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. 2007 Nov;177(3):1439–1444. doi: 10.1093/genetics/177.3.1439

Guido Pontecorvo (“Ponte”): A Centenary Memoir

Bernard L Cohen *,1
PMCID: PMC2147990  PMID: 18039877

IN a memoir published soon after Guido Pontecorvo's death (Cohen 2000), I outlined his attractive, but sometimes irascible, character, his history as a refugee from Fascism, and his most significant contributions to genetics. The centenary of his birth (November 29, 1907) provides an opportunity for further reflections—personal, historical, and genetical.2

Two points of interest arise from the support that Ponte received from the Society for the Protection of Science and Learning (SPSL). Formed in 1933 as the Academic Assistance Council, SPSL aimed to assist the refugees who had started to arrive in Britain from the European continent (among them Max Born, Ernst Chain, Albert Einstein, Rudolf Peirls, and Max Perutz). In 1938, facing dismissal on “racial” grounds from his position in Tuscany (a copy of the letter of dismissal is available from the author), Ponte arranged with Alick Buchanan-Smith,3 whom he had met in Edinburgh during a 1937 tour of animal breeding centers, to work at the Institute of Animal Genetics (IAG) while he sought another post and, on Buchanan-Smith's recommendation, Pontecorvo applied for and was awarded a small, short-term SPSL scholarship. Thus, he could again apply a genetical approach to a problem related to animal breeding (Pontecorvo 1940a), the branch of agriculture in which he had most recently specialized with a series of data-rich articles (e.g., Pontecorvo 1937). But Ponte was stranded in Edinburgh by the outbreak of war and the cancellation of a Peruvian contract and continued for about 2 years to be supported by SPSL. The first point of interest is a prime example of the power of chance and opportunity. Renting a small room in the IAG guest house, Ponte there met Hermann Joseph Muller, who had recently arrived from Russia. Muller was almost certainly the most able, perhaps the only “pure” geneticist that Ponte had met (Pontecorvo 1968a,b) and the effect of this meeting was so strong that Ponte promptly changed direction, registering as Muller's Ph.D. student (July 1939–April 1941). The second point of interest is that in 1955, when appointed to the newly founded chair of genetics at the University of Glasgow, Ponte immediately offered to repay his SPSL scholarship, something that only a few others had done. There being no more refugees, the money was put toward a history of the SPSL.

For someone with no background in experimental genetics, Ponte's Ph.D. project must have provided a challenging experience: the sterility of melanogaster × simulans hybrids was overcome by mating heavily X-irradiated simulans males with triploid, multiply marked, females. Viable, diploid, “pseudobackcross” progeny occasionally appeared in which one or more chromosomes were homozygous for melanogaster markers while the remainder were heterozygous, enabling at least some further analysis of the genetical correlates of species divergence in the form of interchromosomal and chromosomal/cytoplasmic interactions (Muller and Pontecorvo 1940; Pontecorvo 1943a). While this work shed no important and lasting light on genetic isolating mechanisms or on the nature of species differences—other species were more informative—it was a typically ingenious application of Muller's great knowledge of the intricacies of Drosophila genetics and his skill in devising experiments (Crow 2005) and is still often cited. One report (Pontecorvo 1943b, p. 52) also contains a cryptic reference to Ponte's temporary internment as an “enemy alien” where, referring to Muller and Pontecorvo (1941), he writes, “Unfortunately their work was suddenly interrupted before completion.” The experience of X-ray-induced chromosome loss was soon put to use in another context (see next paragraph) and again almost 30 years later (e.g., Pontecorvo 1971).

Ponte is most widely known for his pioneering work in microbial genetics. This resulted in the development of Aspergillus nidulans as a genetically tractable model organism (Pontecorvo 1949; Pontecorvo et al. 1949, 1953) and in recognition of the parasexual cycle (Pontecorvo 1954; Pontecorvo and Sermonti 1954) and the possibilities that this offered for genetic analysis in imperfect fungi and, via somatic cells, in humans (Pontecorvo and Käfer 1956; Pontecorvo 1958b, 1959). Less widely known, Ponte introduced the term “mutational site” and first applied Haldane's terms cis and trans to the genomic arrangement of alleles (Pontecorvo 1950). Surprisingly perhaps, the turn to microbes owed something to Pontecorvo's Ph.D. studies: he started work with Penicillium around 1943 and proposed to use X-rays to create high-yielding penicillin-producing strains. But because all British penicillin research and development had been officially transferred to the United States, this initiative was discouraged. E. Sansome and M. Demerec, along with others in the United States, soon discovered or developed high-yielding strains, thereby enabling penicillin to be made available in quantity. Despite the official discouragement, some fundamental studies of Penicillium were pursued in collaboration with a member of the botany department staff (later a noted horticulturalist and broadcaster). Genetic evidence for Penicillium heterokaryosis was obtained (Pontecorvo and Gemmell 1944b), the significance of sectored colonies explored (Pontecorvo and Gemmell 1944a), and, with a later collaborator, parasexual cycle genetics was demonstrated (Pontecorvo and Sermonti 1954). Although the earlier work with Penicillium might have been a dead end, the search for a genetically tractable micro-organism that followed was largely inspired by realization of the opportunities for genetic analysis created by heterokaryosis (Pontecorvo 1947) and by clear ideas on how the resolving power of genetic analysis was limited by the number of offspring that could be screened (Pontecorvo 1956a). It is notable that, of the novel processes of gene recombination that came to light among micro-organisms between 1936 and 1946, i.e., bacterial transformation, conjugation, transduction, and the parasexual cycle, only the latter does not generally involve merozygosis (Pontecorvo 1958b).

The power of somatic recombination for the recovery from diploids of segregants homozygous for recessive alleles and for mapping gene loci in the absence of meiotic sexual processes was soon applied by plant pathologists to the study of host–pathogen relations (Day 1960) and these techniques have been more recently embraced by mouse geneticists. For example, in recognition of his pioneering work, Haigis and Dove (2003) dedicated their publication “to the memory of Guido Pontecorvo, who insisted long ago on the importance of somatic recombination for mammalian genetics.” Pontecorvo also emphasized “fields of higher order” in which neighboring functional units may be integrated into one field of action (Goldschmidt 1955; Pontecorvo 1958a, pp. 54–55). He thought the T locus in mice was a particularly good example, about which Salome Gluecksohn Waelsch (1989, p. 724) chose to quote his words: “… this is probably only a foretaste of what we are likely to find when we pass from the analysis of ‘simple’ genes like those providing the information for such a minor matter as the mere synthesis of an enzyme, to the genetic organization necessary to carry the information for morphogenetic processes.”

Chance and opportunity were also involved in forging a human connection between the early work on Aspergillus and later developments. Among the new biochemical ideas at the time (when the molecular nature of genes and enzymes was still largely obscure) was one due to Henry McIlwain (McIlwain 1946), then professor of biochemistry at the University of Sheffield, who suggested that some enzymes (especially those synthesizing metabolites like vitamins, required in small quantities) might be organized in a sort of assembly line so that substrates and products of milli-micromolar reaction chains would be produced and consumed in close proximity (as is indeed the case with bacterial polyketide synthetases, e.g., Llewellyn and Spencer 2007). This was of direct interest because of the use of auxotrophy for vitamin-like substances (e.g., biotin) as genetic markers. Were the genes also so arranged and, if so, could fine genetic analysis reveal traces of the organization? This would provide a plausible rationale for an Aspergillus genetics research program. In 1946, when Ponte went to Sheffield to consult McIlwain about these ideas, a young student, J. A. Roper, was detailed to escort him from the railway station. Roper was present during the discussions and later, when the Aspergillus work started to flourish, it was he who became one of the most important graduate collaborators and the discoverer of stable diploids (Roper 1952, 1994).

Among the many remarkable features of Ponte's writings is the section of an article on the resolving power of genetic analysis (Pontecorvo 1956b) in which he sought to give “chemical meaning to what we are resolving” by applying Benzer's (1955) argument. With an estimated genome size of 4 × 107 bp and a total genetic map length of >500 cm, Ponte found that the minimum recombination frequency so far measured in Aspergillus (1 × 10−6 or 10−4 cm) corresponded to “8 nucleotides … compared with 216 for Drosophila and 12 for phage” and that “a whole section of allelism (a gene) could include as a minimum 1,000–8,000 nucleotides in Aspergillus … and … further that this is the sort of template required for making a protein” (p. 84). Evidently, these “very crude estimates” (later reduced to 3, 40, and 2 nucleotides; Pontecorvo 1958a, Table 4) were prescient. When sequenced, the two closest pairs of sites separated by recombination in meiotic crosses were found to be 11 and 12 nucleotides apart, respectively (Glatigny et al. 1998; Wilson and Arst 1998). The frequency of recombination in the 11 nucleotides separating hxA143 and hxA101 was 6.3 × 10−6, corresponding to 5.7 × 10−7 between adjacent nucleotides (C. Scazzocchio, personal communication, based on S. Lee, H. M. Sealy-Lewis and C. Scazzocchio, unpublished data). In the sequenced genome, linkage map/recombination distances are rather variable between regions, but suggest a recombination frequency of 1.3 × 10−7 between adjacent nucleotides (A. J. Clutterbuck, personal communication). Considering that the number of meiotic progeny screened for recombination is extrapolated from viable counts, Ponte's early calculations agree surprisingly well with the specific fine-structure experiments and with the overall genomic data. His claims for the (then) unparalleled resolution attained by fine genetic analysis were not misplaced, but they had been anticipated—remarkably, Haldane (1920, pp. 7–8) had calculated that in Drosophila, the shortest distance between loci is “… about 100 times the diameter of a hydrogen atom … .”

As noted above, Ponte's other most important idea was the realization (Pontecorvo and Käfer 1956) that the parasexual cycle could be applied to human genetic analysis through the use of cultured somatic cells, a possibility to which his own research efforts were dedicated from about 1958 until long after official retirement age. These attempts to get somatic cell genetics under way were unsuccessful through prematurity, but were influential through collaborators, some of whom later helped the field to flower, and one of whom, George M. Martin (personal communication), provided this reminiscence:

I came to work with Ponte in 1961 because of my interest in mapping human genes via mitotic recombination using cultures of human diploid fibroblasts. It quickly became apparent that we could not do serial sub-cloning or repeated replicate plating because of what later became known as the “Hayflick limit” of proliferation, but Ponte's view was always that we had not yet devised the correct culture conditions. (He was at least partially correct, as culturing these cells under ambient oxygen is now known to be sub-optimal.) 

Even would-be human geneticists were expected to work with Aspergillus as a form of training in the parasexual cycle, but Ponte determined that this should be scaled down to increase the efforts with human cells, to which Martin says: “… our response was to continue that valiant effort during the day, and to sneak back to the lab at night to do REAL genetics with A. nidulans.”

Despite the immediate failure of Ponte's approach to human genetics in vitro, one of his articles in this field (Pontecorvo 1975) became a citation classic. This gave him sardonic and impish pleasure, mainly because it was rejected by the Proceedings of the Royal Society B on the basis of review by an eminent Oxford cell biologist and fellow FRS who should have known better, but who thought this valuable technical advance was too trivial for such an august journal. Not only was the polyethylene glycol (PEG) fusion method (already in use for protoplast fusion of plants and some bacteria) a considerable improvement over the Sendai virus technique then in use for human cells, but also Hopwood (2007, p. 86-88) recounts how it came to be used for Streptomyces protoplast fusion and how the conditions optimized by Ponte for human cells proved to be beyond improvement. However, it may be unrealistic to claim that the wider use of PEG protoplast fusion in bacteriology (most recently the transplantation of a bacterial genome: Lartigue et al. 2007) owes much to Ponte's work.

The many visitors who came to work with Ponte in Glasgow carried away varied impressions, but one common theme has been expressed by K. M. Dronamraju (personal communication), who wrote that: “Before my departure for Glasgow, I recall reading Haldane's copy of Ponte's book, Trends in Genetic Analysis (Pontecorvo 1958a), and was struck by its simplicity and clarity. Indeed, his writing style was much better than that of many who were born and brought up in England, and it reminded me of Dobzhansky, who was equally skilled in English although his mother tongue was Russian.” 4 L. Cavalli-Sforza (personal communication) stressed other qualities: “Two extraordinary things about Ponte were his intelligence, and the human warmth he kept under his sharply critical mind … . But he was not the self-promoting kind.”

Not self-promoting perhaps, but neither was he inclined to hide his light under a bushel, as shown by his consistent record of contributing (when he had something useful to say) to meetings of genetics, biochemistry, and microbiology societies. With European scientists struggling to reestablish themselves after the war, attendance at the 1946 Cold Spring Harbor symposium (no. 11, “Heredity and Variation in Microorganisms”) provided an important opportunity to meet one another again and to get to know, and be known by, the North American proponents of the burgeoning discoveries in microbial genetics. Leading figures at the 1946 meeting (with benefit of hindsight) included D. Bonner, S. Cohen, M. Delbruck, B. Ephrussi, A. D. Hershey, J. Lederberg, the Lindgrens, S. E. Luria, A. Lwoff, J. Monod, D. D. Perkins, J. Preer, F. J. Ryan, T. M. Sonneborn, G. Streisinger, E. L. Tatum, and E. M. Witkin. Important new results were reported on virus and phage structure and function; mutation in bacteria, bacteriophage, and fungi; and biochemical variation. Ponte's contribution, a review of genetic systems based on heterokaryosis, was rather different from most of the rest, many of which were reports of specific experiments, some in tedious detail. A small selection from his apparently unique record of the participants is presented in Figures 1 and 2.

Figure 1.—

Figure 1.—

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Pontecorvo with Jacques Monod at Cold Spring Harbor, 1946 (© Lisa Pontecorvo).

Figure 2.—

Figure 2.—

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Ugo Fano, Salvador Luria, and Pontecorvo at Cold Spring Harbor, 1946 (© Lisa Pontecorvo).

Although Ponte had a reputation for filing official papers in the wastepaper basket, he was (fortunately) more particular with his personal correspondence, of which a substantial archive exists and which throws interesting light on this symposium. From letters of February 1946, it is clear that he was in close, collaborative contact with Boris Ephrussi, who did not initially receive an invitation to attend, although one had arrived by early March. More unexpectedly (to the writer) is the revelation that Ephrussi was requesting scientific advice and support from Pontecorvo (e.g., on X-ray and chemical mutagenesis in Drosophila) and that the idea of trying to select yeast mutants resistant to acriflavine originated in Glasgow. Thus, Ponte was indirectly responsible for the discovery of “petite” yeast which, in the hands of Ephrussi and his colleagues, played such an important role in the clarification of organelle inheritance.

The Pontecorvo legacy in Glasgow has been mixed. When the Department of Genetics was established and for many years thereafter, Glasgow operated like most United Kingdom universities on a model in which the professor was not only primus inter pares among colleagues but also the King-Emperor of a Protectorate. Thus, the very creation of a genetics department, although welcome on academic grounds (except, apparently, to the dean of medicine!) entailed a potential loss of empire by botany, zoology, microbiology and biochemistry, and it would have been both impolitic and uncongenial for Ponte to seek to greatly broaden the range of the department's undergraduate courses, which were devoted mainly to teaching core genetics to science students. While there was a good flow of postgraduates, the undergraduate honors class remained small, largely because Ponte thought it irresponsible to train people for nonexistent employment; thus, only the most determined one or two supplicants were allowed to enroll each year, later entrants having to surmount the challenge of a full double-degree course of genetics with botany or zoology. This extraordinary burden ensured that, from 1959 until 1972 or 1973, Glasgow produced no genetics graduates at all. This weakness, however, was somewhat balanced by a pioneering interdepartmental course in molecular biology, which flourished from the mid-1960s, the setting up of which contributed substantially to interdepartmental harmony. Ponte's key role (with Michael Stoker, John Paul, Bob Williamson, and later, Adam Curtis) in the origination of this course perhaps underlines a paradox of his promotion to “Professor and Head of Department” because this traditionally implied an empire-building role for which he was temperamentally and philosophically unsuited. Generally at odds with the entrenched, conservative elements of the academic administration, the form filling, letter writing, and committee sitting required of someone in his professorial position were anathema (see his letter about the library committee in Cohen 2000) and contributed much to his later decision to resign from the Glasgow professorship.

Glasgow genetics remains a flourishing undergraduate and postgraduate department of study, and the parallel but overlapping undergraduate honors courses in genetics and in molecular biology continue to flourish. But in 1994 all the separate biology departments were absorbed into the present conglomerate (IBLS, now a faculty headed by a dean), and current plans entail the abandonment of the 1960s Pontecorvo Building (with its continuously moving Paternoster lift) and the move of genetics staff into refurbished accommodation nearer the campus center, allowing more opportunity to mingle (or even collaborate!) with other biologists. Pontecorvo, who believed that the essential unity of biology “makes a nonsense” (a favorite phrase) of its division into independent, semicompeting academic departments, might well have welcomed such changes.

Despite Cavalli-Sforza's kind words above, Ponte was no wallflower when it came to protecting or promoting science and position, as shown by the following extracts from a 1953 letter to the university principal:

Dear Sir Hector,

[paraphrased] … I found the enclosed report of the Institute of Biology on The Remuneration of Biologists most instructive … the remuneration of the other members of the Genetics Department staff worked out to be about average for the corresponding age-groups in universities. [from here not paraphrased] However, my own salary is grossly below the average for my age-group. I find that at my age (46) the average is about £1,700. I have £1550, yet I am not a biologist of ability 10% below average.

There are two ways in which the University could put this matter straight. One would be only to my own advantage, i.e., raising my salary. The other one would be both to the University's and to my own advantage: i.e., establishing a Chair of Genetics (with adequate salary of course).

I have come to the conclusion that the time is now ripe for the second alternative … . The unpleasant, but unfortunately true, fact is that in spite of the personal reputation that my collaborators and I have, especially abroad, people in my field do not connect my name with Glasgow at all … . By establishing a Chair of Genetics this would soon be put straight, and it would be easier to attract visiting research workers, lecturers, etc… . There are already 5 chairs in the country: Edinburgh (Waddington), Birmingham (Mather), London, U.C. (Haldane and Penrose), and Cambridge (Fisher). I think it would do Glasgow good to go ahead.

Yours sincerely, G. Pontecorvo

And fortunately the university agreed with him about as promptly as its procedures allowed.

Although very grateful to Glasgow University for the shelter and support that it offered, Ponte did occasionally apply for other posts and received several unsolicited inquiries and offers. However, except for the final move to the Imperial Cancer Research Fund (described in Cohen 2000), which gave complete freedom from administration, all came to nothing. The possibilities (between 1948 and 1964) for which I have some evidence included: an invitation from Demerec to join the Cold Spring Harbor Laboratory; inquiries from Imperial Chemical Industries and from the Institute of Brewing Research; an application for the Quick Chair of Biology at Cambridge; and an inquiry from Leiden University. And, when asked by University College, London, to comment on the suitability of a chair candidate (a former Ph.D. student of whom he held a not-so-high opinion), Ponte proposed himself instead (!). Remarkably, he was elected to, but did not actually succeed, R. A. Fisher in the Balfour Chair of Genetics at Cambridge. This came about because the Electors were powerless except to elect, without even the candidate's agreement, and the relevant powers would not or could not satisfy his teaching and accommodation needs. Ponte was put forward by Medawar as a possible candidate for Haldane's chair at University College, London; was offered a position at the John Innes Institution; invited to apply for the Edinburgh botany chair, invited to consider Harvard; invited by Medawar to join the National Institute for Medical Research at Mill Hill; and finally, consulted about a possible move to botany at Imperial College, London. By contrast, a 1962 offer from the Albert Einstein Medical College in New York was accepted and many preparations for the move were undertaken, only for it to fall through for family and visa reasons. It is rare for history of this sort to be made public, but we may read it as clear evidence of the high regard in which Ponte was held in academic circles. Medawar's involvement is known to have reflected anxieties about “the brain-drain”; the scientific establishment that he represented was anxious not to lose Ponte from the United Kingdom.

Acknowledgments

I am indebted to Lisa Pontecorvo for the suggestion that her father's centenary should be marked in some way and for access to and permission to use unpublished material. People who contributed particular reminiscences or information are named in the text, and their help is gratefully acknowledged.

2

Additional sources for the stories of the refugee scientists include recorded interviews by Ponte and others in the sound archives of the Imperial War Museum's oral history project, “Britain and the Refugee Crisis, 1933–1947” (reference 004505/13) and Medawar and Pyke (2001). There is also a recorded interview with Ponte in the Edinburgh University Library collection on the history of the Institute of Animal Genetics. For authentic accounts of the early history of Aspergillus work, see Roper (1994) and Pontecorvo (1994). For a wider-ranging account, see Roper and Hopwood (1988).

3

A. D. Buchanan-Smith (1899–1984), who trained in animal breeding at Iowa State University, was a remarkable character from a distinguished family. As well as being a lecturer in genetics he was a practical farmer and pig breeder (at Balerno) and was called on at various times in a long career to serve at senior levels in national agricultural administration, national politics, the Scottish church and universities, and several educational charities. In the wartime army, he rose to the rank of brigadier and was director of personnel selection at the War Office. Much honored, he was eventually elevated to the House of Lords as Lord Balerno of Currie.

4

A flavor of the (sometimes amusingly quirky) text is conveyed by the following quotations: On the misuses of genetics (a favorite hobbyhorse): “… the closer relationship between biochemistry and genetics. For this we are indebted mostly to Beadle and Tatum … a technique of immense and versatile power… . Unfortunately, by and large, this technique has not been put to the best possible use in one of the directions for which it has exceptional value, i.e., the study of the primary actions of the genetic material and their relations to its fine structure. In this respect, it has been made mainly into a tool for the unexciting description of intermediary metabolism …” (Pontecorvo 1958a, p. 4). Self-deprecation: “A discussion on the relations between arrangement and activity of the chromosomal material should, of course, include a consideration of heterochromatin. The trouble here is that the study of heterochromatin is at a prescientific level (e.g., Pontecorvo 1944)” (Pontecorvo 1958a, p. 67). Almost an invocation of alternative splicing: “… the same idea which I tried to convey very crudely some years ago, that is that new integrations (i.e., genes) would arise epigenetically as a consequence of minor changes in chromosome spiralisation resulting from previous activities.” (Pontecorvo 1958a, p. 69). A tribute to (or a sly dig at) an influential member of the British genetics establishment: “The study of recombination suffers from the same disease as Italian literature: that of having reached its highest peak too early in its life. The result is subsequent scholasticism. The Divine Commedia of recombination was Darlington's Recent Advances in Cytology.” (Pontecorvo 1958a, p. 132). And finally, on the resolution of genetic analysis: “It is quite possible, therefore, that “negative interference” could be the result of the closeness of the mutants necessary to detect it. If this were so we would have reached in biology a situation of indeterminacy analogous to that well-known in physics: beyond a certain limit we could not increase at the same time the accuracy of determination of the relative positions and of the recombination between two markers.” (Pontecorvo 1958a, p. 89).

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