Mutation and hybridization in plant | Lecture notes Pharmacognosy

Mutation

“A knowledge of the laws of mutation must sooner or later lead to the possibility of inducing mutations at will and

so of originating perfectly new characters in animals and plants. And just as the process of selection has enabled us

to produce improved races, greater in value and in beauty, so a control of the mutative process will, it is hoped,

place in our hands the power of originating permanently improved species of animals and plants.” - De Vries, 1909

Mechanics of mutation

A mutation is any change within the genome of an organism that is not brought on by normal recombination and

segregation. Causative agents are many, but include exposure to mutagenic agents such as radiation or certain

chemicals, and mistakes made during normal cell division and replication. Most of these act upon the genome at

random and are occurring all the time. These mutations are usually benign and go unnoticed in the organism due to

the many cellular mechanisms that protect against these sorts of genetic mistakes. Mutations that are not caught b y

DNA repair mechanisms in the cell can go on to affect the organism and be present in future progeny.

Spontaneous mutations are those that occ ur without human intervention. These types of mutation happen randomly

and the cause of them therefore not easily traceable.

Types of mutation

At the most basic level, there are only a handful of classifications of mutations regardless of causative event.

Deletions and insertions involve the removal or addition of segments of DNA respectively. These segments can

range from individual base-pairs to several thousand base-pairs long. Substitutions occur when a particular base is

replaced with one of the other three nucleotide bases. Inversions are instances where a segment of chromosome is

rotated and replaced in the opposite direction that the segment was facing. The last major classification is a

reciprocal translocation. T his involves the excision of segments from two non-homologous chromosomes. These

excised portions are then inserted into the other chromosome. Chromosome A will gain the segment from

Chromosome B and Chromosome B gains the segment from chromosome A.

Mutagens and implications of each

HJ Muller first discovered and used the mutagenic properties of X-ray radiation to study the genetics of Drosophila

flies and the mechanics of heredity (Muller, 1928).

Ionizing Radiation

Ionizing radiation includes ultra-violet (UV) light, X-ray, Gamma rays, and neutrons. These high-energy forms of

radiation cause double-strand breaks of the DNA double helix. Once pieces of the DNA are broken, cellular repair

mechanisms stitch the pieces back together. These DNA repair systems can only handle low rates of radiation,

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Mutation

“A knowledge of the laws of mutation must sooner or later lead to the possibility of inducing mutations at will and so of originating perfectly new characters in animals and plants. And just as the process of selection has enabled us to produce improved races, greater in value and in beauty, so a control of the mutative process will, it is hoped, place in our hands the power of originating permanently improved species of animals and plants.” - De Vries, 1909 Mechanics of mutation A mutation is any change within the genome of an organism that is not brought on by normal recombination and segregation. Causative agents are many, but include exposure to mutagenic agents such as radiation or certain chemicals, and mistakes made during normal cell division and replication. Most of these act upon the genome at random and are occurring all the time. These mutations are usually benign and go unnoticed in the organism due to the many cellular mechanisms that protect against these sorts of genetic mistakes. Mutations that are not caught by DNA repair mechanisms in the cell can go on to affect the organism and be present in future progeny. Spontaneous mutations are those that occur without human intervention. These types of mutation happen randomly and the cause of them therefore not easily traceable. Types of mutation At the most basic level, there are only a handful of classifications of mutations regardless of causative event. Deletions and insertions involve the removal or addition of segments of DNA respectively. These segments can range from individual base-pairs to several thousand base-pairs long. Substitutions occur when a particular base is replaced with one of the other three nucleotide bases. Inversions are instances where a segment of chromosome is rotated and replaced in the opposite direction that the segment was facing. The last major classification is a reciprocal translocation. This involves the excision of segments from two non-homologous chromosomes. These excised portions are then inserted into the other chromosome. Chromosome A will gain the segment from Chromosome B and Chromosome B gains the segment from chromosome A. Mutagens and implications of each HJ Muller first discovered and used the mutagenic properties of X-ray radiation to study the genetics of Drosophila flies and the mechanics of heredity (Muller, 1928). Ionizing Radiation Ionizing radiation includes ultra-violet (UV) light, X-ray, Gamma rays, and neutrons. These high-energy forms of radiation cause double-strand breaks of the DNA double helix. Once pieces of the DNA are broken, cellular repair mechanisms stitch the pieces back together. These DNA repair systems can only handle low rates of radiation,

however, and increases in the rate of exposure to ionizing radiation causes permanent mutations to occur and accumulate in an organism’s genome. Radiation causes deletions of nucleotides from the DNA sequence. These deletions can cause reading-frame shifts, inactive protein products, or faulty transcripts. This typically results in null mutations, which are those in which a particular gene is inactivated. Chemical Mutagens Chemical mutagens affect the DNA molecule through chemical reactions within the genome. Base analogs are chemicals with similar properties to the DNA bases. They can be incorporated by the cell into the genome, replacing the proper base. Alkylating agents such as ethyl methanesulfonate (EMS), react with guanine or thymine by adding an ethyl group which causes the DNA replication machinery to recognize the modified base as an adenine or cytosine, respectively. Nitrous acid, a deaminating agent, removes the amine group from adenine or cytosine. When the cell replicates this altered area, it matches adenine to the deaminated cytosine, and cytosine to the deaminated adenine, resulting in similar effects to that of alkylating agents. The last type of chemical mutagen, intercalating agents, causes deletions, reading frame shifts, or random base insertions. These compounds insert themselves into the DNA between adjacent base pairs, thus disrupting replication and transcription machinery. Transposable Elements They are self-replicating segments of DNA that excise and/or insert themselves within the genome. Also known as transposons, these strange sequences were first proposed by the pioneering Barbara McCLintock working on maize. Transposable elements, unlike other forms of mutagenesis, do not act upon the genome in a completely random fashion. Rather, they have certain “hot-spots” where they are more likely to insert or replicate themselves. By their insertion or deletion, they act upon the genes in which they are located or those adjacent to them. Transposable elements can cause gene disruptions, protein product alterations, or large-scale genome rearrangements. If inserted into the intron of a gene, they can cause transcriptional inefficiency (Hartwell et al., 2008). Use of Mutation in Plant Breeding Mutagenesis, the act of inducing mutations within an organism’s genome, has been used in plant breeding since Muller’s discovery of the mutagenic effects of X-rays on Drosophila flies. Dose, Rate, Species/Genotypes, Conditions of application, etc. Much of the early work done with ionizing radiation and chemical mutagens was an effort to determine efficient doses and exposures of the various agents to effect high percentages of mutations without causing lethality. The researchers noticed that the rates and doses varied tremendously for species, genotype, ploidy level, and the conditions in which treatment were conducted. Grays (Gy) are the measurement unit of radiation dose. For the sterilization of food products, processors typically use rates as high as 10 kGy.

Mutation breeding in seed propagated species Seeds treated with mutagenic agents give rise to chimeric plants. Chimeric plants produce both mutant and non- mutant seed. This can be problematic; however, one just needs to plant more seeds to find the desired mutants. Mutagenic treatment of seed is by far the most popular method in mutation breeding programs. Ploidy and how it affects mutation breeding Mutagenesis of polyploid plant species is difficult. Because most mutations are recessive, plants must be homozygous to display the trait. Polyploid conditions can further complicate the process of reaching homozygosity for the mutation, so must be selfed for additional generations to ensure presence of the mutation. Importance of variation in plant breeding Variation is the source from which plant breeders are able to produce new and important cultivars. Alleles of varying forms at given loci in a population can be selected and fixed within a new individual or line. We depend on recombination and independent assortment of favorable alleles to produce new and unique individuals from which to select and produce the lines that will serve as our cultivars. Consider that variation within a population can be exploited by selecting individuals with new combinations of desirable traits or alleles. Importance of mutation in creating variation Recombination does not of itself produce novel traits. This ability is only attainable through the act of mutation, which can ultimately lead to new species. These changes in our target plant can be passed on to progeny and used for human benefit through breeding. The occurrence of mutations within the genome of plants is rare, and in natural settings can be lethal. Through breeding and selection, beneficial mutants can be identified and used to improve target species Naturally occurring mutations Mutations occur spontaneously in natural settings quite frequently. They can happen due to mistakes made during cell replication or exposure to mutagens such as radiation. It is estimated that a mutation occurs every 10-8 base pair per generation in eukaryotic genomes. In corn (Zea mays), mutations occur from 10-6 to 5 x 10-4 per base pair per generation.

Hybridization

Hybridization is the process of interbreeding between individuals of different species (interspecific hybridization) or genetically divergent individuals from the same species (intraspecific hybridization). Offspring produced by hybridization may be fertile, partially fertile, or sterile. Crop yields increase dramatically when hybridization is used to exceed one or more of the parents in size and reproductive potential. Plants hybridize much more frequently and successfully than animals do. Pollen from flowering plants disperses widely and may land on flowers of other species. One of the first persons to study plant hybridization was Josef Kölrueter, who published the results of his experiments on tobacco in 1760. Kölrueter concluded that interspecific hybridization in nature is rare unless humans disturb the habitat. Since that time, many instances of hybridization among various plant species have been documented. One good example of plant hybridization involves hybridization between the elegant sego lily ( Calochortus selwayensis ) and a mariposa lily ( C. apiculatus ) in western Montana. Types of Hybridization: Based on the taxonomic relationships of the two parents, hybridization may be classified into two broad groups:

Intervarietal Hybridization: The parents involved in hybridization belong to the same species; they may be two strains, varieties or races of the same species. It is also known as intraspecific hybridization. In crop improvement programmes, intervarietal hybridization is the most commonly used. An example would be crossing of two varieties of wheat (T. aestivum), rice (O. Sativa) or some other crop. The intervarietal crosses may be simple or complex depending upon the number of parents involved.

Mutation and hybridization in plant, Lecture notes of Pharmacognosy

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