Mutation and Mutation Breeding

Mutation Sudden change in the hereditary material of a cell Can be: Chromosomal Genic Molecular level Mostly Deleterious Harmful Lethal

Mutation New genetic variability – new adaptation An organism that exhibits a novel phenotype resulting from a mutation = mutant

b c a Mutation (a) Variegated plant Each green cell has a mixture of normal chloroplast and plastids with a mutation that prevents chlorophyll synthesis. Cells in the white area have only mutant plastids. b (b) Cells usually have many plastids and mitochondria, so random distribution of the organelles is usually sufficient to ensure both cells receive some. c a (c) If mutant plastids are present, then occasionally when cytokinesis occur, one end of the cell may have only mutant plastids.

Type of mutation Micromutation / point mutation/ microlessions Macromutation/ Chromosomal aberrations

Miromutations or Point mutations Small effects on the organisms Change in one base Types: Base substitutions Transition = Pyrimidine to Pyrimidine or A-G = Purine to Purine T-C Transversion = Purine to Pyrimidine or = Pyrimidine to Purine Base insertion Base deletion Frameshift

Maromutations Big effect on the organism Involved big chunk of DNA Types Inversions - Fliped Deletion - Lossed Duplication - Duplicated Translocation - Moved

Chimera When mutations occur in somatic tissue, only a small sector of the plant will carry the mutant gene. In this case, that small sector of the plant is called chimera.

Chimera

Chimera Tunica-corpus model

Chimera Mutated cell do not entirely cover the apical dome. Involve limited number of cells. Generally, one cell layer. Giving rise to chimeral and nonchimeral shoots.

Chimera Relatively stable and can be vegetatively propagated. Mutated cell is positioned near the apical dome. Meristem contains one layer which is genetically different from the remainder of the meristem.

Chimera mutations affect sections of the apical meristem Altered genotype extending through all the cell layers. Giving rise to chimeral and nonchimeral shoots.

Recessive Vs. Dominant mutation Recessive mutation Change from a dominant allele to recessive allele Common Dominant mutation Change from a recessive allele to dominant allele rare

Recessive mutation M1 generation M2 generation Mutagen selfed AA No mutation AA AA Seeds or Plants AA AA Aa Mutation – Chimeric 1AA:2Aa:1aa Mutation – Non-chimeric Aa

Recessive mutation Chimeric plant Mutagen treatment of wheat seeds induces a recessive mutation in an embryo meristemic cell. A tiller (M1) that arises from that affected cell is heterozygous for the mutant gene. When M1s are selfed, the M2s from the affected tiller will show 3:1 segregation ratio. One out of four M2 plants will exhibit the recessive mutant character and seeds from this tiller will produce mutant plants in the M3. From the same M2 population, two plants out of four are heterozygous. The recessive mutant will segregate out in the M3 generation. Heterozygous 3:1 1-AA 2-Aa 1-aa

Dominant mutation M1 generation M2 generation Mutagen selfed aa No mutation aa aa Seeds or Plants aa aa Aa Mutation - Chimeric 1AA:2Aa:1aa Mutation – Non-chimeric Aa

Germinal mutation Occur in germline cells Occur at any stage in the reproductive cycle of the organism Dominant mutation which occur in germline cells – effects is expressed immediately in progeny

Somatic mutation Occur in somatic cells Resulting mutant phenotype occur only in the descendants of that cells Mutation will not be transmitted to the progeny Only through vegetative propagation the mutant can be maintained and perpetuated

Somatic mutation In ‘Red delicious’, a mutant allele determining the colour of the ovary wall to be golden arose in a flower. This developed into golden apple. As vegetative propagation is feasible for apple, this mutant was developed into a variety called the ‘Golden delicious’ variety – from muntant branch

Spontaneous Vs. Induced mutations Spontaneous mutation Occur in nature or without a known cause. Associated with natural forces. Result from normal chemical processes in the cell. Induced mutation Occurs as the result of the influence of any artificial factor, i.e. physical and chemical agents that cause changes in DNA.

Mutagens Ionizing radiations X-rays Neutrons Gamma ray Ultraviolet Laser beam Chemicals EMS (Ethyl methanesulfonate) dES (Diethyl methanesulfonate) EI (Ethyl Imine)

Ionizing radiation X-ray Widely available Easily operated Seeds, plants or pollen can be treated with fairly accurate doses. Neutron radiation Produces more severe damage to the chromosome than X-ray. It is used mainly with seeds.

Ionizing radiation Gamma rays Emitted from radioactive cobalt or radioactive isotopes Cause less injury to the plant cells Frequently used for whole plants or plant parts including pollen. Laser beams is a more recent event.

Ionizing radiation The radiation dose is determined by: Intensity of the radiations length of the exposure. IAEA (International Atomic Energy Agency) in Vienna provides training course and publishes manuals on the use of ionizing radiation in mutation breeding.

Chemical mutagens Simpler to use and produce less damaging effects. The most widely used is EMS. It is a powerful carcinogen and must be used with extreme caution. Seeds, buds, roots and dormant cuttings can be treated by soaking in a solution of the chemical mutagen. Chemical mutagens are less drastic in their effects then ionizing radiations, producing more gene mutations and fewer chromosome disruptions. However, it is not possible to direct the mutation process so that a specific type of mutation can be produced.

Use of mutagens What dose should be used? Use the dose that kill 50% of the plants Too high – Kill too many plants Mutagens harmful to plants Mutations are lethal Too low – Not enough mutations

Tissue for mutagen treatment Seeds - easy to treat large number and easy to handle - multicellular, M1 plants chimeric Gametes or zygotes - leads to non-chimeric heterozygous M1 plants Vegetative propagules - shoot meristem, adventitious bud Cell and Callus cullture - leads to non-chimeric regenerated plants

Example of mutations breeding Sorghum – Example of spontaneous mutation A dwarfed mutant plant was found in a farmer’s field. Seeds increased and gave rise to the ‘Dwarf’ cultivar. A second dwarfed mutant was found among the population of ‘Dwarf’ cultivar and a new cultivar called ‘Double dwarf ’ was developed. Mutant dwarf form facilitate mechanized harvesting of the grain sorghum crop.

Plant cultivars from induced mutation According to 1363 (FAO/IAEA, 1991) Novel plant architecture Earlier or later flowering time Different flower colour or shape Different fruit colour or size Resistance to pathogens and pests Changes in chemical composition, e.g. oil, fatty acid, protein, amino acid, starch quality etc. Changes in reproductive characters e.g. male sterility, fertility restoration, self-incompatibility, seedlessness