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MOHD SHAMSHADL-2012-A-91-M
OVERCOMING THE BARRIERS OF ALIEN
GENE TRANSFER
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INTRODUCTION
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TYPES OF BARRIER
PRE-FERTILIZATION BARRIERI. Failure of pollen germination on stigmaII. Slow pollen tube growth III. Pollen tube unable to reach the styleIV. Arresting of pollen tube in style , ovary
and ovuleV. Failure to obtain sexual hybridsVI. Differences in ploidy level
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POST-FERTILIZATION BARRIERI. Hybrid inviability and weakness, embryo
abortionII. Embryo abortion at very young stagesIII. Chromosome elimination IV. Hybrid sterilityV. Hybrid breakdownVI. Lack of recombination
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Technique for overcoming barrier to alien gene transfer
Pre-fertilization barrier Technique to overcome
Failure of pollen germination 1. Mechanical removal of pistil followed by pollination of the exposed end of the style
2. Use of recognition pollen
Slow pollen tube growth 1. Use of recognition pollen 2. In vitro fertilization 3. Use of growth hormones and
immunosuppressant's Pollen tube is unable to reach the style
Shortening the style
Arresting of pollen tube in the style, ovary and ovule
In vitro fertilization
Failure to obtained sexual hybrids Protoplast fusion
Differences in ploidy level 1. Chromosomal doubling of species or species hybrid before hybridization with recipient species
2. Bridging species technique
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Post fertilization barrier Technique to overcome
Hybrid in-viability and weakness (i) Embryo abortion Embryo rescue (ii) Embryo abortion at very young stages
1. Ovary culture2. Ovule culture 3. In vitro fertilization
(iii) Lethality of F1 hybrids 1. Reciprocal crosses 2. Grafting of hybrids3. Regenerating plant from callus
(iv) Chromosome elimination 1. Altering genomic ratios of two species 2. Inducing chromosomal exchanges before on set of
eliminationHybrid sterility 1. Chromosomal doubling (amphipoild production)
2. Back crossing Hybrid break down Growing larger F2 population
Lack of recombination 1. Inducing choromosomal exchange through tissue culture
2. By irradiation3. Homoelogous recombination through genetic
manupulation of chromosomal pairing system
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TECHNIQUES FOR OVERCOMING PRE-FERTILIZATION BARRIER
Manipulation of the chromosome numberMany difficulties arise when hybridization is attempted between species with different ploidy level e.g wheat, cotton, potato, tobacco, oat, Brassica napus and B. junceaChromosome doubling in either of parents helps overcome such barrier Example
o PotatoSolanum chacoense (chromsomal doubling) XS.tuberosum by livermore and johnstone 1940
o Barely Hordeum chilense X H. vulgare by Martin 1982
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BY USE OF BRIDGING SPECIES TECHNIQUE
Crossing between two species with same or different ploidy levels is difficult third species are used .
Helpful in wheat, Tobacco, lettuce Sears (1956)
Aegilops umbellulata 2n= 14
Triticum astivum2n=42
No viable seed
Use of third species T. dicoccoides (2n= 28) as bridging species
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SHORTENING THE STYLE Several species that differ widely in stylar length are
difficult to hybridize .Style of one species is to long Example - Corn zea mays (30 cm) X Tripsacum dactyloides (2 cm)To overcome this shortening the style of maize plantLess than 2 cm each of ear treat as set some seeds by
Mangelsdorf and Reeves 1939
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USE OF RECOGNITION MENTOR POLLEN
Pollen grains unable to germinate on the stigma of other plants
Use incompatible mixed with killed maternal pollen grains germination of incompatible pollen grains is obtained.
Pollen wall contain extracellular protein which are released on the stigma after pollination.
Used by Knox et al 1972 in Populus
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USE OF GROWTH HORMONES AN IMMUNOSUPPRESSANTS
Various growth harmones have been found to stimulate pollen tube growth and embryo development .
Prolong receptivity of stigma and prevent early abscission of pollen flower
Use of 75 ppm gibberellic acid (GA3 ) to the maternal plant 1 or 2 days before and after pollination for producing interspecific hybrid and intergenic hybrid
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IN VITRO FERTILIZATION Any manipulation of excised maternal and paternal
tissue to accomplished pollen tube penetration to the embryo sac to accomplish fertilization is referred to as in vitro fertilization .
Whole gynoecia are excised and placed on medium for 24 -48 hr followed by dusting of pollen on the stigma
Kanta et al. 1962 first try invitro pollination in Papaver somniferum.
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SOMATIC HYBRIDIZATIONDevelopment of hybrid plants through the fusion of somatic protoplasts of two different plant species/ varieties is called somatic hybridization
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Somatic hybridization technique
1. Isolation of protoplast
2. Fusion of the protoplasts of desired species/varieties
3. Identification and Selection of somatic hybrid cells
4. Culture of the hybrid cells
5. Regeneration of hybrid plants
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PROTOPALST FUSIONProtoplast It is cell without cell wall created by
degrading the cell wall using enzymes
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Cell wall
plasma membrane
Production of protoplast by enzymatic treatment (enzyme aredepicted above arrow) osmoticum (enzyme are depicted belowarrow) to stabilized the protoplast
N Npectinase 0.1-1% cellulase 1-2 %
500- 800 m mol/l serbitol 50-100 m mol/l CaCl2
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Isolation of protoplast (Separation of protoplast form plant tissue)
1. Mechanical method
2. Enzymatic method
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Mechanical method
Plant tissue
Release of protoplasm
Cell plasmolysis
Microscope observation of cells
Cutting cell wall with knife
Collection of protoplast
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Plasmolysed cells Plasmolysed cells
Protoplast released
Release of isolated cells
Leaf sterilization removal of epidermis
ENZYMATIC METHODS
Isolation of protoplast
Pectinase + cellulase Pectinase
cellulase
Protoplast released
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Protoplasm fusion ( Fusion of two different genome )
Spontaneous fusion Induced fusion
Intraspecific
Intergenric chemofus
ionMechanical fusion
Electric fusion
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INDUCED PROTOPLAST FUSION Electro fusion : A high frequency of AC field is
applied between two electrodes immersed in the suspension of protoplast this induced charges on the protoplast and causes them to arrange themselves in lines between the electrodes. They are the subjected to high voltage discharge which causes them membranes to fuse where they are in contact.
Polyethylene glycol (PEG) causes agglutination of many types of small particles, including protoplasts which fuse when centrifuged in its presence.
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A schematic representation of the 3 most successful protoplast fusion strategies.
PROTOPLASTS OF SPECIES(A)
PROTOPLAST OF SPECIES(B)
(SUSPENDED IN ENZYME MIXTURE)
HIGH Ca2 + , HIGH pH TREATMENT
PEG- INDUCED FUSION
ELECTROFUSION
Ca2+ 50 m mol 1-1
Ph 10.5,Temp.37OC
PEG 28-50% (MW 1,500-6,000)
LOW VOLTAGE15 -30 min PROTOPLAST AGGREGATION
30 min
WASHING MEDIUM Ph 9-10
Ca2+ 50 m mol 1 -1
PROTOPLAST CHAIN FORMED (DESIRED PROTOPLAST PAIR ALINGED WITH A MICRO –MANIPULATOR)
HIGH VOLTAGE (FEW MILLI-SECONDS)
PROTOPLAST FUSION
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PROTOPLASTS (SPECIES A)
PROTOPLASTS (SPECIES B)
PRODUCTS THAT INVOLVE TWO PROTOPLASTS ARE DEPICTED HERE
FUSOGEN TREATMENT
A B A + A
B +B
B +A
UNFUSED PROTOPLASTS
HOMOKARYONSHETEROKARYON
USEFUL ONE
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Possible Result of Fusion of Two Genetically Different Protoplasts
Fusion
heterokaryon
Cybrid Cybrid Hybrid Hybrid
= Nucleus
= Mitochondria
= Chloroplast
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Identification and Selection of somatic hybrid cells
Hybrid identification- Based on difference between the parental cells and hybrid cell with respect to- Pigmentation Cytoplasmic markers
Fluorochromes like FITC (Fluoroscein Isothiocyanate) and RITC (Rhodamine Isothiocyanate) are used for labelling of hybrid cells.
Presence of chloroplast Nuclear staining
Heterokaryon is stained by carbol-fuschin, aceto-carmine or aceto-orcein stain
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TECHNIQUES FOR OVERCOMING POST-FERTILIZATION BARRIER
Hybrid inviability and weaknes Abortion embryo is most common among post fertilization barrier Different stages of development depending upon the genomic
relationship between two speciesEmbryo rescue :
When embryos fails to develop due to endosperm degeneration, embryo culture is used to recover hybrid plants; this is called hybrid rescue.
ExampleH. vulgare x Secale cereale. Embryo rescue generally used to overcome endosperm degeneration.
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EMBRYO IMPLANTATIONUsed when embryo abortion starts at very young stages of development.Endosperm of female parent serve as nurse tissue for the hybrid embryo Normal endosperm from the female parent is placed on the surface of nutrient media but not direct contact to hybrid embryos. Sometimes the endosperm extract is added to the nutrient media
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OVARY CULTURE Ovary culture is often used either for in vitro pollination and
fertilization or for embryo rescue, when the embryo culture and ovule culture either fail, or are not feasible due to very small ovules. Interspecific hybrids, using ovary culture, have successfully been obtained in several genera including Brassica (B. campestris x B. oleracea).
This technique is consist of excising ovaries 2-15 days after pollination depending upon the cross combination and culturing them on nutrient medium. The calyx, corolla, and stamen are removed. Before culturing the tip of distal part of the pedicel is cut off and ovary is implanted with the cut end inserted in to the medium. After embryo become visible they are excised aseptically on nutrient media followed by embryo rescue.
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OVULE CULTURE
For a variety of difficult interspecific / intergeneric crosses involving members of the families Malvaceae, Fabaceae, Cruciferae, Solanaceae, etc., ovules after fertilization have been successfully cultured to obtain mature embryos /seeds.Depending upon, when the embryo aborts, the ovules have to be excised any time from soon after fertilization to almost developed fruits, which may sometimes be lost due to premature abscission.However, ovule culture is mainly tried only in those cases, where embryo aborts very early, and embryo culture is not possible due to difficulty of its excision at a very early stage. In some cases, the medium may need to be supplemented with some fruit/Vegetable juice to accelerate initial growth.Used affecting growth of zygote in early stages of development
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Contd………
Ovaries are Harvested 1-12 days after pollination Surface are sterilized and cut open with sterilized scalpel. Fertilized ovule is scooped out placed as evenly as possible on nutrient mediumE.g. cotton
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GRAFTING HYBRIDSGrafting of hybrid on the normal plant for successful production of hybrids
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RECIPROCAL CROSSES Reciprocal differences in wide crosses are also vary common and can be due to chromosomal imbalance in the endosperm, the role of the sperm nucleus in differential endosperm development or the alteration of endosperm development by pollen through the effect of antipodal cells, which are assumed to supply nutrients during early endosperm development.If disharmony between the genome of one species and cytoplasm of another species is a cause of a fertilization barrier ,reciprocal crosses is successful in recovery of hybrids.
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ALTERATION GENOMIC RATIOChromosome elimination sometime occur in wide crosses.In Hordeum vulgare H. bulbosumIn wheat maize crosses May be avoid by altering the ratios of parental genome Hordeum vulgare (diploid ) H. bulbosum (tetraploid)
No chromosomal elimination in the zygote (kasha 1974)Followed by mutagenic treatment of fertilized egg cell
could be used to induced chromosomal exchanges before the onset of chromosome elimination
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CHROMOSOME DOUBLING OVERCOMES STERILITY OF F1 HYBRID
Crosses betweenTriticum aestivum X Secale cerealsPennisetum glacum X P.setaceum Wild Oryza spp. X Oryza spp. Etc
Lead to sterile F1 hybridsDoubling the chromosome by use of colchicine result formation
of restitution nuclei produces amphidiploids in which pairing occur between respective diploid genome present in F1 hybrid.
E.g. Triticale
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Triticale Production
Durum wheat2n=28 AABB
Rye 2n=14 RR
Haploid embryo n= 21 ABR
Haploid plantlet n= 21 ABR
Embryo rescue
Chromosomal doubling
Primary hexaploid Triticale2n= 42 AABBRR
“WIDE’’ CROSSING OF WHEAT AND RYE REQUIRES EMBRYO RESCUE AND CHEMICAL TREATMENTS TO DOUBLE THE NO. OF CHROMOSOMES TRITICAL
Triticale
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BACKCROSSING Brar D S and Khush G S (1997) Alien introgression in rice. Plant Molecular Biology 35–47
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HYBRID BREAKDOWN
F1 hybrids are fertile Recombination in F2 or later generation are lethal,
weaker and are gradually eliminated ,resulting in only parental types.
Cause Centromeric affinityCryptic structural hybridity Gene substitutionUnfavorable nuclear-cytoplasm interactions
E. g . Cotton
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LIMITED RECOMBINATION Tissue culture of wide crosses Irradiation-induced chromosomal translocationManipulation of chromosome pairing
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FUTURE STRATEGY FOR ALIEN GENE INTROGRESSION
Advanced backcross –QTL strategyLooking for gene based on molecular mapsRecombination DNA technologyDouble haploids
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ADVACED BACKCROSSED-QTL STRATEGY
Productivity enhancing gene / QTLs are introgressed inoat from Avena sterilis (Frey et al 1983) in tomato fromLycopersicon pimpinellifolium and L. parviflorum(Tanksley et al 1996, Fulton et al 2000) and in chick peafrom Cicer reticulatum ( Singh et al 2005)
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RECOMBINATION DNA TECHNOLOGY
Plant genetic transformation technique such as Agrobacterium-mediated transformation and direct gene delivery system (biolistics) allow the precise transfer of genes from any organism into either plant nucleus or chloroplast genome. Many isolated plant genes are now being transferred between sexually incompatible plant species.
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CONCLUSION Wild species are an important reservoir of useful genetic variability for traits of economic importance to diseases and insects, tolerance for abiotic stresses, improved quality characteristics and new source of male sterility. There are several pre and post-fertilization barrier that hinder the transfer of useful alien gene into crop plant. The techniques of embryo rescue, ovary and ovule culture , invitro fertilization, protoplast fusion, use of growth hormones, chromosomal doubling , induced chromosomal exchange and recent approch Markered assisted selection (MAS), recombination DNA technology and Advaced backcrossed-QTL strategy wider the scope of introgression of alien gene in cultivated plant not only from plant kingdom but also from animal kingdom.
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