Hybridization | Additional Study Material for NEET PDF Download

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. It is performed very often to combine character from two different plants.

Types of Hybridization :

Hybridization is divided in following categories.

(i)   Intravarietal hybridization : The crosses between the plant of same variety.

eg., Triticum aestivum var. kalyansona  × T. aestivum var. K. sona

(ii)  Intervarietal hybridization : The crosses are made between the plant belonging to two different varieties of same species (also known as intraspecific hybridization).

eg., Triticum aestivum var. kalyansona  × T. aestivum var. sonalika

(iii) Interspecific hybridization : The plants or two different species belonging to the same genus are crossed together

eg., Solenum tuberosum × S. acuale

(iv) Intergeneric hybridization : The crosses are made between the plant belonging to two different genera. e.g. Triticale, Raphanobrassica are the example of intergeneric cross.

Development of Rabbage :

Rabbage or Raphano brassica was developed by Karpechanko

• 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.

The main steps of hybridization are following-

(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.

Two 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 ancestry 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. Thus productivity is  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 up on number of chromosomal sets, the individual are given different names-monoploid, diploid, triploids, 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.
• Polyploidy can be induced artificially by Colchicine treatment. 
• Colchicine is an alkaloid obtained from Colchicum automanale (fam. Liliaceae) plant from Bulb and Seeds.
• In some plants triploids are having much vigour and increased fruit size e.g. apple & pear.

(6)  Mutational breeding :- 

Mutation – Sudden and inheritable change in genetic material of an organism.

Mutagens – The agents, which are used to induced mutation.

Chemical mutagen – Ethyl methane sulphonate (EMS), sodium azide.

Physical mutagen – X-rays, Gamma rays, UV rays

Induced mutation – Mutation which are induced artificially with the help of mutagens.

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 roja.

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 : Remei & Atomita-2 are developed through mutation breeding.

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 useless 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), b-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.

Biofortification :- 

Breeding crops with higher levels of vitamins and minerals or higher proteins and healthier fats is known as biofortification. Breeding for improved nutritional quality is undertaken with the objective of improving.

1.   Protein content and quality

2.   Oil content and quality

3.   Vitamin content

4.   Micronutrient and mineral content

Maize hybrid with twice the amount of amino acid, lysine and tryptophan was developed in 2000. High protein content variety Atlas 66 has been used as a donor for improving cultivated wheat. It has been possible to develop an iron fortified variety containing over five times as much iron as in commonly consumed varieties.

The IARI New Delhi has also released several vegetable crop that are rich in vitamins and minerals eg., vitamin A enriched carrot, spinach, pumpkin, Vitamin C enriched bitter gourd, bathua, mustard tomato, iron and calcium enriched spinach and bathua and protein enriched beans, lablab and garden peas. 

Breeding for disease resistance :

Methods of breeding for disease resistance : Breeding is carried out by conventional hybridization method and by mutation breeding.

Conventional breeding is often constrained by the availability of limited number of disease resistance genes that are present and identified in various crop varieties or wild relatives. Thus mutation breeding technique is used.

Yellow mosaic virus resistant and powdery mildew resistant moong varieties are developed through mutation breeding.

Yellow mosaic virus resistant variety of Okra, Parbhani Kranti was developed by gene transfer from wild variety.

Plant breeding for developing resistance to insect pests : 

Another major cause for large scale destruction of crop plant and crop produce is insect and pest infestation.

Insect resistance in most crop plants may be due to morphological, biochemical or physiological characteristics.

Hairy leaves in several plants are associated with resistance to insect pests, e.g, resistance to jassids in cotton and cereal leaf beetle in wheat.

In wheat, solid stems lead to non-preference by the stem sawfly and smooth leaved and nectar-less cotton varieties do not attract bollworms.

High aspartic acid, low nitrogen and sugar content in maize leads to resistance to maize stem borers.

Selection : 

This is the most primitive method for crop improvement.

This is practiced in crop improvement and it is selection of pheno typically superior plants from a mixed population. This step yields plants that are superior to both of the parents.

Selection acts on genetic variation, present in a population and produces a new population with improved characters. Selection is of three 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 a method of crop improvement in self pollinated crops (wheat).

In this method phenotypically superior plants are selected, these are harvested separately and their produce is maintained separately.

The seeds so obtained from different plants are shown separately and selection is made for 4 to 5 generation 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 practiced in cross pollinated crop plants.
• 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 in same field 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 3 to 4 generations or more then desired phenotypically similarity 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 vagetatively propagated plant is called clone.
• Here selection is made between different plant clones and not within the same plant clone.
• Kufri Red in potato and green banana of Bombay varieties are produced through clonal selection.

Limitation of clonal selection :- 

(1)  Only applicable for vegetatively propagated crops.

(2)  Creates no new variation

Single Cell Protein : Conventional agricultural production of cereals, pulses, vegetables, fruits, etc., may not be able to meet the demand of food at the rate at which human and animal population is increasing. The shift from grain to meat diets also creates more demand for cereals as it takes 3-10 Kg of grain to produce 1 Kg of meat by animal farming. Can you explain this statement in the light of your knowledge of food chains? More than 25 per cent of human population is suffering from hunger and malnutrition. One of the alternate sources of proteins for animal and human nutrition is Single Cell Protein (SCP).

Microbes are being grown on an industrial scale as source of good protein. Microbes like Spirulina can be grown easily on materials like waste water from potato processing plants (containing starch), straw, molasses, animal manure and even sewage, to produce large quantities and can serve as food rich in protein, minerals, fats, carbohydrate and vitamins. Incidentally such utilization also reduces environmental pollution.

It has been calculated that a 250 Kg cow produces 200 g of protein per day. In the same period, 250g of a micro-organism like Methylophilus methylotrophus, because of its high rate of biomass production and growth, can be expected to produce 25 tonnes of protein. The fact that mushrooms are eaten by many people and large scale mushroom culture is a growing industry makes it believable that microbes too would become acceptable as food.