At this Symposium we are remembering and honouring the great and revolutionary discoveries of Gregor Mendel, presented to the world 100 years ago this year, which for the first time expressed the basic phenomena of heredity in a concise, analytical and, above all, numerical form and thus laid the foundation of the science of genetics. However, as the title of this symposium implies, we are not met here merely in adulation of Mendel’s genius but rather to review and discuss the fruits which have now matured on the tree which he planted. I think some of these fruits would seem very strange and incomprehensible to Mendel, for over the last two decades we have witnessed another revolution in genetics as dramatic and as pregnant with new potentialities as that of 100 years ago. The basic ingredients of this revolution were, first, the disclosure of systems of genetic recombination in micro-organisms, and especially in bacteria and their viruses, which enormously increased the resolution of genetic analysis; and, secondly, the elucidation by Watson & Crick, in 1953, of the physico-chemical structure of the genetic material, deoxyribonucleic acid—undoubtedly the most important and provocative biological discovery since Mendel. As a result we are now recapitulating the cytogenetical studies and correlations which marked the turn of the century, but this time the precision and refinement of our tools and techniques have increased more than 1000-fold so that we are looking at genetic behaviour and interaction at the level of molecular structure. In this lecture I would like to discuss what, from the Mendelian viewpoint, must be one of the most bizarre forms of sexual heredity, namely, the process of conjugation and genetic recombination in the bacterium
Escherichia coli
. My reasons for choosing this rather esoteric topic, apart from personal interest, are three. First, the whole mechanism of sexuality in this organism is mediated and controlled by a new kind of genetic element called the sex factor which, like some temperate bacteriophages, is able to exist in alternative states in the cell, either free in the cytoplasm or as an integral part of the bacterial chromosome, and which can properly be construed as a virus with a novel mode of infectivity, as I hope to show. Secondly, a number of essentially similar elements have recently been discovered in bacteria, masquerading under such different disguises as the genetic determinants of antibiotic substances called colicins or as carriers of transmissible drug resistance, so that the sex factor is far from being a unique entity among the bacteria. Thirdly, genetic interactions occur between the sex factor and the bacterial chromosome which confer great flexibility on this system. Some of the situations which are generated by these interactions mimic those found in the cells of higher organisms so that it is possible to construct plausible, though very speculative, models for the evolution of more stable and highly organized genetic systems.
Recurrent DNA virus domestication leading to different parasite virulence strategies