Genetic Enhancement of Crop Plants | Crops | Plant Breeding | Botany

Everything you need to know about genetic enhancement of crop plants !

Q.1. Define genetic enhancement and give its main features.

Ans. The term genetic enhancement was first used by Jones in 1983. It refers to transfer of useful genes from exotic or wild types into agronomically acceptable background. In 1984, Rick used the term pre-breeding to describe the same activity. Now terms genetic enhancement and pre-breeding are used as synonyms and interchangeable. However, the term genetic enhance is in more usage.

Main points related to pre-breeding/genetic enhancement are briefly discussed below:

i. Main Purpose:

Genetic enhancement is carried out for broadening the genetic base of the population. It increases genetic diversity in the population. In other words, it helps in developing population with more genetic diversity.

ii. Main Steps:

GE consists of three main steps, viz.:

(a) Identifying a useful character

(b) Capturing its genetic diversity, and

(c) Putting those genes into usable form.

iii. Base Material:

For launching genetic enhancement programme, three types of material viz. agronomic base, exotic germplasm and wild species are required. The former is used as the recipient parent and last two as donor parents.

iv. Breeding Methods used:

The backcross method is widely used for genetic enhancement, which helps in transferring useful genes from exotic germplasm and wild species into acceptable agronomic base. Now biotechnological approaches are also used for genetic enhancement.

v. Impact:

The major impact of genetic enhancement is value addition in the germplasm. In other words it leads to genetic improvement of germplasm for various economic characters. It also leads to creation of new variability. It improves adaptation of the population.

vi. End product:

The end products of genetic enhancement programme are improved germplasm lines which can be used as parents for developing productive cultivars/hybrids in traditional breeding.

vii. Time Frame:

The genetic enhancement is a long term programme as compared to traditional plant breeding or sustainable plant breeding.

viii. Undertaken by:

The genetic enhancement work is undertaken by public sector plant breeding organizations. It is seldom undertaken by private sector plant breeding organizations.

ix. Phase of Breeding:

Pre-breeding is the early component of sustainable; plant breeding which refers to the transfer or introgression of useful genes and gene combinations from non-adapted sources into breeding material (FAO, 1996). The non-adapted material mainly, consists of exotic germplasm and wild species/relatives of the crop plants. It is considered as prior step of sustainable plant breeding.

x. Adaptation:

Pre-breeding helps in improving the genetic adaptation of the population in two ways. Firstly by creating genetic diversity and secondly by broadening the genetic base of the population.

Q.2. What are base materials used for genetic enhancement?

Ans. The breeding material that is required for launching genetic enhancement programme consists of an adapted cultivar, exotic germplasm, land races and wild species.

Q.3. Explain briefly main features of adapted cultivars.

Ans. Commercially cultivated varieties are referred to as adapted cultivars. Such cultivars are also known as popular varieties or good agronomic bases. Adapted cultivars are of two types, viz. obsolete varieties and modern cultivars. Improved varieties of recent past are known as absolute cultivars and currently cultivated high yielding varieties are known as modern cultivars.

The main features of adapted cultivars are given below:

(i) They are successful under specific environmental conditions.

(ii) They are also known as popular varieties or good agronomic bases.

(iii) They are high yielding and uniform.

(iv) They have narrow genetic base due to less genetic diversity.

(v) They have local or narrow adaptation.

(vi) They have danger of uniformity due to narrow genetic base.

(vii) They possess most of the desirable traits and are deficient in one or two characters.

(viii) They are generally used as female parents or recurrent parents in pre- breeding programmes.

Q.4. Give main points related to exotic germplasm.

Ans. The germplasm which is collected from other countries is known as exotic germplasm. Such germplasms do not have immediate use without selection for adaptation in a given environment. Exotic germplasm has to undergo conversion before use in plant breeding.

Main points about exotic germplasm are given below:

(i) Exotic germplasm is also known as un-adapted material which possesses several inferior alleles in addition to some desirable alleles.

(ii) Use of such material may lead to introduction of inferior alleles into adapted genotypes. Several backcrosses will be required to eliminate undesirable alleles.

(iii) Sometimes, there is linkage between desirable and undesirable alleles which restricts the use of exotic germplasm in breeding programmes.

(iv) It may negatively affect adaptedness when introduced into locally adapted genetic.

(v) It may lead to disruption of co-adapted gene complexes in the elite breeding material.

(vi) Exotic germplasm is less productive and has poor adaptation.

(vii) It cannot be used as such in traditional breeding. It requires some genetic modification or enhancement before use in traditional breeding.

(viii) In Pre-breeding exotic germplasm is generally used as male parents or donor parent,

Q.5. What do you mean by landraces? Give their main features.

Ans. Land races refer to primitive crop cultivars which were selected and cultivated by the farmers for many generators.

Main features of land races are given below:

(i) Land races evolved under subsistence agriculture. They were not deliberately bred like modern cultivars.

(ii) Land races have high level of genetic diversity which provides them high degree of resistance to biotic and abiotic stresses.

(iii) Land races have broad genetic base which provides them wider adaptability.

(iv) Main drawbacks of land races are that they are less uniform and low yielders.

Q.6. Explain main features of wild species.

Ans. Wild species refer to wild forms of cultivated crop plants. Wild species have high degree of resistance to biotic and abiotic stresses.

Main features of will given below:

(i) They are wild relatives of cultivated plants.

(ii) They have common ancestry with crop plants.

(iii) They can cross with crop plants.

(iv) They are perenials, less adapted and poor yielders,

(v) When crossed with cultivated species, three types of problems viz. hybrid sterility, hybrid in-viability and transfer of undesirable genes; are faced. Because of these problems, wild species are being used to lesser extent in pre-breeding.

Q.7. What are methods of genetic enhancement?

Ans. Breeding methods used for genetic enhancement are slightly different from those which are used in traditional breeding. The breeding methods/procedures which are commonly used for genetic enhancement include back cross, convergent improvement and marker assisted backcross.

Q.8. Explain the role of backcross method in genetic enhancement.

Ans. Backcross refers to crossing of F₁ with either of its parents. Backcross is widely used for introgression and incorporation of desirable genes from exotic germplasm and wild species into well adapted cultivars. Introgression refers to transfer of one or a few alleles from exotic stocks to adapted breeding populations; Incorporation refers to transfer of alleles from exotic germplasm to develop material with broad genetic base and adaptation to local conditions.

Main points about backcross method are given below:

(i) It is used for genetic enhancement of both self and cross pollinated species.

(ii) It is used for transfer of both oligogenic and polygenic traits. However, it is more effective with oligogenic traits than polygenic characters.

(iii) It is more successful when the character under transfer has high heritability.

(iv) In this method, cultivated variety or adapted variety is used as maternal parent and the exotic germplasm or un-adapted line as donor parent. The recipient parent is also known as recurrent parent because repeated backcrossing is done with recipient parent.

(v) Generally 5 to 6 backcrosses are sufficient for the transfer of a character.

(vi) Continuous backcrosses are made when the character is governed by dominant gene. Each backcross is followed by one selling when the character is governed by recessive gene.

(vii) The new line differs from the parental line only for the character under transfer. In other words, the end product resembles the parent variety in all the characters except for the character under transfer.

(viii) Various modifications of backcross method have been suggested by different workers to achieve specific objectives. To get intermediate expressions alternate backcross can be made with recurrent and donor parents. To obtain several populations of common genetic backgrounds, 2-3 backcrosses are made and then selfing is done for several generations.

Q.9. How convergent Improvement is useful in genetic enhancement.

Ans. In this method two genotypes soy A and B are selected for crossing. The crossing is done between these genotypes and the F₁ is backcrossed to both A and B parents. The main objective of this scheme is to improve both the lines simultaneously. The desirable character of A is transferred to B and vice versa. This method is used when each of the two parents is deficient in one character which is present in the other parent.

Main features of this method are given below:

(i) Each parent is deficient in a character which is present in the other parent. In other words, deficiency of one parent is rectified from the allele present in the other parent.

(ii) It simultaneously improves two populations.

(iii) The progress is rapid when the heritability of both the characters is high,

(iv) It can be used for both self and cross pollinated species.

Q.10. Describe the role of Marker Assisted backcrossing in genetic enhancement.

Ans. Indirect selection for a desired plant phenotype on the basis of banding patterns of linked DNA (molecular) markers is known as marker assisted selection and improvement of crop plants using such markers is called molecular breeding.

Main features of marker assisted selection (MAS) are briefly presented below:

(i) MAS are applicable to both self and cross pollinated species.

(ii) Molecular markers that are commonly used in plant breeding include RFLP, AFLP, RAPD, SSR etc.

(iii) Molecular markers have very high accuracy.

(iv) MAS permit identification of recessive alleles even in heterozygous condition and thus speeds up the progress of crop improvement work.

(v) DNA markers are free from environmental effects.

(vi) DNA markers permit tagging of quantitative trait loci (QTL).

(vii) MAS require well trained manpower for handling of equipment’s, isolation of DNA molecule and study of DNA markers.

(viii) It requires sophisticated and well equipped laboratory to initiate work on marker assisted selection.

(ix) This is very expensive method of genetic enhancement.

In backcross breeding method, use of DNA markers helps in identification of genotypes having desired allele. Thus it helps in speeding up the progress of gene transfer through backcross method. The DNA markers that are linked to the desired phenotypic trait are used for selection.

Q.11. What are types of gene pool used for genetic enhancement?

Ans. There are various types of gene-pool that are used for genetic enhancement. Each type of gene-pool has its own significance and limitations. Four types of gene-pool have been identified so far. Their brief description is presented in Table 9.2.

The first three types of gene-pool are found in the nature while the fourth type (GP4) is created by biotechnological approaches. The primary gene-pool is used for transfer of desirable alleles and broadening the genetic base of breeding populations. The secondary and tertiary gene-pools are used for creating vast genetic variability for various oligogenic and polygenic traits.

Q.12. What are advantages of genetic enhancement?

Ans. In the past, more emphasis has been laid in developing productive cultivars and hybrids. This has resulted in narrowing down of genetic base in some crop plants. The improved cultivars are productive and uniform but have narrow genetic base and narrow adaptation. The land races on the other hand are less productive and less uniform but have broad genetic base and wide adaptation. The exotic germplasm is not of much use without selection for local adaptation and enhanced yield potential.

The genetic enhancement is essential because of following reasons:

i. It helps in broadening the genetic base of the population which is essential for achieving stability in yield over regions and seasons.

ii. The genetic diversity has depleted in the improved cultivars which invites danger of uniformity. Genetic enhancement is useful in restoring genetic diversity in such cultivars.

iii. It helps in combining useful genes or gene combination from landraces, perennials and wild species into the cultivated or well adapted genotypes. Such traits include resistance to biotic and abiotic stresses, earliness and improvement in quality parameters.

iv. It helps in developing plant types which are suitable for machine harvesting.

v. It also leads to creation of new genetic variability in various economic traits, Thus it leads to value addition in the germplasm.

vi. The germplasm lines developed through genetic enhancement become usable in traditional breeding programmes for development of productive cultivars/hybrids.

Q.13. What practical achievements of genetic advancement?

Ans. The genetic enhancement or pre-breeding work has been carried out in almost all the important field crops and remarkable results have been achieved.

Some examples are cited below:

i. Maize:

In maize new inbred lines have been developed through pre- breeding which are used as parents for development of productive single cross hybrids. These single cross hybrids are superior to double cross hybrids. The heterotic pool has been developed.

ii. Sorghum:

In sorghum, desirable traits have been introgressed from Ethiopian and Sudanense land races into adapted Indian cultivars,

iii. Sugarcane:

In sugarcane, pre-breeding has resulted in development of interspecific genetic stocks of S. spontaneum, S. sinense and S. offcinarum.

iv. Barley:

In barley, pre-breeding has resulted in creating vast genetic diversity and broadening the genetic base of breeding populations.

v. Potato:

In potato, day long tretraploid populations have been developed through introgressive hybridization. The new germplasm is being used widely in potato Improvement. In potato, genetic enhancement has been achieved mainly through polyploidy breeding.

The genetic enhancement has also been achieved in other crops such as oil seeds, lentil, tomato, chick pea, ground nut, cassava etc. This work is in progress at various International and National Crop breeding centres. Various plant characters such as yield, quality parameters, resistance to biotic and abiotic, stresses, adaptation, genetic base, crop maturity, plant type, photo and thermo insensitivity, etc. are kept in mind during pre-breeding programmes.

v. Cotton:

In cotton, fibre quality parameters and resistance to biotic and abiotic stresses have been introgressed from wild species.

Q.14. What are limitations of genetic enhancement?

Ans. There are several problems that are associated with genetic enhancement programmes particularly when genes are introgressed from wild species.

Some problems are listed below:

1. In the Introgressive hybridization using wild species, there are problems of cross incompatibility, hybrid in-viability and hybrid sterility.

2. Linkage between desirable and undesirable alleles poses problems in utilization of desirable alleles.

3. Generally, the genetic re-combinations are restricted in introgressive breeding.

4. Small populations are available due to poor seed setting in interspecific crosses.