Mutation Induction: creating novel genetic diversity using radiation

Many geneticists consider that one of the most important breakthroughs in the history of genetics was the discovery that mutations can be induced. The Nobel Prize in Physiology or Medicine 1946 was awarded to Hermann J. Muller for the discovery of the production of mutations by means of X-ray irradiation. In the late 1920’s, around the same time Muller did his experiments with fruit flies, Lewis John Stadler in his studies on maize, barley and wheat has demonstrated that radiation can induce novel genetic variability in plants. The basis for more than 80 years of successful mutation breeding was founded. Physical mutagens, mostly ionizing radiations, have been used widely for inducing hereditary genetic changes and more than 70% of released mutant varieties were developed using physical mutagens. Since the 1960’s, gamma rays have become the most commonly used mutagenic radiation in plant breeding; during the past two decades, ion beam radiation has also emerged as an effective and unique mutagen. Other types of mutagenic radiation, e.g. X-rays, α- and β- particles, fast neutrons, UV light and even space radiation, have also demonstrated usefulness in plant mutation induction, either for particular types of material, or for particular purposes (e.g. fast neutrons in inducing large deletion mutations).

Principle of mutation induction - Gamma rays and x-rays

Gamma rays and x-rays are electromagnetic radiation like visible light, radio waves, and ultraviolet light, however, they are much more energetic. If these highly energetic rays hit cells of living organisms they can directly or indirectly, through generation of reactive oxygen species cause a change in the DNA (see Fig. 1). If not repaired by the cell’s own repair mechanism a heritable mutation has been generated. For mutation breeding seeds or other plant propagules are typically treated for seconds or minutes in a gamma cell with a Co60 source (Fig. 2) or they are irradiated in X-ray machines. Alternatively whole plants or seedlings are irradiated in a gamma greenhouse (Fig 3) or a gamma field (Fig 4), a process called chronic irradiation.

Mutation Induction: creating novel genetic diversity using radiation Radiation can increase the natural mutation rate by 1000 to 1 million fold, making the generation of genetic variation very effective. Due to the nature of gamma- or x-rays (electromagnetic and particulate) plant material that has been irradiated, at no time is radioactive. Before using radiation for mutation breeding the optimal dose has to be determined: high enough to cause mutations, low enough to allow germination and growth of the plantlet. In most cases the mutations are recessive and remain invisible until the plants undergo subsequent generations. From many thousands of plants the rare cases have to be selected where the mutation imparted a new desirable character. Thus, the detection of the mutations is considered the actual “art” of mutation breeding.

Direct and indirect action of gamma rays and X-rays on the DNA (source: T. Lawrence, Slideshow is from the University of Michigan Medical; http://www.slideshare.net/openmichigan/010709tlawrenceintroradoncologypreclin

Devices for mutation induction

Mutation Induction: creating novel genetic diversity using radiation Fig 2: A GammaCell. A: A Cobalt-60 gamma source with a raised loading stage. B: Close-up of the raised loading stage of a Cobalt-gamma source showing rice grains in a Petri dish.

Mutation Induction: creating novel genetic diversity using radiation Fig 3: The gamma greenhouse at the Malaysian Nuclear Agency. a: An aerial view of the facility and immediate environs; the markings of the different security perimeters are overlaid on the picture. b: A close-up of the aerial view of the gamma greenhouse. (Courtesy of Dr. R. Ibrahim)

Mutation Induction: creating novel genetic diversity using radiation Fig 4: The gamma field at the Institute of Radiation Breeding, Ohmiya, Japan. a: An aerial view of the facility showing concentric rings of crops grown in terraces around the irradiation source located at the centre of the field; b: A close-up image of the facility showing the irradiation tower, crops being exposed to chronic irradiation and freshly prepared field for planting; c: A close-up image of the facility showing the reinforced steel casing housing the 60Co source. (Courtesy of Dr. H. Nakagawa)

References:

Plant Mutation Breeding and Biotechnology. Edited by Q. Y. Shu , B. P. Forster and H. Nakagawa . Wallingford, UK: CABI (2012), pp. 608, ISBN 978-178064-085-3

Muller Hermann J. (1930). Types of visible variations induced by X-rays in Drosophila. J Genet 22: 299–334

Stadler, L.J. 1928a. Genetic effects of X rays in maize, Academy of Sciences of the USA 14:69-75

Stadler, L.J. 1928b. Mutations in barley induced by X-rays and radium. Science. 68: 186-187