GERMPLASM Flashcards
Germplasm
Germplasm is a collective term for seeds, planting materials, or any living tissue of a plant that can regenerate to produce a plant
In other words, Germplasm is living tissue from which new plants can be grown
It can be a seed or plant part – a leaf, a piece of stem, pollen or even just a few cells that can be turned into a whole plant
Germplasm contains the information for a species’ genetic makeup, a valuable natural resource of plant diversity.
Germplasm is important for the maintenance of biological diversity and food security
Germplasm is basis for plant breeding without which breeding is impossible to conduct
It is the genetic material that can be used to perpetuate a species or population
Germplasm provides the materials (parents) used to initiate a breeding program
Sometimes, all plant breeders do is to evaluate plant germplasm and make a selection from existing biological variation.
Sources of germplasm for plant breeding
Domesticated plants-Domesticated plants are those plant materials that have been subject to some form of human selection and are grown for food or other uses
There are various types of such materials such as released varieties, traditional varieties, introduced germplasm, breeding stock and breeding materials.
Commercial varieties-There are two forms of this material: released (notified) cultivars and obsolete (denotified) cultivars
These are products of formal plant breeding for specific objectives
HYVs have superior gene combinations, adapted to a growing area, and have good performance
The denotified cultivars are taken out of commercial production because they may have suffered a set back (e.g., susceptible to disease) or higher performing cultivars were developed to replace them.
Breeding materials-Established breeding programs maintain variability from previous projects
A breeder may release one genotype as a commercial cultivar after yield tests
Many of the genotypes that made it to the final stage or have unique traits will be retained as breeding materials to be considered in future projects
Similarly, genotypes with unique combinations may be retained.
Landraces-
Landraces are farmer-developed and maintained cultivars
They are developed over very long periods of time and have co-adapted gene complexes
They are adapted to the growing region and are often highly heterogeneous
Landraces are robust, having developed resistance to the environmental stresses in their areas of adaptation
They are adapted to unfavorable conditions and produce low but relatively stable performance.
Plant introductions-
The plant breeder may import new, unadapted genotypes from outside the production region, usually from another country (called plant introductions)
These new materials may be evaluated and adapted to new production regions as new cultivars, or used as parents for crossing in breeding projects.
Genetic stock-This consists of products of specialized genetic manipulations by researchers (e.g., by using mutagenesis to generate various chromosomal and genomic mutants)
Genetic resources are sometimes called the “first resource” of the natural resources on this planet - the others being land, air, and water
Therefore, attention to genetic resources means attention to the vast diversity among and between species of animals, plants, and micro-organisms.
Undomesticated plants(wild relatives) Other species and genera-When desired genes are not found in domesticated cultivars, plant breeders may seek them from wild populations Wild relatives may possess traits that have an advantage for survival in the wild (e.g., disease resistance, drought resistance etc) which are then incorporated in cultivated varieties Wild germplasm have been used as donors of several important disease and insect resistance genes and genes for adaptation to stressful environments.
Other species and genera
Gene transfer via crossing requires that the parents be cross-compatible or cross-fertile
Crossing involving parents from within a species is usually successful and unproblematic
However, as the parents become more genetically divergent, crossing is less successful, often requiring special techniques (e.g., embryo rescue
Modern biotechnology tools allow gene transfer between unrelated species today.
Concept of gene pools of cultivated crops
J. R. Harlan and J. M. J de Wet proposed a categorization of gene pools of cultivated crops according to the feasibility of gene transfer or gene flow from those species to the crop species
Three categories are defined:
Primary genepool
Secondary genepool
Tertiary gene pool
Primary gene pool (GP1)
Secondary gene pool (GP2)
Tertiary gene pool (GP3)
Primary gene pool (GP1)
GP1 consists of biological species that can be intercrossed easily without any problems with fertility of the progeny
That is, there is no restriction to gene exchange between members of the group
This group may contain both cultivated and wild progenitors of the species.
Secondary gene pool (GP2)
Members of this gene pool include both cultivated and wild relatives of the crop species
They are more distantly related and have crossability problems
Nonetheless, crossing produces hybrids and derivatives that are sufficiently fertile to allow gene flow
GP2 species can cross with those in GP1, with some fertility of the F1.
Tertiary gene pool (GP3)
Gene transfer by hybridization between GP1 and GP3 is very problematic resulting in lethality, sterility, and other abnormalities
To exploit germplasm from distant relatives, tools such as embryo rescue and **bridge crossing **may be used to obtain fertile plants
However, gene transfer techniques enable breeders to transfer genes beyond the tertiary gene pool.
Conservation of plant genetic resources
Plant breeders manipulate variability in various ways – they assemble, recombine, select, and discard
The preferential use of certain genetic stock in breeding programs has narrowed the overall genetic base of modern cultivars
Such a narrow genetic base makes agriculture vulnerable to disasters (e.g., disease epidemics, climate changes)
Hence, national and international efforts are required to conserve plant genetic resources.
Why conserve plant genetic resources?
Plant germplasm is exploited for food, fiber, feed, fuel, and medicines by agriculture, industry, and forestry
As a natural resource, germplasm is a depletable resource
Without genetic diversity, plant breeding and crop improvement is not possible
Genetic diversity determines the boundaries of crop productivity and survival
Genetic erosion is occurring and it needs to be minimized.
Genetic erosion
Genetic erosion may be defined as the decline in genetic variation in cultivated or natural populations largely through the action of humans
Loss of genetic variation may be caused by natural factors, and by the actions of farmers, plant breeders, curators of germplasm repositories, and others in society at large
Natural factors: Genetic diversity can be lost through **natural disasters **such as floods, wild fires, and severe and prolonged drought. These events are beyond the control of humans.
**Action of farmers: **
Clearing of **virgin land **(e.g. germplasm-rich tropical forests) and the choice of planting material
Farmers, especially in developed economies, primarily grow improved seed, having replaced most or all landraces
Extending grazing lands into wild habitats by livestock farmers destroys wild species and wild germplasm resources.
Action of breeders:
Farmers plant what breeders develop. Some methods used for breeding (e.g., pure lines, single cross) promote uniformity and a narrower genetic base
When breeders find superior germplasm, the tendency is to use it as much as possible in cultivar development
In soybean most of the modern cultivars in the USA can be traced back to about half a dozen parents
This practice causes severe reduction in genetic diversity.
Approaches to germplasm conservation
There are two basic approaches to germplasm conservation – in situ and ex situ.
In situ conservation: This is the preservation of variability in its natural habitat in its natural state
It is most applicable to conserving wild plants and entails the use of legal measures to protect the ecosystem from encroachment by humans
These protected areas are called by various names (e.g., nature reserves, wildlife refuges, natural parks).
Ex situ conservation:
In contrast to in situ conservation, ex situ conservation entails planned conservation of targeted species
Germplasm is conserved not in the natural places of origin but under supervision of professionals off site in locations called germplasm or gene banks
The advantage of this approach is that small samples of the selected species are stored in a small space
However, it halts the natural process of evolution.
Types of plant germplasm collections
Base collections
These collections are not intended for distribution to researchers, but are maintained in long-term storage systems
Storage conditions are low humidity at sub-freezing temperatures (−10 to −18°C) or cryogenic (−150 to −196°C), depending on the species
Materials may be stored for many decades under proper conditions.
Backup collections
The purpose of backup collections is to supplement the base collection
In case of a disaster at a center responsible for a base collection, a duplicate collection is available as insurance
Active collections
Active collections usually comprise the same materials as in base collections, however, the materials in active collections are available for distribution to plant breeders or other users upon request
They are usually stored at 0°C and about 8% moisture content.
Managing plant genetic resources
The key activities of curators of germplasm banks include regeneration of accessions, characterization, evaluation, monitoring seed viability and genetic integrity during storage
Regeneration: Germplasm needs to be periodically rejuvenated and multiplied. The regeneration of seed depends on the life cycle and breeding system of the species.
Characterization: Users of germplasm need some basic information about the plant materials to aid them in effectively using these resources. Curators of germplasm banks characterize their accessions, an activity that entails a systematic recording of selected traits of an accession
Evaluation: Genetic diversity is not usable without proper evaluation. Preliminary evaluation consists of readily observable traits. Full evaluations are more involved and may include obtaining data on cytogenetics, evolution, physiology, and agronomy.
Exchange: The ultimate goal of germplasm collection, rejuvenation, characterization, and evaluation is to make available and facilitate the use of germplasm
There are various computer-based genetic-resource documentation systems worldwide, some of which are crop-specific
These systems allow breeders to rapidly search and request germplasm information
There are various laws regarding international exchange of germplasm.
