Everything you need to know about maintenance breeding !
Q.1. What is maintenance breeding?
Ans. A branch/area of plant breeding which deals with principles and methods of breeder seed production is referred to as maintenance breeding. In other words, it deals with ways and means of maintaining genetic and physical purity-of released and notified varieties and parents of hybrid. It is also referred to as varietal maintenance technology.
Q.2. What are main features of maintenance breeding?
Ans. Thus main points related to maintenance breeding are listed below:
1. It deals with principles and methods of pure seed production.
2. It deals with continuous breeder seed production of released and notified varieties, In other words, it leads to fresh breeder seed production every year,
3. It also undertakes breeder seed production of the parental lines of released hybrids.
4. Genetic purity, physical purity and germination are the main points taken into account.
5. The seed health is also taken into account.
6. The breeder and foundation seed is used as base material for starting maintenance breeding programme.
7. The main objective of maintenance breeding is to enhance the life span of released and notified varieties and commercial hybrids,
8. Its entire focus is on the planting seed quality of a crop species rather than milling seed quality,
9. It prevents varietal deterioration
Q.3. What are genetic causes of varietal deterioration?
Ans. Genetic causes include:
(i) Cross pollination,
(ii) Mutations,
(iii) Residual variability, and
(iv) Adaptive changes.
These are briefly discussed below:
(i) Natural cross pollination or outcrossing with other genotypes leads to genetic deterioration of a variety in subsequent cycles of seed multiplication.
(ii) Spontaneous mutations after crossing with the original variety lead to its deterioration in subsequent years.
(iii) Sometimes residual variability remains in a variety at the time of its release. Such variability leads to deterioration of a variety in few years,
(iv) Sometimes shift in gene frequency either due to natural selection or artificial selection leads to varietal deterioration.
Q.4. What are non-genetic causes of varietal deterioration?
Ans. Non-genetic causes of varietal deterioration include:
(i) Mechanical mixture,
(ii) Disease infestation, and
(iii) Adverse agroclimatic conditions.
These are briefly discussed below:
(i) Mixing of different varieties may take place during thrashing, ginning, sowing with seed drills. Planting of such mixed seed leads to varietal deterioration due to multiplication and natural outcrossing.
(ii) Development of new virulent races of a pathogen may lead to deterioration of a variety. The variety may become susceptible to such races and deteriorate.
(iii) Adverse agro-climatic conditions such as flood, drought, soil salinity etc. may lead to deterioration of a variety.
Q.5. What are important activities of plant breeding?
Ans. Plant breeding has three important activities, viz.:
(i) Varietal development,
(ii) Seed multiplication, and
(iii) Varietal maintenance.
These are defined below:
(i) Varietal Development:
It makes use of various plant breeding methods (selection, hybridization, mutation and genetic engineering) to develop new plant varieties.
(ii) Seed Multiplication:
It deals with multiplication/production of successive generations of various categories of seeds such as nucleus seed, breeder seed, foundation seed, registered seed and certified seed. The seed multiplication/increase is regulated by seed certification agencies and breeders.
(iii) Varietal Maintenance:
It deals with continuous fresh supply (annually) of breeder seed which used to start another cycle of seed multiplication. It makes use of various selection/maintenance procedures.
Q.6. What are types of maintenance procedures?
Ans. Maintenance procedures are of two types, viz.:
(i) Selection procedures and
(ii) Non-selection procedure.
The second approach deals with maintenance of breeder seed in cold storage.
Merits and demerits of these methods are given below:
1. Selection Procedures:
(i) It involves continuous extensive field testing.
(ii) The material is exposed to outcrossing, spontaneous mutations and change in gene frequencies.
2. Storage Methods:
(i) Field testing is not required.
(ii) The seed material is not exposed to outcrossing, mutations, diseases, insects, mechanical mixtures, change in gene frequency caused due to natural selection and selection exerted by breeders.
Q.7. What breeding approaches are used for varietal maintenance?
Ans. In crop plants various breeding methods/approaches are used for maintaining the genetic purity of cultivars and hybrids.
In crop plants four methods, viz.:
(i) Positive mass selection,
(ii) Negative mass selection,
(iii) Model bulk selection, and
(iv) Plant to row selection used for maintaining varietal purity.
Q.8. Describe the role of positive mass selection in varietal maintenance?
Ans. Selection of desirable plants (true to type) from the field of a variety and mixing the seed of individual selected plants to grow next generation is referred to as positive mass selection. This is also known as bulk of large number of true to type plants. This method is widely used for varietal purification in various field crops. In cotton, this method was first used by Menbane in Texas, USA. Now this method is widely used for maintenance of varietal purity in cotton.
Merits and demerits of this method are listed below:
Merits:
(i) It is a simple method of varietal maintenance.
(ii) It is used for purification of improved varieties.
(iii) It can also be used for purification of parental lines of hybrids.
(iv) It retains diversity in the variety.
Demerits:
(i) Selection is based on phenotype which may not always be an indication of superior genotypes.
(ii) The progeny test is not carried out.
Q.9. What is the role of Negative Mass Selection in varietal maintenance?
Ans. When only undesirable off type plants are removed from the field of a variety and rest are allowed to grow further, it is known as negative mass selection. It is also known as roguing of off type plants. It is widely used for maintaining varietal purity in various field crops. In cotton, it has been found to be a satisfactory maintenance procedure. This is a simple method. Selection is based on phenotype and progeny test is not conducted.
The main advantages of this method are presented below:
Merits:
1. It is a simple method of varietal maintenance and is widely used for maintenance of varietal purity in different crops.
2. The maintenance procedure is easy and requires little skill.
3. The number of discarded plants are very few in proportion to plants retained.
4. It can be used even for maintenance of inbred lines used for developing hybrids.
5. The genetic diversity of the variety is not affected.
6. It is used both in self pollinated and cross pollinated species.
Demerits:
1. The progeny test is not conducted.
2. Sometimes, off type plants may escape which may lead to contamination of the variety in later cycles of seed multiplication.
3. It requires careful examination of off type plants.
Q. 10. Give comparison of positive and negative mass selections.
Ans. A Brief comparison of positive mass selection and Negative mass selection is presented below:
Q.11. Describe the role of Model Bulk Selection in maintenance breeding.
Ans. This method was first used by Manning in 1955, According to this method large number of single plants (300-500) which are productive and true to the type are selected in a variety. Then these plants are examined under laboratory conditions for three parameters, viz. fibre length, lint per seed and seed weight.
Some additional characters which were not used by Manning (1955) such as lint index and ginning out turn can also be included. Those plants which conform to the mean of a variety for above three parameters are selected and their seed is bulked to grow new generation, rests of the plants are discarded. This method is widely used for maintenance breeding.
Merits and demerits of this method are given below:
Merits:
(i) This is a simple method of varietal purification.
(ii) This is more scientific than earlier two methods.
(iii) It is based on both field and laboratory observations.
(iv) It used in cotton and some other crops.
Demerits:
(i) Large population has to be grown for selection.
(ii) Selection is based on phenotype.
(iii) It requires more labour, space and time.
Q.12. Describe the role of Plants to Row method in maintenance breeding.
Ans. This method of maintenance breeding is based on progeny test. It is also known as progeny row selection. In this method large numbers of single plants (500-1000) true to the type are selected from the parent variety. The selected plants are screened for fibre length and lint percentage in the laboratory. Plants conforming to the norms of a variety are selected and rests are discarded. In the next crop season, progeny rows of selected plants are grown. Those progenies which fulfill the norms of the variety are bulked and rests are discarded.
Several modifications of this scheme have been suggested by various researchers as follows:
1. Some researchers are of the opinion that progeny test should be conducted atleast for one year to assess the performance of the variety.
2. Others are of the opinion that one year progeny test is not sufficient. Progeny test should be conducted for two, three or more seasons. When several years progeny testing is carried out, it is referred to as a pedigree system or pedigree method of selection.
The merits and demerits of plant to row method are given below:
Merits:
(i) The selection is based on breeding value i.e. progeny performance.
(ii) It is more scientific method than earlier three methods.
(iii) It involves both field and laboratory evaluation.
(iv) Progeny tests are based on large samples which provide more precession.
Demerits:
(i) It is more laborious and time consuming method.
(ii) It requires more experimental area for evaluation and testing.
(iii) It requires two or more crop seasons for testing.
Q.13. What are advantages of maintenance breeding?
Ans. There are several advantages of maintenance breeding which are briefly presented below:
1. It helps in purification of improved cultivars and parental lines of hybrids.
2. It prevents cultivars from genetic deterioration and thus prolongs life of a variety.
3. It is useful in studying the efficacy of various maintenance procedures.
4. It helps in quality seed production which in turn leads to higher crop yields.
Q.14. What are the limitations of maintenance breeding?
Ans. There are some limitations of maintenance breeding which are briefly presented below:
1. Some of the maintenance procedures require lots of experimental material for evaluation.
2. Large number of single plants has to be evaluated in terms of agronomic performance and quality parameters. Hence only limited number of cultivars can be handled at a time.
3. Maintenance procedures are used for varietal purification. Hence chances of evolving new varieties through such procedures are rare.
4. Progeny row method require more time (2-3 crop seasons) for evaluation of purity of a variety.
5. Most of the testing procedures are based on phenotypic performance only.
6. In positive mass selection major portion of the plants are rejected which adds to the expenditure.