TOPICS IN GENETIC ANALYSIS G. PONTECORVO Digitized by the Internet Archive in 2018 with funding from Public.Resource.org https://archive.org/details/topicsingeneticaOOunse i - .j,. - "I •a < ' :’ ■ :^ ■ / k f-: - :;•■■ ^ . > .>4^ .-.t'- ^ . - •V Topics in Genetic Analysis Selected Papers of G. Pontecorvo Raman Professor, 1982-83 INDIAN ACADEMY OF SCIENCES BANGALORE 560 080 1985 1>; 1985 INDIAN ACADEMY OF SCIENCES, BANGALORE I'hc copyright ol individual papers rests with the original publishers viz., Macmillan Journals I.united, London; Cold Spring Harbor Laboratory, New York; Interscience Publishers Inc, New York; Academic Press, New York; Cambridge University Press, Cambridge; Annual Reviews, Calilornia; Royal Society, London; Columbia University Press, New York; Alternative bookshop, London; British Council, London; Elsevier Science Publishers, Amsterdam; Raven Press, New York; Plenum Publishing Corporation, New York. Prinu-d at Sharadh Enterprises. 19/6 Nagappa Street, Palace Guttahalli, Bangalore 560 003, Phone: 364311 FOREWORD Prof. Guido Pontecorvo visited India for three months during December 1982-February 1983, as Raman Professor of the Indian Academy of Sciences. Prof. Pontecorvo and Mrs. Pontecorvo spent much of this period in Bombay (the Tata Institute of Fundamental Research) and in Bangalore (the Indian Institute of Science). He also visited and lectured at Universities and Institutions in Delhi, Poona, Hyderabad, Calcutta, Madras, Ahmednagar, Patna and other cities. The Academy takes great pleasure in bringing out this selection of papers by one of its distinguished Honorary Fellows. It also includes his Gandhi Memorial Lecture which he delivered at the Raman Research Institute, Bangalore. We also thank the original publishers-of these articles — Macmillan Journals Limited, London; Cold Spring Harbor Laboratory, New York; Interscience Publishers, New York; Academic Press Inc, New York; Cambridge University Press, Cambridge; Annual Reviews, California; Royal Society, London; Columbia University Press, New York; Alternative Bookshop, London; British Council, London; Elsevier Science Publishers, Amsterdam; Raven Press, New York; Plenum Publishing Corporation, New York. — for permission to reprint them in this volume. S. RAMASESHAN President Indian Academy of Sciences Bangalore Prof. G. Pontecorvo BIOGRAPHICAL SKETCH Guido Pontecorvo was born in Pisa (Italy) in the year 1907. He studied classics and graduated from the University of Pisa in Agricultural Science, summa cum laude, in 1928; two years later, he entered the Agricultural Advisory Service in the city of Florence as an animal breeder. Fascism in Italy and the gathering clouds of war in Europe forced Pontecorvo to emigrate to Scotland. In 1938 he joined a select band of students who had gathered at the University of Edinburgh around the great American geneticist H.J. Muller. Pontecorvo took his Ph.D. from the Institute of Animal Genetics at Edinburgh and Joined the Zoology Department at Glasgow University in 1941 on a Carnegie grant. Two years later he was appointed lecturer at the University of Edinburgh. In 1945, Pontecorvo returned to Glasgow. His earlier researches were concerned with animal breeding and radiation effects and genetics of speciation with Drosophila. At Glasgow Pontecorvo began his work with Aspergillus nidulans a line of research which was to earn him a place of his own in the annals of genetics. He was succes¬ sively appointed Lecturer, Reader and then Professor of Genetics as the first occupant of the Chair in genetics at Glasgow. In 1956 he was appointed Director of the Institute of Genetics and Head of the Unit of Somatic Cell Genetics created by the Medical Research Council. In 1968, Pontecorvo moved to London to join the staff of the Imperial Cancer Fund Laboratories at Lincolns Inn Fields. He continues to work there after his formal retirement in 1980. The years in Glasgow were very fertile. The Genetics Department was located in the old medical school, a somewhat dilapidated and morose structure, in which Pontecorvo’s group occupied a few rooms. It was, in the fifties, a highly international group with research scholars and visitors drawn from many parts of the world, attracted by Pontecorvo’s ideas and reputation. There was a continuous stream of visitors. Some came to talk about their own work, others to learn the intricacies of mitotic genetics or to apply its methods to mammalian cell genetics. There were animated conversations on the mutation theory, gene regulation and recombination and, above all, on DNA and the newly arrived idea of the genetic code. Pontecorvo is a fervent believer in working with one’s own hands. His professorial office was a small desk in his one-room laboratory. Sir Michael Stoker records that when he offered Pontecorvo a position at the Imperial Cancer Fund, Pontecorvo’s answer was that he would come providing his laboratory was small enough and there were no assistants. Among Pontecorvo’s varied contributions to genetics, the two that stand out as most consequential are his discovery of the parasexual cycle in fungi; this led him to develop methods of genetic analysis that were the forerunners of modern somatic cell genetics, an approach to genetics of higher animals that has revolutionised our knowledge of human genetics. The other is his work on intragenic recombination and the seminal paper in 1952 on the organisation of the genetic material. This paper outlined a new theory of the gene, a year before the discovery of DNA structure by Watson and Crick and the revision of the gene concept by Seymour Benzer. First about the parasexual cycle; classical genetics was restricted to organisms with a biparental sexual cycle. The development of microbial genetics, in the forties, brought into prominence new and unconventional genetic systems of fungi, bacteria and viruses which greatly extended the scope and power of genetic analysis. The ground for ,h.s new flowenng of gene.ics .n .he fifties was wo* and Tatum on Neurospora crassa. Pontecorvo entered the fie d of biochemic^ gene with a homothaUic fungus, Asper.Ulus mdulans. The life cycle to conventional sexual genetics, provided appropriately marked strains ca crossed To do this Pontecorvo made use of “forced heterokaryosis . or mg heterokaryons Roper and Pontecorvo discovered spontaneous diploidisation of nuclei during vegetative growth and the formation of diploid heterozygous conidia. Pontecorvo realised the importance of this discovery m that, it freed genetic ana ysis from the restriction of an obligatory sexual cycle. His colleagues and he denionstrate mitotic crossing-over and segregation m diploid myceha and worked out the modalities of gene mapping through mitotic crossing-over and haploidisation. Crossine-over in somatic cells had been studied by Curt Stern m Drosophila. Pontecorvo foresaw that mitotic segregation could be harnessed to extend genetics to asexual species. This was a decisive step in the development of somatic genetics. In the later fifties, Pontecorvo himself switched to working with human cell cultures. His ideas thus played a critical role in the rapid development of human genetics through the parasexual cycle. In 1975, Pontecorvo applied to human cells the method of fusion by polyethylene glycol, and a technique for selectively eliminating chromosomes from fused cells by irradiation and other radiomimetic treatments. A second development of considerable importance in modern genetics to which Pontecorvo made an important contribution, concerns our understanding of the nature of gene. The picture of gene that was nearly universally accepted before the forties was that of “beads strung along a thread”. In practice a gene was recognised by different sorts of operations; tests of allelism depend upon function, mutations indicate the parts of a gene that can undergo a stable change, crossing- over indicates separation of mutable loci. It was tacitly assumed that these different operations referred to the same entity, “the particulate gene”. A chink in this simplistic picture appeared when E.B. Lewis, in 1945, found that the mutations of the bithorax locus oi Drosophila, although allelic from a functional point of view, could, nevertheless, recombine with each other. Recombination between alleles, in classical genetics, was a paradox. Lewis himself believed, as it indeed turned out to be the case, that the bithorax locus was a string of functionally related genes. He termed the apparent allelism of such genes “pseudoallelism . Pontecorvo saw, in this phenomenon, the possibility of a radically different interpretation. He assumed that allelic recombination was a much more general phenomenon, a universal property of all genes. In a seminal paper entitled “Genetic formulation of gene structure and gene action” published in 1952, in Advances in Enzymology, Pontecorvo pointed out that different operational definitions of a gene need not refer to the same entity and the gene as a unit of function could be a segment of the chromosome stretched over a considerable length. Crossing-over could occur within this length. The picture of genes as a linear array of recombinable units was vindicated a year later by the structure of DNA proposed by Watson and Crick and by the brilliant work of Seymour Benzer on the rll genes of Phase T4 that was to follow. In his Jessup lectures entitled “Trends in Genetic Analysis”, Pontecorvo traces several of his insights into the nature of the genetic material to his teacher H.J. Muller. Indeed he has expressed