Radiation hybrids: A valuable tool for genetic, genomic and functional analysis of plant genomes

Radiation has been used as a mean to break and transfer fragments of DNA from one plant species to another. Early examples include the experiments by Sears, (Brookhaven Symp Biol 9:1–22, 1956) to transfer rust resistance genes from Aegilops umbellulata to wheat. Radiation found its niche as a mutagen due to advances in nuclear technology and formation of the International Atomic Energy Agency and their sponsorship of developing mutation breeding through “Mutation Enhanced Technologies for Agriculture”. Mutation breeding has resulted in the release of several important cultivars. Although radiation was used in plants for the mutation and introgression of genes from related species (Sears, Brookhaven Symp Biol 9:1–22, 1956; Driscoll and Jensen, Genetics 48:459–468, 1963; Riley and Law, Stadler Genet Symp 16:301–322, 1984; Sears, Crop Sci 33:897–901, 1993), this approach was not used for mapping. This aspect of radiation application was first utilized in animal cell culture lines to generate radiation hybrid (RH) panels. In the beginning these panels were generated for single chromosomes but evolved to the development of whole genome panels. This technology matured in animal systems with the onset of genomics era by its use in the development of high resolution RH-based physical maps for many species before or during the development of complete genome sequence information. The advantages of this system are: (1) radiation-induced breaks are independent of recombination events providing higher and more uniform resolution, (2) radiation dosage could be adjusted to provide varied resolution without greatly affecting the population size and (3) all markers regardless of their polymorphism can be mapped on RH panels. Plant scientists followed these studies by the development of RH panels for individual chromosomes or whole genomes. However, early RH panels in plant systems were of low to medium resolution and of limited use in physical mapping. Recently, RH panels have been produced resulting in map resolutions of 200–400 Kb. These high resolution panels promise the same value as animal systems in helping generate a complete genome sequence with a fraction of the cost of traditional methods. But the use of radiation in plants has matured to go beyond physical mapping by its application to gene cloning and forward/reverse genetic studies. These applications take advantage of plasticity offered by many plant species in tolerating radiation to produce seed and live progeny. This ability allows scientists to phenotype RH lines and to associate the phenotypic data with the genotypic data. The great potential of this system is just being realized. Genetic variation is the key to phenotypic improvement in plants.Variation in the genome provides individuals the potential tonucleus and interact with DNA adapt to changing environmental pressures and improve their chances of survival. Genetic variation also lays the foundation of genomic studies and efforts made towards crop improvement. Genetic variability in any population can be the result of natural processes such as recombination, mistakes in DNA replication and repair, gene flow or induced mutagenesis which results from chemical or radiation treatment. In this chapter, we discuss the use of radiation-induced changes on genes and chromosomes to understand the structure and function of plant genomes, dissect genetic mechanisms, and the potential use of these changes for plant improvement.