Hybridization | Biology for Grade 12 PDF Download

NEW AND UNDER-UTILIZED CROPS

•  Out of 3, 50, 000 known plants at this time, a few i.e about 100 plants are being used for fulfilling man's daily requirement.

•  Scientists are in search of less known and under-utilized crop plants, which can be used for food and other purposes. Such under-utilized and under-exploited plants are known as new crops.

(1) Triticale (Muntzing) :- It is the first man made cereal or crop.

•  Triticale is not suitable for purpose of bread making due to low gluten content, but it is a good forage crop.
  Triticale (Muntzing)

(2) Winged bean :- (Psophocarpus tetragonoloba) 

•  This is a herbaceous plant, which has capacity of nitrogen fixation.

•  The ripe seeds contain about 34% proteins and 13% oils (similar to soyabean).

(3) Jojoba = (Simmondsia chinensis) :-

•  This is a shrub, which is native of Mexican deserts. It is important drought desert plant and hence is being grown in deserts.

•  The seeds of this plant contain about 50% liquid wax, which is similar to sperm whale oil (spermaceti).

•  This liquid wax was originally used in cosmetics, but now is also being used in high-performance lubricants required to withstand extreme pressures.

•  So growing of this plant, can reduce the pressure on sperm whales, which are killed for their oil.

•  Jojoba can thrive under poor soil and low moisture conditions. Its cultivation in arid regions of the world help in economic development of the poor.

(4) Guayule = (Parthenium argentatum) :-

•  This plant commonly known as "Congress grass". It is a terrestrial weed in India.

•  This plant is now a days used in obtaining rubber, which is called "Guayule rubber".

•  Guayule rubber is similar to Para rubber or hevea rubber. 

•  The plant contains 12-20% rubber on dry weight basis.

•  This plant can be a natural source of rubber in future.

(5) Leucaena or Subabul :-

•  Leucaena leucocephala (fam.leguminosae)

•  This plant is nowadays being planted on a large scale under "social forestry".

•  This plant is used as "wind breaks, fire break, shade plant for deforested tropical regions.

(3) Plant introduction – 

•  This is the most rapid method of crop improvement. 

•  The process of introducing plants with specific characters from one area to a new and changed climatic condition is called"Plant introduction".

•  If plants are brought from foreign country, it is called exotic collection(EC) 

•  If plants are brought from same country then it is called "indigenous collection"(IC).

•  Primary introduction :- If introduced plants may be used directly for cultivation, it is called primary introduction.

•  Secondary introduction :- If introduced plants may be used after subjecting to selection/hybridization, it is called secondary introduction.

•  Acclimatization :- It means adaptation of introduced or transferred plant in the changed or new climatic conditions is called acclimatization.

•  Introduced plant material are subjected to "Quarantine laws/plant protection inspection. To check the entry of pathogen (All the introduced plant materials is thoroughly inspected for contamination with weeds, diseases and insect pest)

•  If the plant material is found suitable, phytosanitary certificates are issued and only then the plant material is introduced in our country.

•  Uncontrolled plant introduction in the past are responsible for introduction of hazardous diseases like late blight of potato, leaf rust of coffee, fire blight of apple, bunchy top of banana etc.

•  Many weeds like Argemone maxicana, Eichornia crassipes and Parthenium argentatum etc introduced in our country due to uncontrolled plant introduction.

•  New plants like potato, groundnut, coffee, rubber, guava, grapes, papaya, litchi, gulmohar, bougainvillea etc are the result of plant introduction.

•  Wheat variety "Ridley" was introduced in India from Australia

•  Many improved varieties of different crop plants are also outcome of these introduction e.g Sonora-64 varieties of wheat, Palmetto varieties of soyabeans, Kent variety in Oat, Bonneville var. in Pea, Taichung native 1(TN1) of rice etc.

(4) Hybridization – Mating between two or more individuals or lines, which are different in genotype.

• Hybridization is the most common method for creation of genetic variations.

Lines – A group of individuals related by descent and often similar in genotype.

Hybridization takes place between two different lines.

• The plants, which are crossed together may belong to the same species, or different species or different genera.

• Hybridization is divided in following categories

(1) Intra-varietal hybridization :- The cross between the plants of same variety

(2) Inter-varietal hybridization :-

• The cross is made between the plant belonging to two different varieties of same species (also known as intra-specific hybridization)

(3) Inter-specific hybridization :- The plants or two different species belonging to the same genus are crossed together.

Ex. All present cultivated varieties of sugar cane.

(4) Inter-generic hybridization :- The crosses are made between the plant belonging to two different genera. e.g. Triticale, Raphanobrassica, Rabbage are the example of intergeneric cross.

• First natural hybridization was reported in corn (maize) by Cotton Mather

• First artificial hybrid was obtained by crossing Sweet william and carnation by Thomos Fairchild(1717) and was known as Fairchild's Mule

• Hybridization was first of all practically utilized in crop improvement by kolreuter(1760)

• Hybridization or crossing leads to hybrid vigour or heterosis, which is defined as superiority of hybrid over it's parents

• The term heterosis was given by G.H.Shull in 1914.

• Hybrid vigour is due to heterozygosity

• Heterosis is lost by inbreeding

• Vegetatively reproducing plants are most suited for maintaining hybrid vigour because once a desired hybrid has been obtained, there are very few chances of losing it.

The main steps of hybridization are:

(a) Selection of parents

(b) Selfing of parents to induce homozygosity

(c) Emasculation i.e removal of anthers or male reproductive part from flower before maturity

(d) Bagging :- Female flowers are covered with bags, so that no undesirable pollen may fall on stigma.

(e) Crossing of desired & selected plants.

• Thus 2-main aspects of hybridization are :-
  (i) To combine characters of two plants in to one
  (ii) To utilize hybrid vigour Inbreeding – When two individuals of a species, which have common ancestory are mated together. Most extreme form of inbreeding is self pollination.

Inbreeding depression – Loss of vigour due to inbreeding.

• Cross pollinated species shows inbreeding depression, but self pollinated species do not show inbreeding depression.

• Plants of cross pollinated species are highly heterozygous, therefore they contain recessive alleles of most of the genes in heterozygous state.

• When these are subjected to inbreeding there is ­ in homozygosity and many recessive harmful alleles also become homozygous.

• While in self pollinated crops alleles, become rapidly homozygous & then harmful alleles are removed by selection. So here inbreeding depression does not occurs.

(5) Polyploidy –

• The organism (plant) which contains more than two complete sets of chromosomes is called polyploid.

• Depending upon number of chromosomal sets, the individuals are given different names-monoploid, diploid, triploids, tetraploids pentaploids and hexaploid (eg- wheat)

• Polyploids are characterized by gigantism.

• These polyploids are used in crop improvement.

• Triploids are present naturally in different crop plants and generally triploid crop plants are seedless.

• Most of the varieties of banana are triploids, so their fruit are seedless.

• In sexually reproducing organism polyploidy is induced, as a result of fusion of egg with more than one male gametes.
   

•  Polyploidy can be used artificially by Colchicine treatment.

• Colchicine is an alkaloid obtained from Colchicum autumnale(fam. Liliaceae)

• In some plants triploids are having much vigour and increased fruit size e.g: apple & pear.

(6) Mutation breeding – Plant breeding –It is the purposeful manipulation of plant species in order to create desired plant types that are better suited for cultivation, give better yields and are disease resistant.

• Plant breeding may be defined as "the branch which deals with improvement in heredity of crops and production of new varieties, which are superior to earlier ones in all respects.

Improvement may be in following respects :-

(i) Increase in yield of seed, fibre oil etc.

(ii) Resistance to disease, insects, pests, drought, frost & cold etc.

(iii) Adaptability to wider range of conditions etc.

(iv) Earliness or lateness in maturity period or change in maturity behaviour. 

Important plant breeders :-

(i) N.E. Borlaug :- Famous Mexican plant  breeder, who was awarded Nobel peace or prize (1970) for developing high yielding dwarf wheat varieties like Sonora-64 and Lerma rojo-64 etc. He is known as "Father or green revolution".

(ii) Dr. M.S. Swaminathan :- He is pioneer in mutation breeding. He has produced Sharbati sonora variety of wheat by mutation, which is responsible for green revolution in India. He is known as"Father of green revolution in India"

• Crop improvement means combining all the good characters in a single plant and multiplying them. The main aim of the plant breeder is to produce new superior crop varieties.

Mutation breeding – Use of induced mutations in plant breeding to develop improved varieties. Induced mutations are useful in specific situations, when the desired alleles are absent in the germplasm.

• Many important varieties in different crop plants have been produced by mutation breeding

• In wheat : Sharbati sonora and pusa lerma are two important varieties of wheat produced by gamma rays treatment of sonora-64 and lerma rojo (Mexican dwarf wheat varieties)

• Sharbati-sonora is amber grain coloured variety of wheat produced by Dr. M.S. Swaminathan and is responsible for green revolution in India.

• In rice : About 45 varieties up to 1992 have been produced by mutation breeding eg Remei & Atomita-2.

• In barley : Erectoids and erectiferum.

• In Castor : World famous variety Aruna has been produced in which life span has been reduced from 270 days to 102 days and also having high oil content and disease resistance.

• Penicillin production has been increased enormously  by UV-rays treatment of Penicillium notatum & P. chrysogenum.

In mung bean, resistance to yellow mosaic virus and powdery mildew were induced by mutations.

Mutation breeding has some important limitations as :

(i) Most of the mutations are recessive.
(ii) Mutation rate is extremely low.
(iii) Most of the induced mutation are invaluable to the breeders and many of them are lethal.
(iv) Stability of mutant is sometimes doubtful, as some mutants have tendency to revert back to original type.

Breeding for nutritional quality –

• A crop product should provide the optimum nutrition to human and does not contain any anti-nutritional factor.

Anti-nutritional factors – Compounds that have harmful effects on animal's/human's growth & development.
Eg. Glucosinolates (present in oils and cakes of rapeseed and musturd), β–N oxalyl aminoalanine (BOAA) neurotoxin found in khesari dal (Lathyrus sativus).

• Cereals & millet proteins are deficient in lysine and tryptophan amino acid. (eg. Maize)

• Pulses are deficient in sulphur containing amino acid (i.e. cysteine & methionine).

• Three varieties of maize have been developed in India which are lysine–rich. Shakti, Rattan, Protina.

• Breeding is also useful to develop disease resistance in plants.

(1) Breeding for disease resistance:

(2) Plant breeding for developing resistance to insect pests –

(B) Selection –

• This is the most primitive and simplest method for crop improvement.

• This is practised in crop improvement and it is selection of phenotypically superior plants from a mixed population.

• Selection acts on genetic variation, present in a population and produces a new population with improved characters.

Selection is of 3 types – (a) Pure line selection (b) Mass selection (c) Clonal selection

(a) Pure line selection :- The progeny of a single self pollinated homozygous plant is called pure line (term by Johanson) and these pure lines are used in selection method of crop improvement, which is called pure line selection

• Pure line selection is method of improvement in self pollinated plants (wheat) In this method a number of phenotypically superior plants are selected, these are harvested separately and their produce is maintained separately.

• The seeds so obtained from different plants are sown separately and selection is made for 4 or 5 generations till the desired improvement is achieved. About 10 years time is needed to develop a new variety by this method

(b) Mass selection :-

• This is practised in self & cross pollinated crops plants but more useful for self pollinated crops.

• The first step involves selecting plants having desirable character from a given population of plants based on phenotypic characters.

• The seeds of selected plants are then mixed and sown the same field (mixed cropping) to allow natural cross pollination.

• The plants are selected from this field by eliminating the undesirable ones and saving the best.

• It is done for 5 to 6 generations or more then desired improvement can be achieved.

• It takes about 8 yrs. time to develop a new variety by mass selection.

(c) Clonal selection :-

• This type of selection is applicable to vegetatively propagated plants eg. sugarcane, banana, potato.

• Clone :- Progeny of a single vegetatively propagated plant is called clone.

• Here selection is made between the clones and not within the clone Limitation of clonal selection :- (1) Only applicable for vegetatively propagated crops (2) Creates no new variation

(C) Evaluation & Release of varieties –

• A newly developed pure line, improved population or hybrid undergoes critical evaluation for yield, quality, disease and insect resistance and for other traits. In India, Indian Council of Agricultural Research, New Delhi (ICAR – New Delhi) carries out the evaluations.

• Ultimately a new pure line, population or hybrid that is superior to the existing varieties as well as to other new material may be released as new variety.