VARIETY DEGENERATION Flashcards
VARIETY DEGENERATION
The genetic purity of a variety or trueness to its type deteriorates due to several factors during the production cycles
The following factors are responsible for deterioration of varieties:
Developmental variations
Mechanical mixtures
Mutations
Natural crossing
Minor genetic variations
Selected influence of pest and diseases
minor genetic variation
The technique of the plant breeder
Developmental Variations
Developmental Variations
When seed crops are grown under environments with differing soil, fertility, climate, photoperiods, or at different elevations for several generations, developmental variations may occur as differential growth responses
It is, therefore, preferred to grow the varieties of crops in the areas of their natural adaptation to minimize developmental shifts
Mechanical Mixtures
Mechanical mixtures, the most important reason for varietal deterioration, often take place at the time of sowing, through volunteer plants of the same crop, or through different varieties grown in adjacent fields
Two varieties growing next to each other field may be mixed during harvesting and threshing operations
The threshing equipment is often contaminated with seeds of other varieties
Similarly, gunny bags and seed bins also often contaminate, adding to the mechanical mixtures of varieties
Utmost care during seed production and processing are necessary to avoid such mechanical contamination.
Mutations
Mutations occur naturally but their effects take time to show
Mutations may not seriously deteriorate varieties
It is often difficult to identify or detect minor mutations occurring naturally.
Natural Crossing
Natural crossing can be an important source of varietal deterioration in sexually propagated crops
The extent of contamination depends upon the magnitude of natural cross-pollination
The deterioration sets in due to natural crossing with undesirable types, diseased plants, or off types
In self-pollinated crops, natural crossing is not a serious source of contamination
In cross-pollinated crops, it is a serious problem and necessary steps to avoid crossing should be taken
Selected Influence of Pest and Diseases
New crop varieties often are susceptible to newer races of pests and diseases and thus selectively influence deterioration
The vegetatively propagated stock also can deteriorate quickly if infected by virus, fungi or bacteria
Seed production under strict disease free conditions is therefore essential.
VARIETY DEGENERATION
The Techniques of the Plant Breeder
Serious instabilities may occur in varieties owing to improper assessments prior to the release of new varieties
Premature release of varieties, still segregating for resistance and susceptibility to diseases or other factors can cause significant deterioration of varieties
This failure is attributed to the variety-testing program.
MAINTAINING VARIETAL PURITY
Adopt following measures to safeguard genetic purity:
Adoption of Crop: Growing crops only in areas of their adaptation to avoid genetic shifts.
Approved Class of Seeds: Use of only approved class of seed in seed multiplication.
Isolation: Isolation of seed crops from various sources of contamination by natural crossing or mechanical mixtures.
Roguing: Roguing of off types differing in characteristics from those of the seed variety.
Field Inspection: by qualified personnel of seed certification agency at all appropriate stages of growth.
Periodic testing of varieties for genetic purity.
MAINTENANCE BREEDING
A branch/area of plant breeding which deals with principles and methods of breeder seed production
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.
methods of genetic purity
In crop plants various breeding methods or approaches are used for maintaining the genetic purity of varieties. These are:
(i) Positive mass selection
(ii) Negative mass selection
(iii) Model bulk selection
(iv) Plant to row selection
Positive Mass Selection
Positive Mass Selection
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.
Negative Mass Selection
Negative Mass Selection
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 widely used for maintaining varietal purity in various field crops
This is a simple method where selection is based on phenotype.
Model Bulk Selection
Model Bulk Selection
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 various parameters
Those plants which conform to the mean of a variety for the parameters are selected and their seed is bulked to grow new generation, and the rest of the plants are discarded.
This method of maintenance breeding is based on progeny test (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 known traits 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 rest are discarded.
