DOCSLIB.ORG

Natural Variation of Large Plasmids in Bacterial Populations

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Natural Variation of Large Plasmids in Bacterial Populations NATURAL VARIATION OF LARGE PLASMIDS IN BACTERIAL POPULATIONS by LAURA E. WILLIAMS (Under the Direction of Anne O. Summers) ABSTRACT Plasmids encode an extraordinary range of adaptive functions, including antibiotic resistances, virulence factors and degradative enzymes. Plasmids also mediate horizontal gene transfer, which impacts evolution of bacterial chromosomes. Despite their important ecological and evolutionary roles, we lack a comprehensive picture of the variation and evolution of plasmids themselves. In this dissertation, I investigated the natural variation of large plasmids on two different scales: 1. individual plasmid sequences from Gram- negative and -positive bacteria and 2. total plasmid content of E. coli and Salmonella reference collections. For the sequence-based studies, I sequenced four large plasmids from E. coli, Staphylococcus and Corynebacterium. Whereas 98% of the E. coli plasmid was highly similar to other plasmid sequences, 40-60% of the three Gram-positive plasmids had no apparent similarity to known sequences. The E. coli plasmid’s backbone, which encodes “core” functions such as replication, was almost identical to that of a Salmonella enterica sv Typhimurium plasmid. Subsequent analysis identified this backbone in five other enterobacterial plasmids. All seven plasmids differed in accessory gene content, leading to variation in plasmid-encoded phenotypes. For the collection-based studies, I analyzed 228 E. coli and Salmonella strains from four reference collections. Plasmids, especially large plasmids, were abundant. Replicon typing identified two common families, IncF and IncI1, by the presence of characteristic backbone genes. At least 20% of strains with large plasmids in each collection were untypable, suggesting variation in plasmid backbones. Restriction fragment length polymorphism (RFLP) analysis showed high variation in large plasmid genomes. The majority of E. coli and Salmonella large plasmids (87.5% and 56%, respectively) had unique RFLP patterns. The lack of widespread RFLP patterns suggested that the prevalence of IncF and IncI1 is due to similar backbone genes on otherwise different plasmids. The only exception was the IncFII virulence plasmid pSLT, which was detected in multiple Typhimurium strains from different hosts and geographic locations, suggesting that virulence plasmids evolve differently from other plasmids. These results show that plasmid evolution is a significant force shaping the horizontal gene pool. Rather than existing as static elements, plasmids are dynamic and flexible genetic scaffolds driving gene flux in host bacteria. INDEX WORDS: plasmid evolution, horizontal gene transfer, mobile genetic element NATURAL VARIATION OF LARGE PLASMIDS IN BACTERIAL POPULATIONS by LAURA E. WILLIAMS B.S., Illinois Wesleyan University, 2001 A Dissertation Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY ATHENS, GEORGIA 2009 © 2009 Laura E. Williams All Rights Reserved NATURAL VARIATION OF LARGE PLASMIDS IN BACTERIAL POPULATIONS by LAURA E. WILLIAMS Major Professor: Anne O. Summers Committee: Jessica Kissinger Mark Schell Patricia Sobecky Electronic Version Approved: Maureen Grasso Dean of the Graduate School The University of Georgia May 2009 DEDICATION Dedicated to Bram Tucker and Joan Williams. They made a hard journey easier. iv ACKNOWLEDGEMENTS Most importantly, I would like to thank my advisor Anne Summers for guiding me through my time here at UGA with constant enthusiasm. I admire how she fully integrates her students into the scientific community, which is one of the most important but intangible purposes of graduate school. From the beginning, she included me as a full participant in interactions with our collaborators and faculty colleagues at UGA and elsewhere. I felt very welcomed into the community of scientists, and I am grateful to her for that. My committee members Jessica Kissinger, Mark Schell and Patricia Sobecky have given me valuable help and advice during this journey. They have been supportive and patient, and I am very glad to have benefited from their expertise throughout my graduate work. I am indebted to a number of incredible mentors, including Beth Arthington- Skaggs, David Bollivar, Marcio Chedid, Jonathan Dey, Don LeBlanc, Jo Porter, Slava Titov, and Chris Zook. Each of them played an important role in getting me to this point, and I hope that their experiences were as rewarding as mine. I would also like to thank my fellow graduate students at UGA, especially Caran Cagle, Adrienne Cottrell, Erinn Howard, Bijal Patel and Julie Shearer. I wish that Bijal was still here to see his own work through to the end, but I imagine that he has all the answers now. v Joy Wireman deserves special recognition. It is easy to forget how much work it takes to keep things running smoothly, and Joy is a true master. Beyond that, she is a thoughtful and accomplished researcher. If I ever have a lab of my own, I’ll be lucky to find someone half as good as Joy. Balance in my life has always kept me sane, and during my time in Athens I have been blessed with a truly remarkable group of friends. The GSYPF crew, with special mention to Christy and Paul, and the Killer Tomahtoes tennis team have been a constant and delightful source of hilarity and friendship. I know that I will miss them when I move on. vi TABLE OF CONTENTS Page ACKNOWLEDGEMENTS.................................................................................................v LIST OF TABLES............................................................................................................. ix LIST OF FIGURES .............................................................................................................x CHAPTER 1 Introduction and Literature Review...................................................................1 Genetic Structure of Plasmids .......................................................................3 Backbone Systems.........................................................................................4 Accessory Genes .........................................................................................21 Plasmid Ecology and Evolution ..................................................................23 Objectives of the Dissertation .....................................................................35 References ...................................................................................................40 2 Facile Recovery of Individual High-Molecular-Weight, Low-Copy- Number Natural Plasmids for Genomic Sequencing.......................................63 Abstract .......................................................................................................64 Introduction .................................................................................................65 Materials and Methods ................................................................................67 Results .........................................................................................................72 Discussion ...................................................................................................79 References ...................................................................................................92 vii 3 Large Plasmids in Escherichia coli and Salmonella Reference Collections Are Highly Variable Despite Common Replicon Families .............................97 Abstract .......................................................................................................98 Introduction .................................................................................................99 Materials and Methods ..............................................................................102 Results .......................................................................................................109 Discussion .................................................................................................126 References .................................................................................................150 4 Conclusions....................................................................................................159 Update of Sequence Analysis from Chapter Two.....................................160 Broad Implications of Dissertation Research............................................165 Future Research.........................................................................................170 References .................................................................................................173 viii LIST OF TABLES Page Table 2.1: Bacterial strains and plasmids ..........................................................................83 Table 2.2: Sequencing coverage resulting in single, circularized contigs .........................84 Table 2.3: Highlights of the chimeric character of novel plasmid genomes .....................85 Table 3.1: Plasmid prevalence in reference collections of natural E. coli and Salmonella.......................................................................................................137 Table 3.2: Plasmid replicon families in reference collections of natural E. coli and Salmonella and in NCBI GenBank ................................................................138 Table 3.3: Summary of plasmid clusters by RFLP analysis ............................................139 Table 3.4:
Load more

Recommended publications
  • A Viewpoint of Aspects of a History of Genetics: an Autobiography
    12. The Postgraduate Medical School,London University. The Head of the Bacteriology Department at the PGMS(later the Royal PGMS) was Professor Lord(Trevor) Stamp who did'nt seem to bother what research I did so long as I did something. My closest colleague was Dr.Denny Mitchison(Lecturer),one of the three biologist sons of authoress Naomi Mitchison and nephew of J.B.S.Haldane,whose main interest was typing tubercle bacilli under MRC!auspices. In addition there were at least two non-tenured medical graduates on the staff. My only departmental responsibilities were to take my turn at clinical bacteriology which amounted to the mornings of one week in every three or four,and to share in teaching the bacteriology course for the Diploma in Clinical Pathology which occupied me for not more than a few weeks in the year. There was,in fact,a positive obligation to spend at least half one's time at research. I decided to return to the mechanism of somatic phase variation in S.enteritidis,but had begun to realise the potentiafities of genetic analysis inherent in the recent discovery by Lederberg and Tatum(1946) of conjugation in Escherichia coli ,and wondered if Salmonella also could conjugate. But it was clear that I should first become acquainted with the phenomenon in --E.coli. Towards the end of 1950 I went on a bacterial chemistry course at Cambridge,organized by Prof.E.H.Gale,and there met Luca Cavalli(later Cavalli-Sforza) who had worked with Joshua Lederberg on conjugation at Wisconsin and,being mainly interested in population genetics,was currently a visitor in Prof.R.A.Fisher's Department of Mathematics.
    [Show full text]
  • Phage Integration and Chromosome Structure. a Personal History
    ANRV329-GE41-01 ARI 2 November 2007 12:22 by Annual Reviews on 12/13/07. For personal use only. Annu. Rev. Genet. 2007.41:1-11. Downloaded from arjournals.annualreviews.org ANRV329-GE41-01 ARI 2 November 2007 12:22 Phage Integration and Chromosome Structure. A Personal History Allan Campbell Department of Biological Sciences, Stanford University, Stanford, California 94305-5020; email: [email protected] Annu. Rev. Genet. 2007. 41:1–11 Key Words First published online as a Review in Advance on lysogeny, transduction, conditional lethals, deletion mapping April 27, 2007 Abstract by Annual Reviews on 12/13/07. For personal use only. The Annual Review of Genetics is online at http://genet.annualreviews.org In 1962, I proposed a model for integration of λ prophage into the This article’s doi: bacterial chromosome. The model postulated two steps (i ) circular- 10.1146/annurev.genet.41.110306.130240 ization of the linear DNA molecule that had been injected into the Annu. Rev. Genet. 2007.41:1-11. Downloaded from arjournals.annualreviews.org Copyright c 2007 by Annual Reviews. cell from the phage particle; (ii ) reciprocal recombination between All rights reserved phage and bacterial DNA at specific sites on both partners. This 0066-4197/07/1201-0001$20.00 resulted in a cyclic permutation of gene order going from phage to prophage. This contrasted with integration models current at the time, which postulated that the prophage was not inserted into the continuity of the chromosome but rather laterally attached or synapsed with it. This chapter summarizes some of the steps leading up to the model including especially the genetic characterization of specialized transducing phages (λgal ) by recombinational rescue of conditionally lethal mutations.
    [Show full text]
  • The Eighth Day of Creation”: Looking Back Across 40 Years to the Birth of Molecular Biology and the Roots of Modern Cell Biology
    “The Eighth Day of Creation”: looking back across 40 years to the birth of molecular biology and the roots of modern cell biology Mark Peifer1 1 Department of Biology and Curriculum in Genetics and Molecular Biology, University of North Carolina at Chapel Hill, CB#3280, Chapel Hill, NC 27599-3280, USA * To whom correspondence should be addressed Email: [email protected] Phone: (919) 962-2272 1 Forty years ago, Horace Judson’s “The Eight Day of Creation” was published, a book vividly recounting the foundations of modern biology, the molecular biology revolution. This book inspired many in my generation. The anniversary provides a chance for a new generation to take a look back, to see how science has changed and hasn’t changed. Many central players in the book, including Sydney Brenner, Seymour Benzer and Francois Jacob, would go on to be among the founders of modern cell, developmental, and neurobiology. These players come alive via their own words, as complex individuals, both heroes and anti-heroes. The technologies and experimental approaches they pioneered, ranging from cell fractionation to immunoprecipitation to structural biology, and the multidisciplinary approaches they took continue to power and inspire our work today. In the process, Judson brings out of the shadows the central roles played by women in many of the era’s discoveries. He provides us with a vision of how science and scientists have changed, of how many things about our endeavor never change, and how some new ideas are perhaps not as new as we’d like to think. 2 In 1979 Horace Judson completed a ten-year project about cell and molecular biology’s foundations, unveiling “The Eighth Day of Creation”, a book I view as one of the most masterful evocations of a scientific revolution (Judson, 1979).
    [Show full text]
  • 6 Women Scientists Who Were Snubbed Due to Sexism Despite Enormous Progress in Recent Decades, Women Still Have to Deal with Biases Against Them in the Sciences
    6 Women Scientists Who Were Snubbed Due to Sexism Despite enormous progress in recent decades, women still have to deal with biases against them in the sciences. BY JANE J. LEE, NATIONAL GEOGRAPHIC PUBLISHED MAY 19, 2013 In April, National Geographic News published a story about the letter in which scientist Francis Crick described DNA to his 12-year-old son. In 1962, Crick was awarded a Nobel Prize for discovering the structure of DNA, along with fellow scientists James Watson and Maurice Wilkins. Several people posted comments about our story that noted one name was missing from the Nobel roster: Rosalind Franklin, a British biophysicist who also studied DNA. Her data were critical to Crick and Watson's work. But it turns out that Franklin would not have been eligible for the prize—she had passed away four years before Watson, Crick, and Wilkins received the prize, and the Nobel is never awarded posthumously. But even if she had been alive, she may still have been overlooked. Like many women scientists, Franklin was robbed of recognition throughout her career (See her section below for details.) She was not the first woman to have endured indignities in the male- dominated world of science, but Franklin's case is especially egregious, said Ruth Lewin Sime, a retired chemistry professor at Sacramento City College who has written on women in science. Over the centuries, female researchers have had to work as "volunteer" faculty members, seen credit for significant discoveries they've made assigned to male colleagues, and been written out of textbooks.
    [Show full text]
  • The Roles of Moron Genes in the Escherichia Coli Enterobacteria Phage Phi-80
    THE ROLES OF MORON GENES IN THE ESCHERICHIA COLI ENTEROBACTERIA PHAGE PHI-80 Yury V. Ivanov A Dissertation Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2012 Committee: Ray A. Larsen, Advisor Craig L. Zirbel Graduate Faculty Representative Vipa Phuntumart Scott O. Rogers George S. Bullerjahn © 2012 Yury Ivanov All Rights Reserved iii ABSTRACT Ray Larsen, Advisor The TonB system couples cytoplasmic membrane-derived proton motive force energy to drive ferric siderophore transport across the outer membrane of Gram-negative bacteria. While much effort has focused on this process, how energy is harnessed to provide for transport of ligands remains unknown. Several bacterial viruses (“phage”) are known to require the TonB system to irreversibly adsorb (i.e., establish infection) in the model organism Escherichia coli. One such phage is φ80, a “cousin” of the model temperate phage λ. Determining how φ80 is using the TonB system for infection should provide novel insights to the mechanisms of TonB-dependent processes. It had long been known that recombination between λ and φ80 results in a λ-like phage for whom TonB is now required; and this recombination involved the λ J gene, which encodes the tail-spike protein required for irreversible adsorption of λ to E. coli. Thus, we suspected that a φ80 homologue of the λ J gene product was responsible for the TonB dependence of φ80. While φ80 has long served as a tool for assaying TonB activity, it has not received the scrutiny afforded λ.
    [Show full text]
  • Host Growth Rate Affects Bacteriophage Lambda Burst Size
    Host Growth Rate Affects Bacteriophage Lambda Burst Size Aida Abbasiazam Submitted in fulfillment of the requirements for the degree of Master of Arts in Biology in the Graduate Division of Queens College of The City University of New York January 2020 Approved by: ________________________ (Sponsor) Date: _______________________________ 1 TABLE OF CONTENTS Abstract Acknowledgements Introduction................................................................................... 1 1.1 Bacteriophage Lambda Genome and Structure 1.2 Phage Life Cycle and Replication 1.3 Phage Burst Size Determination 1.4 Role of Nutrient Concentration on Phage Burst Size 1.5 The Effect of Growth Media on Host Cell and Concentration of Phages Objectives.......................................................................................14 Materials and Methods.................................................................15 Results............................................................................................ 19 Discussion ..................................................................................... 29 References...................................................................................... 33 2 ABSTRACT Life history traits such as growth rate and burst size involve the multiplication of phages in the host bacterial cell as a means of transmission and survival. As a consequence phage fitness depends on the physiological state of the host based on nutrient availability. In this study I tried to elucidate the effect of E. coli growth
    [Show full text]
  • An Interview with Werner Arber
    Interview The Inventiveness of Nature: An Interview with Werner Arber Jane Gitschier* Departments of Medicine and Pediatrics and Institute for Human Genetics, University of California San Francisco, San Francisco, California, United States of America Image 1. Werner Arber stands outside the Biozentrum at the University of Basel, Switzerland. doi:10.1371/journal.pgen.1004879.g001 I admit to having a soft spot for the years, restriction and modification was not spawned extremely useful technology. In bacterial phenomenon of host-specific only a remarkable story in itself, but also past issues, I have interviewed Hamilton modification and restriction. I vividly remember first learning about this mech- Citation: Gitschier J (2014) The Inventiveness of Nature: An Interview with Werner Arber. PLoS Genet 10(12): anism to stave off marauding DNA as a e1004879. doi:10.1371/journal.pgen.1004879 beginning graduate student in the mid- Published December 18, 2014 1970s. Preceding the discovery of its Copyright: ß 2014 Jane Gitschier. This is an open-access article distributed under the terms of the Creative modern-day defensive counterpart, Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, CRISPR (clustered regularly interspaced provided the original author and source are credited. short palindromic repeats) by about 50 * Email: [email protected] PLOS Genetics | www.plosgenetics.org 1 December 2014 | Volume 10 | Issue 12 | e1004879 Smith, who isolated the first type II Edouard Kellenberger, who ran the La- new preparation techniques. You could restriction enzyme, as well as Herb boratoire de Biophysique in the basement spread the lysate out on the film and see Boyer, whose production of recombinant of the Institute of Physics there.
    [Show full text]
  • Esther Lederberg, Pioneer in Genetics, Dies at 83
    Esther Lederberg, pioneer in genetics, dies at 83 Stanford Report, November 29, 2006 Esther Lederberg, pioneer in genetics, dies at 83 BY MITZI BAKER Courtesy of Matthew Simon • The Lederberg Plasmid Collection • Little Lambda, Who Made Thee? • Pioneer of microbial genetics: Joshua Lederberg Esther Lederberg gives a lecture in Japan in 1962. Esther Miriam Zimmer Lederberg, PhD, professor emeritus of microbiology and immunology, whose more than half-century of studies opened the door for some fundamental discoveries in microbial genetics, died Nov. 11 at Stanford Hospital of pneumonia and congestive heart failure. She was 83. A memorial service will be held at the Faculty Club on Nov. 30 from 4-6 p.m. "She was one of the great pioneers in bacterial genetics," said Stanley Falkow, PhD, the Robert W. and Vivian K. Cahill Professor in Cancer Research. "Experimentally and methodologically she was a genius in the lab." Lederberg is perhaps best known for her collaboration with her first husband, Joshua Lederberg, PhD, who in 1958 won the Nobel Prize for Physiology or Medicine for discoveries on how bacteria mate. But her work was extremely noteworthy in its own right, and she was a trailblazer for women scientists at Stanford and at large. "She was a real legend," said Lucy Tompkins, MD, PhD, the Lucy Becker Professor in Medicine and of Microbiology and Immunology. Among Lederberg's achievements was the discovery of lambda phage, a virus that infects E. coli bacteria. She published the first report of it in Microbial Genetics Bulletin in 1951, and it quickly became a significant and widely used tool for studying genetic recombination and gene regulation.
    [Show full text]
  • Joshua Lederberg's Legacy to Bacterial Genetics
    GENERAL ⎜ ARTICLE Joshua Lederberg’s Legacy to Bacterial Genetics R Jayaraman Joshua Lederberg (1925-2008) was an extraordinarily gifted person. Starting his professional career at the age of 17 as a dish washer in Francis Ryan’s laboratory in Columbia Uni- versity, he rose to be the President and later University Professor Emeritus at Rockefeller University, occcupying chairs of Genetics at Wisconsin and Stanford Universities. He R Jayaraman is an was only thirty three when he received the Nobel Prize, along Emeritus Professor of with George W Beadle and Edward L Tatum in 1958. He also Molecular Biology at the received the Presidential Medal of Freedom and the National School of Biological Sciences, Madurai Medal of Science. His scientific work encompassed not only Kamaraj University, bacterial genetics but also astrobiology (exobiology, as he Madurai, where he taught called it) and artificial intelligence. He was part of the Stanford and carried out research in team which developed the artificial intelligence software the area of microbial genetics for three decades. program DENDRAL. With his passing away in February 2008, the last of the founding fathers of bacterial genetics is gone. It is an honour for me to write this small article in his memory. In this article, I will focus on just two of his outstand- ing contributions to bacterial genetics, namely, the spontane- ous, selection-independent origin of bacterial mutations and the discovery of genetic recombination and sexuality in Es- cherichia coli. Introduction The history of bacterial genetics can be divided into two eras: the pre-double helix and the post-double helix.
    [Show full text]
  • Joshua Lederberg – a Remembrance
    c Indian Academy of Sciences SPECIAL FEATURE Joshua Lederberg – a remembrance ABHIJIT A. SARDESAI* and J. GOWRISHANKAR* Laboratory of Bacterial Genetics, Centre for DNA Fingerprinting and Diagnostics, ECIL Road, Nacharam, Hyderabad 500 076, India Joshua Lederberg, an iconic figure in microbiology, passed It was the subsequent work of William Hayes and of away on 2 February 2008 at the age of 83. Many obituar- Lederberg which led to delineation of the underlying mech- ies have since appeared describing his work in and outside anism, namely discovery of the fertility or sex factor (the F- of science (Balaram 2008; Blumberg 2008; Crow 2008a,b; plasmid), mediating conjugative gene transfer, and the word Oransky 2008; Sgaramella 2008). As a practising micro- conjugation gradually came to be applied to the phenomenon biologists, we take retrospective glimpses in this article at (Cavalli et al. 1953;Hayes 1953; reviewed in Firth et al. some of his research contributions that have profoundly in- 1996). It is now recognized that the gene transfer detected fluenced the development and growth of prokaryotic biology. by Lederberg and Tatum was tantamount to their winning For more personal accounts of his career, including specific the jackpot (in more ways than one, see below), since the examples of his legendary intellect, readers may refer to the bacterium E. coli K12 fortuitously chosen by them for their two articles cited above by James F. Crow, a close colleague work differs from the majority of other enterobacteria in that of Lederberg, and the one by Baruch S. Blumberg, a long it carries a natural mutation in the gene encoding an inhibitor time associate.
    [Show full text]
  • Juegative, Downloaded by Guest on September 26, 2021 VOL
    976 GENETICS: BARON, CAREY, AND SPILMAN hoc. N. A. S. 27 Libby, W. F., "Radioactive Strontium Fallout," these PROCEEDINGS, 42, 365-390 (1956). 28 Libby, W. F., "Current Research Findings on Radioactive Fallout," these PROCEEDINGS, 42, 945-962 (1956). 29 Libby, W. F., "Radioactive Fallout," these PROCEEDINGS, 43, 758-775 (1957). 30 Public Hearings on Fallout from Nuclear Weapons Tests, Testimony before the Special Com- mittee on Atomic Energy, Special Subcommittee on Radiation, Joint Committee on Atomic En- ergy, Special Subcommittee on Radiation, Joint Committee on Atomic Energy, May 5-8, 1959. U.S. Government Printing Office (in press). 31 Daily Record of Fission Product Activity Collected by Air Filtration, Naval Research Labora- tory, Washington, D.C. GENETIC RECOMBINA TION BETWEEN ESCHERICHIA COLI AND SALMONELLA TYPHIMURIUM* BY L. S. BARON, W. F. CAREY, AND W. M. SPILMAN DIVISION OF IMMUNOLOGY, WALTER REED ARMY INSTITUTE OF RESEARCH, WASHINGTON, D.C. Communicated by Joshua Lederberg, May 12, 1959 Introduction.-The phenomenon of genetic recombination by sexual mating in bacteria, described by Tatum and Lederberg' has been investigated intensively by Lederberg et al.2 and many other workers. These classical studies were carried out almost entirely with the K-12 strain of Escherichia coli, although comparable re- sults were obtained with a small number of other strains of E. coli.1 4 In further studies, Cavalli5 found that the frequency of recombination could be greatly increased by using a highly fertile mutant of the K-12 strain. Additional
    [Show full text]
  • Perspectives
    Copyright 0 1989 by the Genetics Society of America Perspectives Anecdotal, Historical and Critical Commentaries on Genetics Edited by James F. Crow and William F. Dove REPLICAPLATINGANDINDIRECTSELECTIONOFBACTERIAL MUTANTS: ISOLATIONOFPREADAPTIVEMUTANTSINBACTERIABYSIBSELECTION ISTORICALLY and conceptually, the two about genetic recombination in bacteria (LEDERBERG H themes in these titles (LEDERBERGand LEDER- 1987). BERG 1952; CAVALLI-SFORZA and LEDERBERG 1956) By 1950 it was no longer controversial among ge- are complexly intertwined. Replica plating is a homely neticists. Puce periodic selection, the monotonic in- methodology to ease the technical burdens of screen- crease of mutant number with time in long-term che- ing large numbers of bacterial colonies for mutants to mostat cultures observed by NOVICK and SZILARD use in genetic analysis. This was sometimes easy. For (1950) sealed lacunae in the 1943 experiments. (Oth- example, phage or drug resistance could be readily erwise, one could have entertained some labored hy- obtained with positive selection using phage or a drug, pothetical alternatives like uncontrolled environmen- respectively. But many other kinds of mutants were tal variation from tube to tube, or a subset of that: desired, and replica plating facilitated their discovery. temporal fluctuations within a culture in its overall Furthermore, the broader philosophical question re- propensity to turn resistant on exposure to a selective mained open, whether the resistance was a preadap- agent.) Nevertheless, many, perhaps most, readers of tive or a postadaptive change: did it emerge by natural the 1943 article did not understand its abstruse math- selection of rare preexisting mutants or did the agent ematical argument and would respond either with somehow induce the heritable change? I elaborate uncritical acceptance or uncritical rejection.
    [Show full text]