Mutation, Polyploidy and Hybridization

Objectives

At the end of the lecture the student will be able to

• Understand the process of mutation

• Explain the types of mutation

• Understand the process of mutation breeding

• Discuss the process of polyploidy and hybridization

Mutation

• Sudden changes in the nucleotide sequence of DNA

• May be harmful or beneficial or neutral

• May occur in somatic cells (aren’t passed to offspring)

• May occur in gametic cell and be passed to offspring

• A cell or organisms – shows the effect of mutation –
mutant

• Chemicals & UV radiation cause mutations

• Some mutations may improve an organism’s survival (beneficial)

On the basis of Causative agent of Mutation:

1. Spontaneous Mutation

Mutations that result from natural changes in DNA

• Tautomerization

• Depurination

• Deamination (may also be induced by mutagenic chemicals)

2. Induced Mutation

• Changes caused – environmental chemicals and radiations
any environmental agent that increases the rate of mutation above the spontaneous is called a mutagen

Chemicals Mutagens

Base analog mutagens

• Chemicals with structures similar to that of any of the
four standard bases of DNA

• DNA polymerases cannot distinguish these analogs

• These are chemicals look like normal bases – fool the DNA
replication system

• They base-pair with two different bases thus making
mutations because of their lack of consistency in base-pairing

• 5 – bromo – uracil – should be present during DNA
synthesis

2-amino purine (P)

• Base analog of adenine

• Normally pairs with thymine

• May mispair with cytosine

• Causes a transition mutation

Alkylating mutagens agents

• Chemicals react directly with certain bases and thus do
not require active DNA synthesis

• Ethyl methane sulfonate (EMS), methyl methane sulfonate
(MMS), diethylsulfate (DES), and nitrosoguanidine

• These mutagens tend to prefer G-rich regions, reacting to
form a variety of modified G residues

• It adds an ethyl group to guanine and produces
6-ethylguanine, which pairs with thymine and leads to CG:TA transitions

• Also adds an ethyl group to thymine to produce
4-ethylthymine, which then pairs with guanine, leading to a TA:CG transition

• Mutations produced by EMS can be reversed by additional
treatment with EMS

• Mustard gas is another alkylating agent

Other Chemical Mutagens

Nitrous acid: causes deamination Cytosine à Uracil

Radiations mutagens

• In 1927, Herman Muller demonstrated that mutations   could be induced by X-rays.

• X-rays, gamma rays, and cosmic rays are all capable of
penetrating tissues and damaging DNA

• They remove electrons from the atoms that they encounter,
changing stable molecules into free radicals and reactive ions which then alter the structures of bases and break phosphodiester bonds in DNA

• Ionizing radiation also frequently results in double-strand breaks in DNA.

Types of Mutation

• Chromosomal mutation

• Genetic mutation

Chromosomal Mutation

• Changing the structure of a chromosome

• The loss or gain of part of a chromosome

• Five types

1.   Deletion

2.   Inversion

3.   Translocation

4.   Non disjunction

5.   Duplication

Deletion

• Due to breakage – a piece of a chromosome is lost

Inversion

• Chromosome segment breaks off

• Segment flips around backwards

• Segment reattaches

Duplication

• Occurs when a gene sequence is repeated

Translocation

• Involves two chromosomes that aren’t homologous

• Part of one chromosome is transferred to another
chromosomes

Non disjunction

• Failure of chromosomes to separate during meiosis

• Causes gamete to have too many or too few chromosomes

Gene Mutation

• Change in the nucleotide sequence of a gene

• May only involve a single nucleotide

• May be due to copying errors, chemicals, viruses etc.

• Include:

– Point Mutations

     • Substitutions

     • Insertions

     • Deletions

– Frame shift

Point Mutation

• Change of a single nucleotide

• Includes the deletion, insertion, or substitution of ONE
nucleotide in a gene

• Sickle Cell disease is the result of one nucleotide
substitution

• Occurs in the hemoglobin gene

Frame shift Mutation

• Inserting or deleting one or more nucleotides

• Changes the “reading frame” like changing a sentence

• Proteins built incorrectly

Original:

The fat cat ate the wee rat. Frame Shift (“a” added):

The fat caa tet hew eer at.

Amino Acid Sequence Changed

Spontaneous radiation

• Natural – rare

Artificial mutation

• Induced by substances – mutagenic agents or mutagens

• Radiations or chemicals

Mutation Breeding

• Referred – “variation breeding” – is the process of exposing seeds to chemicals or radiation in order to generate mutants with desirable traits to be bred with other cultivars

• Plants created using mutagenesis – mutagenic plants or
mutagenic seeds

• Used since 1980’s – does not involve genetic modification

• Used to produce traits in crops such as larger seeds, new
colors, or sweeter fruits that either cannot be found in nature or have been lost during evolution, disease resistance, tolerance to harsh growing conditions

Advantage of Mutation Breeding

• Cost effective

• Transferrable, non-hazardous and environmentally friendly

• More than 3200 mutant varieties – officially released for
commercial use – more than 210 plant species from more than 70 countries – mostly of cereals, followed by flowers and legumes

Radiation Breeding

• Type of mutagenic breeding

• Exposing plants to radiation – Mutation with radiation –
radiation breeding

• Radiation breeding was discovered – 1920s – Lewis Stadler
of the University of Missouri used X-rays on maize and barley

• Different types of radiations – α – rays, ß – rays, ɤ
rays, X – rays and UV rays

• ɤ rays (64%) and X – rays (22%) – commonly used

• Part of the plant used – seeds, pollen grains, bud,
cuttings

• Seeds are commonly used – withstand extreme environmental condition

Step 1

Treatment of plant material

• For irradiation breeding – seeds and seedlings are used

• Seeds – should be soaked – normal water

• Seedlings – Neither too old nor too young are irradiated

Irradiation Breeding

• Carried out in Gamma garden

• Area – 3 acres – surrounded by tall wall – protective
shield

• Radioactive cobalt source – pellets –placed in aluminium
capsule – in turn placed on a pole – center of the garden

• When the lid – lead container lifted – entire garden
receives radiations

• Plants are grown – concentric circles

• Garden – sectors – specific group of plants

• Monitors – used to determine the exact dose of radiations
received by the plants

Production of Mutant Strains

• After irradiation – seeds are collected – grown

• Plants with desired characters – selected and grown – M1
generation

• Plants with desired characters – again selected and grown
– M2 generation

• Process – repeated – M5 generation – by which plants will
be more or less homozygous

• Plants – checked for one or more generation – released as
mutant strains

Advantage of Mutant Strains

• Disease resistant plants can be produced

• Higher yield can be achieved

Disadvantage of Mutant Strains

• Can be carried out only in specific area

• Costlier

• May yield beneficial or non-beneficial varieties

Application of Mutant Strains

• Improved varieties – high yield, better quality, earliness, dwarfness, disease resistance and low toxic content are developed

• By irradiation of poppy seeds with cobalt – 60 (60Co) –
mutants with high morphine content – produced

• Mutant strains of Capsicum annum – with increased yield of capsaicin – developed – treating the seeds with sodium hydroazide and ethyl methane sulphonate

• Alkaloid production in various species of Datura raised
from irradiated seeds

List of some Varieties Developed in India – Mutation Breeding

Polyploidy

• Species contains more than two sets of chromosomes

• Polyploidy occurs in a multiple series of 3,4,5,6 etc., of
the basic chromosome

• 3 (Triploid), 4 (tetraploid), 5 (pentaploid) etc

• Polyploid arise naturally as spontaneous chromosomal
mutations –

– aberrations

Classification of Polyploidy:

Plants listed according to their classification as autoploids or alloploids

Induction of polyploidy

• Decapitation

• Indoleacetic acid

• Twin seedlings

• Heat treatments

• Colchicine

• Other chemicals

Colchicine

• Proto Alkaloid – Seeds and corms of Colchicum luteum/C
autamnale

Family: Liliaceae

• Spindle fibre poison or suppressant

MOA of Colchicine –

• Anaphase of Mitosis

• Prevents separation of sister chromatids

• Interacts with disulfide bonds of spindle protein

Treatment of plant materials with Colchicine

Seeds: Soaked in aqueous solution (0.2-2%) for 1-4 days

Roots: By moisturizing with alkaloid solution

Twigs: By dipping in solution

Young shoots: Agar or lanoline paste containing alkaloid

It is also be used in the form of spray

Hybridization

• Process of producing hybrids is called as Hybridization

• Hybrid is an organism that results from crossing of two
species or varieties differing at least in one set of characters

• Monohybrids, dihybrids, polyhybrids

• Helps in inducing in single variety, the favorable  characters of other variety/species and sometimes producing new and favorable characters not present in both the species/varieties

The process of hybridization 

involved following steps:

i) Choice of the Parents:

ii) Evaluation of the parents,

iii) Selfing of parents,

iv) Emasculation,

v) Bagging

vi) Tagging

vii) Pollination

viii) Harvesting

ix) Threshing, drying and storage etc.

Types of Hybridization

– Based on the taxonomical position

• Intravarietal

1. Intraspecific

2. Interspecific

• Distant hybridization

1. Intrageneric

2. Intergeneric

Examples

D. purpurea x D. lanata; D. purpurea x D. lutea

Lanatoside A, but devoid of Lanotoside C and purpurea
glycoside A

Solanum incacum (1.8% solasodine) x S. melongena (Traces of solasodine)

First generation beared more fruits with 0.5 % solasodine

Second generation – high yielding source for solasodine

Summary

• Sudden changes in the nucleotide sequence of DNA

• May be harmful or beneficial or neutral

• May occur in somatic cells (aren’t passed to offspring)

• May occur in gametes (eggs & sperm) and be passed to
offspring

• Chemicals & UV radiation cause mutations

• Some type of skin cancers and leukemia result from somatic mutations

• Some mutations may improve an organism’s survival
(beneficial)

• Species contains more than two sets of chromosomes
–polyploids

• Organism that results from crossing of two species or
varieties differing at least in one set of characters – hybrids – process is hybridization