clevotec! her life to family en c! to his career. He was cleeply grateful to Ruth, for she macle it possible for him to devote himself virtually full time to his scholarly activities. She was clearly the mother en c! personal confidant of all the many students en c! post- doctorals who passed through the Sonneborn Laboratory at Indiana University. When Tracy arriver! home from work, his role of eminent scientist whose every wore! was carefully consiclerec! by his students changer! completely. He was just one more member, albeit a greatly belovec! member, of a very close, well-acljustecI, happy family. At one point cluring Thanksgiving clinner at my first visit to his home, emit! all the Slav conversation. Tracy was finally able to get in an O , , opinion on the topic at hancI. There follower! a suciclen silence arounc! the table follower! by a pronouncement from his youngest son, age five: "OIc! crummy Dacicly." As a new graduate student I was incleec! shockocI, for at the labora- tory his every pronouncement was worthy of the utmost
TRACY MORTON SONNEBORN 273 respect en c! consideration, but here everyone thought it was a splenclic! joke. Throughout his life these close rela- tions within his family never changed. Tracy was vitally interested in the activities en c! accom- plishments of those about him. In conversation he spoke quickly and thought even more quickly. His incisive and often blunt comments were a bit intimidating at first for some of his new students, but his kinciness en c! humor macle him easy to engage in conversation. After a full clay in his office en c! laboratory, he spent almost every evening think- ing, writing, en c! making notes. For most of his life he met for a long session once a week in the evening with his stu- clents en c! others in his research group. Music en c! bircis were his primary hobbies, but they took only a small por- tion of his time. Every task that ciaimec! his attention an experiment, a new course, a research report, a manuscript to review, a stuclent's class paper somehow became the most impor- tant thing in the woric! to him. It hac! to be clone with thoroughness en c! perfection. Nothing was too much trouble. An unclergracluate lecture was as important as a keynote , aciciress at a major scientific meeting. He regularly took his place at unclergracluate registration, interviewing each stu- clent (often 200 or 300), making sure all hac! the appropri- ate backgrounc! en c! interests for his class. He once com- mentec! that teaching en c! research in no way interfered with each other, for all one neecis to clo is devote forty hours per week to each. For him that was clearly an uncler- statement. His lectures, whether for large classes, small classes, un- clergracluates, graduates, or scientific papers presented to his peers, were presented in a clear en c! exciting fashion. His enormous enthusiasm spreac! to all his audience. After a lecture at Goucher College in 1937 describing his first
274 B I O G RA P H I C A L EMOIRS fincling of mating types, his audience was really to follow him out the floor en c! back to his laboratory to fins! what the experiments in progress wouIc! show. His first course at Indiana University, which was supposes! to cover all the in- vertebrates, got no farther than the flatworms. He user! to joke that when his department chairman, Fernanclus Payne, learner! that he hac! only coverer! protozoa, coelenterates, en c! flatworms in his course on invertebrates he almost got firecI. It is noteworthy that two members of the class went on to careers studying protozoa, one even shifting from a commitment in another fielcI. The excitement he gener- atec! was genuine en c! long lasting. For example, when he lecturer! on algae in a course with no formal laboratory, it was routine to see algae appear spontaneously in the vari- ous laboratories in which the graduate students workocI, as they attemptec! to repeat en c! carry some of the experi- ments a step further. In the late 1940s his laboratory enIargecI. He brought in Wilhelm van Wagtenclonk, a biochemist from Oregon, who Sonneborn hoper! wouIc! work out the biochemical basis for the many genetic traits that he was investigating. How- ever, it turner! out that these traits were not reacliTy acces- sible to biochemical investigation. Van Wagtenclonk cleciclec! that it was necessary first to develop a defined medium for culturing Paramecium. This endeavor prover! to be very clif- ficult en c! time consuming. In the enc! he was successful, but it requires! the remaining portion of Van Wagtenclonk's research career to achieve success. Early on, Ruth Dippell became his research technician. Ruth eventually receiver! the doctorate degree en c! became a faculty member, but she always worker! closely with him in her research. As the laboratory enIargec! en c! his Ph.D. students increaser! in number, numerous postdoctoral workers also came, many from Europe en c! some from Japan en c! China. Bloomington
TRACY MORTON SONNEBORN 275 became the Mecca for all who wouic! work on Paramecium. These investigators went on to important positions in uni- versities en c! research institutes throughout the worIcI. Soon most of the work on Paramecium was being clone by those who hac! passer! through his laboratory. He continues! to clo research until his cleath in Bloomington in 1981, following a short illness with cancer. Tracy receiver! many honors cluring his career. He was electec! a member of the National Academy of Sciences in 1946, a foreign member of the Royal Society of London in 1964, a member of the American Academy of Arts en c! Sci- ences in 1946, en c! a member of the American Philosophi- cal Society in 1952. He receiver! the Kimber Genetics Awarc! of the National Academy of Sciences in 1939, the Menclel Centennial Mecial of the Czechoslovakian Academy of Sci- ences in 1965, en c! the Newcomb-Clevelanc! Mecial en c! Prize of the American Association for the Advancement of Sci- ence in 1946. He was an honorary member of the French Society of Protozoology, the Genetics Society of Japan, en c! the American Society of Protozoologists. He receiver! hon- orary cloctor's degrees from Johns Hopkins University, North- western University, Indiana University, the University of Geneva (SwitzerIancI), en c! the University of Westphalia (Ger- many). He server! as president or boars! member of many scientific organizations en c! gave numerous prestigious lec- tures in this country en c! abroad. ~. . .. STENOSTOMUM PROFESSIONAL HISTORY When Tracy began his work for the Ph.D. in 1926, his mentor, Jennings, believer! that, although Menclelian genes were responsible for most of the traits in higher organisms, other genetic mechanisms might also exist. These factors
276 B I O G RA P H I C A L EMOIRS were thought to be especially important in lower organ- isms, en c! it was also thought that they might be localizes! in the cytoplasm en c! susceptible to environmental moclifica- tion. The way to test these speculations was simply to stucly the effects of environment en c! heredity on the clevelop- ment of various traits in selectee! lower forms of life. Such studies were to form the basis of Tracy's whole research career. His Ph.D. problem was on inheritance in Stenostomum, which reproduces asexually by clivicling transversely into an anterior en c! a posterior half. He was able to identify en c! follow these halves in isolation cultures en c! fount! that pro- gressive lines of anterior division products were more likely to age en c! clie than lines of posterior products. He also exposer! Stenostomum to leas! acetate en c! fount! that abnor- malities appeared. After such treatments he was able to iso- late two-heaclec! "monsters" that reproclucec! true to type. Since these traits were maintainer! for many generations, they were jucigec! to have a hereditary basis. However, these variants arose and were lost at a much higher frequency than one woulc! expect if they were clue to mutations in simple Menclelian genes. COLPIDI UM After his Ph.D. work, Tracy stayed for eleven more years in Jennings's laboratory at Johns Hopkins. His first work was on the small ciliate, Colpidium, an organism he hac! user! to few! his Stenostomum. He cultures! Colpidium on a strain of bacteria on which they flourished. When he changer! the bacterium to another less favorable kind, abnormalities appearec! in the belly form. From these abnormal animals he was able to isolate double animals, en c! these doubles reproclucec! true to type indefinitely, even when they were returnee! to culture on the more favorable bacterium. Again, the effect of the environment in inclucing abnormal ani
TRACY MORTON SONNEBORN 277 mats of a particular kinc! in high frequency was not what one would expect on the basis of mutation in Menclelian genes. THE LIFE CYCLE OF PARAMECIUM Sonneborn began his work on Paramecium when the prob- lems of genetics, clevelopment, cell biology, en c! evolution were being attacker! energetically by such workers as Mor- gan, Sturtevant, Bridges, Darlington, Halciane, Wright, Demerec, Jennings, Ephrussi, BeacIle, Tatum, Emerson, McCTintock, en c! StacIler. Sonneborn assumer! his position as one of that group. His plan for research was simple: learn all he conic! about a single organism en c! apply his knowledge generally where applicable. By choosing a single organism, Paramecium, he thought he conic! attain a mas- tery of that organism that wouIc! enable him to carry out sophisticates! experiments impossible for scientists who pick a single problem en c! move from organism to organism. He stuck to his plan faithfully, studying Paramecium almost ex- clusively cluring his whole research career. Sonneborn notes! that, while protozoa are whole organisms, they are also single cells, en c! he recognizec! a rare chance to stucly inheritance inclepenclently of the complex multicellular life cycle that precluclec! investigations of cellular genetics in most organ- isms. While procaryotes are also unicellular, he felt that most studies on bacteria were concernec! with populations of cells, not incliviclual cells. A test for Menclelism by brawling analysis conic! not be macle in the case of either Stenostomum or Colpidium be- cause both lackey! sexual reproduction. By turning to Para- mecium, which is able to conjugate en c! exchange germinal nuclei, he thought clefinitive tests of Menclelism wouic! be possible. The only problem was that mating reactions, while common in both nature en c! the laboratory, conic! not be
278 B I O G RA P H I C A L EMOIRS controller! en c! often occurrec! even in clones (i.e., cultures clerivec! by binary fission from single celIs). So Sonneborn set about learning to unclerstanc! en c! control mating en c! the life cycle. Members of the Paramecium aurelia complex of species have a vegetative polyploic! macronucleus that clirectly con- trols the characters of the cell en c! also two germinal micro- nuclei that perioclically give rise to new macronuclei. Para- mecium reproduces vegetatively by binary fission. The macronucleus clivicles amitotically, en c! the micronuclei cli- vicle by mitosis. At conjugation en c! autogamy, the oIc! ma- cronucleus breaks into fragments en c! normally is lost clur- ing subsequent fissions, while the two micronuclei undergo meiosis. A single haploic! melotic nucleus then clivicles to give a migratory en c! a stationary haploic! nucleus. The mi- gratory nucleus from each conjugant fuses with the station- ary nucleus of its mate, or in the uniparental process of autogamy the two products simply fuse with each other. In each cell the cliploic! zygote micronucleus gives rise by mi- tosis to four micronuclei. Two remain as micronuclei en c! two clevelop inclepenclently into macronuclei. At the next fission the two new macronuclei are segregated one to each daughter cell, while the micronuclei clivicle mitotically en c! are clistributec! two to each daughter cell, restoring the nor- mal vegetative state. Sonneborn was able to control auto- gamy when he fount! that a rapic! fission rate in an excess of fresh culture medium inhibited autogamy while starva- tion inclucec! it, proviclec! the animals hac! undergone a suf- ficient number of fissions since the last conjugation or au- togamy. Note that following the first fission after autogamy en c! conjugation each of the two cells has a macronucleus derived independently from different micronuclei he called the two lines "caryonicles." He cliscoverec! that mating within a caryonicle is seen only rarely, while for the strains of Para
TRACY MORTON SONNEBORN 279 mecium that he was studying, mixing cells of different caryonicles in the proper physiological condition often re- sultec! in immediate en c! massive mating reactions leacling to pair formation en c! conjugation. In this way he not only cliscoverec! mating types in protozoa, but acquirer! the abil- ity to make crosses between different lines. Many different mating types, characteristic of different strains of P. aurelia were clescribecI. The discovery of mating types was an excit- ing discovery en c! won Sonneborn immediate recognition by the academic community en c! even in the press. Later he shower! that sometimes fragments of the oIc! macronucleus are not lost but persist en c! in subsequent asexual generations regenerate into macronuclei. Moreover, he learner! how to incluce this process of macronuclear re- generation at will. Although cytoplasm is not normally ex- changec! at conjugation, he also learner! how to incluce cy- toplasmic exchange. Furthermore, he prover! that the nuclei behaves! as clescribec! above by showing that, after auto- gamy, lines are homozygous in all their genes, en c! after conjugation typical Menclelian ratios conic! be proclucecI. These techniques gave him exquisite control over his or- ganism and made it possible for him to carry out highly sophisticates! genetic experiments. MATING-TYPE INHERITANCE As he continues! his investigations on the genetics of Para- mecium, Sonneborn stucliec! all the character differences he conic! fincI. Unlike students of genetics in organisms like Drosophila, maize, and, later, Neurospora en c! yeast, he fount! that virtually every character he looker! at in those early clays prover! to involve a combination of Menclelian en c! non-Menclelian elements. In some strains of Paramecium two mating types were founcI. Determination occurrec! at the formation of the new macronuclei at conjugation or auto
280 B I O G RA P H I C A L EMOIRS gamy. Other strains, expressed only one mating type. It was shown that the difference in strains was accountec! for by a single genie difference, the first gene clemonstratec! in cili- ates. Tociay we know that massive reorganization of the DNA occurs at macronuclear formation in the ciliates, involving chromosome breakage, cleletions, ant! reordering of se- quences. In the case of mating-type determination, reorga- nization can proceec! in such a way that one mating type is expressed in one caryonicle, while another mating type is expressed in a sister caryonicle. Only tociay are we coming to appreciate the many cases of nuclear differentiation that occur cluring clevelopment in the metazoa. In simple caryonicial inheritance, mating type is cleter- minec! inclepenclently of the parental type en c! inclepen- clently of each of the two sister caryonicles after conjuga- tion or autogamy. However, it was fount! that, in some strains of Paramecium, mating-type inheritance prover! to be caryo- nicial but also shower! a market! tendency for the new caryonicles to be like each other en c! like the mating type of the original cell in which they were formed. The results appearec! to indicate cytoplasmic inheritance, en c! this con- clusion was reinforcer! by crosses involving cytoplasmic trans- fer from one mate to the other. In a brilliant experiment involving conjugation, cytoplasmic exchange, en c! macro- nuclear regeneration, Sonneborn was able to produce incli- viclual cells that container! fragments of the oIc! macronucleus clestinec! to regenerate, micronuclei that were cleveloping into macronuclei, and cytoplasm of the opposite mating type clerivec! from the mate. At subsequent fissions, macro- nuclei of the two kinds segregated, and by means of genetic markers he was able to distinguish those clerivec! from the oIc! macronuclear fragments from those arising from new macronuclei cleveloping in the normal way from micronu- clei. The results clemonstratec! that newly forming macro
TRACY MORTON SONNEBORN 28 nuclei clerivec! from micronuclei responc! to the cytoplasm in which they are fount! en c! become cleterminec! like the cytoplasm that surrounds them. However, the type of frag- ments is always like that of the original macronucleus from which they were clerivecI. In short, in the formation of new macronuclei the cytoplasm determines the macronucleus for mating type, en c! once the macronucleus is cleterminec! it never changes. The cytoplasm, on the other hancI, always reflects the type of macronucleus with which it is founcI. Thus, it appears that genetic information is passer! from the oIc! macronucleus to the cytoplasm to the newly form- ing macronuclei. This mocle of inheritance has since been caller! "macronuclear inheritance" by Meyer. Macronuclear inheritance has been shown to occur for a number of other traits in Paramecium. Its molecular mechanism is still not unclerstoocI. KILLERS Sonneborn also cliscoverec! en c! stucliec! killers, parame- cia that proclucec! a toxin that conic! kill other strains of paramecia yet that are resistant to their own toxin. Crosses shower! the presence of nuclear genes necessary for the perpetuation of the killer trait and also showed the pres- ence of an essential cytoplasmic element that he caller! "kappa." Strains that lost kappa became sensitive to the toxin. Kappa proved puzzling to Sonneborn for many years, but it was finally shown in other laboratories that kappa is an example of a symbiotic bacterium able to live only in Para- mecium. Many such forms have been clescribec! with various degrees of benefit en c! harm to their hosts. They emphasize to all geneticists the difficulties of distinguishing between infection and cytoplasmic heredity. In fact, it is now consid- erec! that all cases of cytoplasmic heredity baser! on the presence of self-replicating cytoplasmic nucleic acids are
282 B I O G RA P H I C A L EMOIRS probably clerivec! evolutionariTy from viruses or bacteria. Even such "normal" organelles as mitochonciria en c! chIoroplasts are thought to have such an origin. SEROTYPES Early workers shower! that antiserum preparer! by inject- ing paramecia into rabbits initially mobilizer! all cells of the injectec! clone. It was also fount! that resistant cells often appearec! en c! that after isolation they proclucec! resistant clones. Sonneborn began a stucly of this phenomenon en c! quickly confirmed! the general features of the early studies. By injecting the resistant paramecia into rabbits, he ob tainec! new sera en c! eventually shower! that from a single clone of Paramecium he conic! obtain many subclones pure for up to a dozen different antigenic types, called sero- types, each reacting only with its own homologous antise- rum. Moreover, he shower! that exposure to antiserum ac- tually inclucec! the shift from one serotype to another. Genetic analysis revealer! a series of inclepenclent genetic loci, each specific for a given serotype, with one gene active at a time. The shift from one serotype to another was clue to switch- ing from the activity of one gene (all the others inactive) to the activity of another. Serotype specificity and the ability of a serotype to be expressed at all were shown to be due to alleles at the different serotype loci. In one set of environ mental conditions, Sonneborn fount! that most of the sero- types wouIc! reproduce stably for many generations. Crosses between serotypes of a single pure genotype always revealer! cytoplasmic inheritance. Early in the investigation of sero types it was pointer! out that serotype inheritance conic! be explained in terms of stable states of gene expression that rely on feedback mechanisms for their perpetuation. This
TRACY MORTON SONNEBORN 283 interpretation was eventually acceptec! by Sonneborn en c! has recently receiver! support from molecular studies. PLASMAGENESE Virtually all the traits Sonneborn encounterer! in his early studies were non-Menclelian, with strong genie en c! strong cytoplasmic components. At this point the evidence seemec! to leac! to the conclusion that cytoplasmic inheritance was an important component in all cases of inheritance in Para- mecium. Perhaps in higher organisms that same was true, but it was being masked by the processes occurring in em- bryological clevelopment. So at this time the plasmagene theory was born: It was postulated that all genes in all or- ganisms produce a self-reproclucing entity that persists through somatic cell divisions but that is lost cluring sexual reproduction. The theory was given support by a number of studies clone by others, especially the studies of Spiegelman on adaptive enzymes in yeast. As work progressed, however, it became clear that the interpretation of the ciata as evidence for plasmagenes was not valicI. Kappa en c! its relatives turner! out to be symbiotic bacteria, dependent upon special genes for their mainte- nance. Further work on the expression of genes for surface proteins seemec! to be best interpreter! as a special inter- play of competing inhibitors en c! activators of protein syn- thesis. Mating-type inheritance was more clifficult to evalu- ate, but Sonneborn was able to show that mating-type inheritance was ultimately uncler nuclear control, the cyto- plasm acting only to transmit information from the old macronucleus to the newly cleveloping macronucleus. There was, in fact, no evidence for self-reproducing cytoplasmic genes.
284 THE CORTEX B I O G RA P H I C A L EMOIRS Facet! with these new finclings, the notion of plasmagenes was cliscarclecI, en c! Sonneborn embarkoc! on his investiga- tions of the structure of the cellular cortex in Paramecium. Sonneborn always viewoc! his early work on Stenostomum en c! Colpidium as incomplete, for, although his two-heaclec! mon- sters in Stenostomum en c! cloublets in Colpidium arose in high frequency en c! in response to environmental stimuli in a cleciclecIly non-Menclelian fashion, the organisms were asexual, en c! decisive brawling tests were not possible. He found, however, that cloublets conic! easily be inclucec! in Parame- cium by exposing conjugating cells to antiserum. He now set about crossing singles with cloubles. The results ruler! out Menclelian genes. He also ruTec! out both the presence of determinants in the Quit! cytoplasm en c! macronuclear inheritance like that observer! for certain mating types. He was left with the cortical structure itself as the basis for the inheritance. Moreover, he en c! his collaborators were able to show that rearrangements in the pattern of the cilia, trichocysts, parosomal sacs, en c! fibrillar structures that make up the cortex also can be inheritec! in the same fashion. Sonneborn said that these instances were based on a new principle of inheritance that he caller! "cytotaxis," the abil- ity of preexisting structures to control the formation and placement of new structures. Cyto taxis has since been stucI- iec! extensively in the ciliate cortex by many workers. Again, Sonneborn proclucec! a brilliant series of experi- ments. They showed without doubt that preexisting struc- ture controls the way new structures are former! in the cor- tex of ciliatec! protozoans. This work was hell! to be a major new phenomenon in genetics and development, applicable to all organisms. Currently, it appears that these principles
TRACY MORTON SONNEBORN 285 are incleec! applicable to other organisms en c! organelles, but its true general significance is yet to be cleterminecI. SENESCENCE AND THE LIFE CYCLE It has been known for many years that after conjugation many ciliates undergo an immature perioc! of many vegeta- tive generations in which they are unable to mate. Then after a perioc! of maturity, if mating floes not occur, there is a perioc! of senescence en c! finally cleath. Jennings pointer! out that each stage lasts for such a long perioc! that one must consoler the stages heritable. The basis for the changes has remainec! unknown, although recent experimental evi- clence involving microinjection reinforces the view that its basis lies within the macronucleus. In any case, it is clear that the mechanism floes not rely on simple Menclelian genetics. Sonneborn investigates! the matter in relation to the unisexual process of autogamy in Paramecium. He fount! that autogamy collie substitute for conjugation in rejuve- nating senescent lines of paramecia. Another life-cycle change he notes! was that after autogamy paramecia must undergo a certain number of fissions, in some cases a large number, before cells can undergo another autogamy. The basis for these life-cycle changes is unknown. THE SPECIES PROBLEM When Sonneborn discovered mating types, he found twenty- eight types among different strains. He was able to show that only mating type I could mate with mating type II, only III with IV, en c! so on, for a total of fourteen different mating pairs. He noted that each pair constituted a single interbreeding group. Since each group shared a common gene pool, it was clear that they constituted a series of sib- ling species. From the beginning he realized the taxonomic problem presenter! by the situation, for mating types can
286 B I O G RA P H I C A L EMOIRS not be readily ascertained in the field. Even in the labora- tory the process is time consuming, requiring the isolation and mixing of clones with standard mating types. He real- ized the problem that would be presented to taxonomists if the groups were given binomial names. His initial solution was to call the groups varieties. Later, in recognition of the genetic isolation of the varieties, he changed the designa- tion to a newly invented term, "syngen." Finally, as more became known about the syngens, particularly their isozymes, responses to different strains of killers, fission rate, and other traits, Sonneborn recognized the syngens of Parame- cium aurelia as separate species and designated them P. primaurelia, P. biaurelia, and so on. In other less-well-charac- terized ciliates, the mating groups are still called syngens. Sonneborn also pointed out that the sibling species of the P. aurelia group, as well as the sibling species of other protozoans that are delimited primarily by their mating types, presented a set of interesting ecological and evolutionary problems. He noted that some species in the P. aurelia group have a long immature period after conjugation, while oth- ers have a very short or no immature period. Since those with a long immature period are less likely to mate with each other in nature, he classified them as "outbreeders," while those with a short immature period he classified as "inbreeders." Outbreeding, which favors genetic diversity, was held to be the ancestral type. He made detailed studies of the properties of the various species and also studied the viability of progeny obtained from crosses. He related this information to the ecology and evolution of the croups. GENE S 0 1 Different Mendelian genes were not readily found in Para- mecium, but the behavior of the first ones that Sonneborn found were sufficient to establish the validity of the com
TRACY MORTON SONNEBORN 287 plex cytological events of the life cycle. Eventually, it was fount! that numerous mutants conic! be isolates! after chemical mutagenesis. Sonneborn then engages! in a large stucly, iso- lating more than 100 different visible morphological en c! behavioral mutants. These were then testec! for linkage. Be- cause of the large number of chromosomes in Paramecium en c! perhaps because of a high rate of recombination, few cases of linkage en c! no maps resultecI. TRICHOCYSTS Sonneborn's last project was the investigation of an aber- rant mutation that reclucec! the ability of trichocysts to clis- charge. Although several simple gene mutations hac! the same effect, this mutant seemec! to follow the cytoplasm in crosses. A more cletailec! analysis revealer! that it was inher- itec! just as mating type was inheritec! in many strains that is, it was macronuclear inheritance as clescribec! above. Since Sonneborn's cleath, aciclitional cases of macronuclear in- heritance affecting other traits have been fount! en c! are now being actively investigated. CONCLUSION While Sonneborn was learning whatever Paramecium conic! teach him about biology, a new generation of microbial geneticists, working with fungi, bacteria, and bacterioph- ages, was establishing the foundations for the new science of molecular biology. Unlike Paramecium, these organisms hac! properties that prover! to be invaluable in the new sci- ence. They had simple nutritional requirements, synthesiz- ing most of the complex substances they neeclec! en c! thereby enabling the investigator to study the genetic control of many of the enzymes of metabolism. They conic! be plater! onto agar, making possible the quick en c! easy examination of innumerable clones. This technique was absolutely es
288 B I O G RA P H I C A L EMOIRS sential for the study of mutations or rare recombinants. In these organisms one could carry out studies on the role of genes in controlling metabolic pathways, enzyme synthesis, and enzyme structure. Investigations of mutations and ge- netic fine structure also were possible. These were the stud- ies that finally led to our knowledge of the roles of DNA and RNA and produced the modern revolution in molecu- lar biology. Paramecium was eminently unsuited for any of these studies. Hence, Sonneborn did not participate in this revolution that was sweeping biology and biochemistry, although it was clear that, like everyone else, he greatly appreciated and admired the work that was going on. He would have loved to be at its forefront. But Paramecium did not lead him there and could not have led him there, for it was simply not useful for such studies. The Nobel prizes that were awarded so generously to the disciples of the new biol- og,v eluded Sonneborn. That is not to say that his work was unnoticed. He was, indeed, widely recognized as an out standing investigator. Nevertheless' a glance at anv current .1 ~· ~. · ~ -7 -- a------- --- ----I textbook of general biology or genetics leads one to the conclusion that he was not the originator of concepts that are basic to the thinking of most biologists and geneticists today. It has been suggested that Sonneborn avoided more con- ventional genetics and focused on the role of the cytoplasm in heredity. In my view, that notion is not correct. Sonneborn concentrated on the inheritance of whatever traits he could find in Paramecium without prejudice. It simply turns out that most of the easily observable traits in Paramecium are inherited in a non-Mendelian fashion. Would he have pur sued his research differently had he known that Paramecium could not take him to the forefront of the great revolution in biology that was just developing? In those early days no
TRACY MORTON SONNEBORN 289 one knew what Paramecium hac! to offer. It was a member of a group of organisms that was simply too big en c! too cliffer- ent to be left unexplorecI. We hac! to know what protozoa were like, just as we hac! to know about bacteria en c! viruses en c! insects en c! mice en c! corn en c! worms en c! zebra fish. We hac! to know about the role of the cytoplasm in genet- ics. Anti, incleecI, Paramecium turner! out to be icleal for the stucly of inheritance at the cellular level en c! for the stucly of nuclear differentiation. Although there are no plasma- genes, there are cytoplasmic entities that contain DNA. There are stable metabolic states that are passer! from one genera- tion of cells to the next. Preexisting structures en c! patterns of structures are important in determining new structures at cell division. Anti, finally, differentiation of new nuclei in ciliates can produce new stable configurations en c! can be influencec! by factors emanating from preexisting nuclei en c! passer! through the cytoplasm. The role of preexisting structure in clevelopmental biology is not yet unclerstoocI, ant! the strange nuclear ant! cytoplasmic effects that Sonneborn uncoverec! are still unexplainec! at the molecu- lar level. Whatever the final outcome of studies of these phenomena, he must take his place among the most bril- liant and devoted experimentalists in the history of biology en c! a true giant, like no other, in the field! of protozoan research. ~ HAVE DRAWN ON unpublished material in my files, much re- ceived from Tracy himself over the years, as well as unpublished material from Ruth Dippell and Ruth Sonneborn, his wife. The reader is also referred to an account of Tracy's life by G. H. Beale in Biographical Memoirs of Fellows of the Royal Society, vol. 28, pp. 537- 74 (London: Royal Society, 1982~.
290 B I O G RA P H I C A L S E L E C T E D EMOIRS B I B L I O G RAP H Y 1930 Genetic studies on Stenostomum incaudatum (nov. spec.), II. The ef- fects of lead acetate on the hereditary constitution. 7. Exp. Zool. 57:409-39. 1932 Experimental production of chains and its genetic consequences in the ciliate protozoan Colpidium campylum. Biol. Bull. 63:187-211. 1937 Sex, sex inheritance and sex determination in Paramecium aurelia. Proc. Natl. Acad. Sci. U.S.A. 23:378-85. 1939 Paramecium aurelia: mating types and groups; lethal interactions; determination and inheritance. Am. Nat. 73:390-412. 1941 Relation of macronuclear regeneration in Paramecium aurelia to ma- cronuclear structure, amitosis and genetic determination. The Collecting Net 16:3-4. Sexuality in unicellular organisms. In Protozoa in Biological Research, ed. G. N. Calkins and F. M. Summers, pp. 666-709. New York: Columbia University Press. 1943 Gene and cytoplasm. I. The determination and inheritance of the killer character in variety 4 of P. aurelia. Proc. Natl. A cad. Sci. U.S.A. 29:329-38. Gene and cytoplasm. II. The bearing of the determination and in- heritance of characters in P. aurelia on the problems of cytoplas- mic inheritance, Pneumococcus transformations, mutations and development. Proc. Natl. Acad. Sci. U.S.A. 29:338-43. 1945 Gene action in Paramecium. Ann. Mo. Bot. Garden 32:213-21.
TRACY MORTON SONNEBORN 1946 291 Experimental control of the concentration of cytoplasmic genetic factors in Paramecium. Cold Spring Harbor Symp. Quant. Biol. 11 :236- 55. 1947 Recent advances in the genetics of Paramecium and Euplotes. Adv. Genet. 1:263-358. 1948 The determination of hereditary antigenic differences in genically identical Paramecium cells. Proc. Natl. Acad. Sci. U.S.A. 34:413-18. With A. LeSeur. Antigenic characters in Paramecium aurelia (variety 4~: determination, inheritance and induced mutations. Am. Nat. 82:69-78. 1950 Methods in the general biology and genetics of Paramecium aurelia. 7. Exp. Zool. 113:87-148. Beyond the gene two years later. In Science in Progress, ed. G. A. Baitsell, pp. 167-203. New Haven: Yale University Press. 1954 The relation of autogamy to senescence and rejuvenescence in P. aurelia. f. Protozool. 1:36-53. 1957 Breeding systems, reproductive methods, and species problems in protozoa. In The Species Problem, ed. E. Mayr, pp. 155-324. Wash- ington, D.C.: American Association for the Advancement of Sci ence. 1959 Kappa and related particles in Paramecium. Adv. Virus Res. 6:229- 356. 1962 Does preformed cell structure play an essential role in cell hered
292 B I O G RA P H I C A L EMOIRS ity? In The Nature of Biological Diversity, ed. T. M. Allen 221. New York: McGraw-Hill. 1965 ,, pp. 165 With T. Beisson. Cytoplasmic inheritance of the organization of the cell cortex in Paramecium aurelia. Proc. Natl. A cad. Sci. U.S.A. 53:275- 82. 1970 Methods in Paramecium research. In Methods in Cell Physiology, vol. 4, ed. D. Prescott, pp. 241-339. New York: Academic Press. 1974 Paramecium aurelia. In Handbook of Genetics, vol. II, ed. R. King, pp. 469-594. New York: Plenum Press. 1975 The Paramecium aurelia complex of fourteen sibling species. Trans. Am. Micros. Soc. 94:155-78. 1977 Local differentiations of the cell surface of ciliates: their determina- tion, effects and genetics. In The Synthesis, Assembly and Turnover of Cell Surface Components, ed. G. Poste and G. L. Nicholson, Cell Surface Reviews, vol. 4, pp. 829-56. New York: Elsevier/North Hol- land. 1979 With M. V. Schneller. A genetic system for alternative stable charac- teristics in genomically identical homozygous clones. Dev. Genet. 1:21-46. 1980 With Y. Brygoo, A. M. Keller, R. V. Dippell, and M. V. Schneller. Genetic analysis of mating type differentiation in Paramecium tetraurelia II. Role of the micronuclei in mating-type determination. Genet- ics 94:951-59.