In this article we will discuss about:- 1. Introduction to Germplasm 2. Kinds of Germplasm 3. Gene Pool System of Classification 4. Activities 5. Organizations Associated with Germplasm.
Introduction to Germplasm:
The sum total of genes in a crop species is referred to as genetic resources or gene pool or genetic stock or germplasm. In other words, gene pool refers to a whole library of different alleles of a species. Germplasm or gene pool is the basic material with which a plant breeder has to initiate his breeding programme.
Some important features of plant genetic resources or gene pool are given below:
1. Gene pool represents the entire genetic variability or diversity available in a crop species.
2. Germplasm consists of land races, modern cultivars, obsolete cultivars, breeding stocks, wild forms and wild species of cultivated crops.
3. Germplasm includes both cultivated and wild species and relatives of crop plants.
4. Germplasm is collected from the centres of diversity, gene banks, gene sanctuaries, farmers’ fields, markets and seed companies.
5. Germplasm is the basic material for launching a crop improvement programme.
6. Germplasm may be indigenous (collected within country) or exotic (collected from foreign countries).
Kinds of Germplasm:
The germplasm consists of various plant materials of a crop such as:
(1) Land races,
(2) Obsolete cultivars,
(3) Modern cultivars,
(4) Advanced (homozygous) breeding materials,
(5) Wild forms (races) of cultivated species,
(6) Wild relatives, and
(7) Mutants.
These are briefly discussed below:
(1) Land Races:
Land races are nothing but primitive cultivars which were selected and cultivated by the farmers for many generations.
Main features of land races are given below:
i. Land races were not deliberately bred like modern cultivars. They evolved under subsistence agriculture.
ii. Land races have high level of genetic diversity which provides them high degree of resistance to biotic and abiotic stresses. Biotic stress refers to hazards of diseases and insects, whereas abiotic stress means, drought, salinity, cold, frost, etc.
iii. Land races have broad genetic base which again provides them wider adaptability and protection from epidemic of diseases and insects.
Land races even respond to selection for high yield, but to certain extent. Since land races possess valuable alleles, their conservation is essential. The main drawbacks of land races are that they are less uniform and low yielders. Land races were first collected and studied by N.I. Vavilov in rice. Now land races have been collected in maize, sorghum, pearl-millet and many other crops especially in South Asia.
(2) Obsolete Cultivars:
Improved varieties of recent past are known as obsolete cultivars. These are the varieties which were popular earlier and now have been replaced by new varieties. These varieties have several desirable characters and constitute an important part of genepool. For example, wheat varieties K68, K65 and Pb 591 were most popular traditional tall varieties before introduction of high yielding dwarf Mexican wheat varieties.
These varieties are well known for their attractive grain colour and chapati making quality. Now these varieties are no more cultivated. They are good genetic resources and have been widely used in wheat breeding programmes especially in India for improvement of grain quality. Now such old varieties are found in the genepool only.
(3) Modern Cultivars:
The currently cultivated high yielding varieties are referred to as modern cultivars. Modern cultivars are also known as improved cultivars or advanced cultivars. These varieties have high yield potential and uniformity as compared to obsolete varieties and land races. Modern cultivars constitute a major part of working collections and are extensively used as parents in the breeding programmes for further genetic improvement in various characters.
Hence these cultivars are in great demand. These varieties are the outcome of scientific plant breeding and have been developed for modern intensive agriculture. However, modem cultivars have narrow genetic base and low adaptability as compared to land races.
(4) Advanced Breeding Lines:
Pre-released plants which have been developed by plant breeders for use in modern scientific plant breeding are known as advanced lines, cultures and stocks. They include advanced cultures which are not yet ready for release to farmers. Sometimes advanced breeding lines and stock are not very much productive, but constitute valuable part of genepool for various economic characters.
(5) Wild Forms of Cultivated Species:
Wild forms of cultivated species are available in many crop plants. Such plants have generally high degree of resistance to biotic and abiotic stresses and are utilized in breeding programmes for genetic improvement of resistance to biotic and abiotic stresses. They can easily cross with cultivated species. However, wild forms of many crop species are extinct. Moreover, entire range of diversity of available wild forms is rarely tapped. They constitute small part of genepool.
(6) Wild Relatives:
Those naturally occurring plant species which have common ancestry with crops and can cross with crop species are referred to as wild relatives or wild species. Wild relatives are important sources of resistance to biotic (diseases and insects) and abiotic (drought, cold, frost, salinity, etc.) stresses.
However, wild relatives are used as the last resort in crop improvement programmes, because their use in crossing leads to: (i) hybrid sterility, (ii) hybrid inviability and (iii) transfer of several undesirable genes to the cultivated species along desirable alleles. This group constitutes a minor part of gene pool. Interspecific derivatives are added to the gene pool.
(7) Mutants:
Mutation breeding is used when the desired character is not found in the genetic stocks of cultivated species and their wild relatives. Mutations do occur in nature as well as can be induced through the use of physical and chemical mutagens. The extra variability which is created through induced mutations constitutes important component of genepool.
Mutants for various characters sometimes may not be released as a variety, but they are added in the genepool. For example, mutant genepool Dee-Geo-Woo-Gen in rice and Norin 10 in wheat proved to be valuable genetic resources in the development of high yielding and semi dwarf varieties in the respective crop species. In seed propagated crops, 410 varieties have been released through the use of mutants in the crosses. (IAEA, 1991)
The Gene Pool System of Classification:
Gene pool of a crop includes all cultivars (obsolete and current), wild species and wild relatives containing all the genes available for breeding use.
Based on degree of relationship, the gene pool of a crop can be divided into three groups, viz.:
(1) Primary gene pool,
(2) Secondary gene pool, and
(3) Tertiary gene pool
These are briefly discussed below:
(1) Primary Gene Pool (GP1):
The gene pool in which intermating (crossing) is easy and leads to production of fertile hybrids is known as primary gene pool. It includes plants of the same species or of closely related species which produce completely fertile offspring on intermating. In such genepool, genes can be exchanged between lines simply by making normal crosses. This is also known as genepool one (GP1). This is the material of prime breeding importance.
(2) Secondary Gene Pool (GP2):
The genetic material that leads to partial fertility on crossing with GP1 is referred to as secondary gene pool. It includes plants that belong to related species. Such material can be crossed with primary gene pool, but usually the hybrids are sterile and some of the progeny to some extent are fertile. Transfer of gene from such material to primary gene pool is possible but difficult. This type of gene pool is also known as genepool two (GP2).
(3) Tertiary Gene Pool (GP3):
The genetic material which leads to production of sterile hybrids on crossing with primary gene pool is termed as tertiary gene pool or genepool three (GP3). It includes material which can be crossed with GP1, but the hybrids are sterile. Transfer of genes from such material to primary gene pool is possible with the help of special techniques.
Germplasm Activities:
There are six important activities related to plant genetic resources: viz.:
(I) Exploration and collection,
(II) Conservation
(III) Evaluation,
(IV) Documentation,
(V) Distribution, and
(VI) Utilization.
A brief description of these activities is presented below:
(I) Exploration and Collection:
Exploration refers to collection trips and collection refers to tapping of genetic diversity from various sources and assembling the same at one place.
The exploration and collection is a highly scientific process.
This process takes into account six important items, viz.:
(i) Sources of collection,
(ii) Priority of collection,
(iii) Agencies of collection,
(iv) Methods of collection,
(v) Methods of sampling and
(vi) Sample size.
These aspects are briefly discussed below:
(i) Sources of Collection:
There are five important sources of germplasm collection: viz., (a) centers of diversity, (b) Gene banks, (c) Gene sanctuaries, (d) Seed companies, and (e) Farmers’ fields. Moreover, collections can be made by local exploration trips to the regions of crop diversity.
(ii) Priority of Collection:
The next important step in the germplasm collection is to fix priority of collection. Some areas of diversity have been threatened more than others by the danger of extinction. Similarly, some crop species have more risk of extinction than others. Hence, endangered areas and endangered species should be given priority for germplasm collection.
(iii) Agencies of Collection:
The task of germplasm collection is undertaken by crop research institutes and state agricultural universities in collaboration with National Bureau of Plant Genetic Resources, New Delhi for indigenous collections. For global collection the task is undertaken in collaboration with International Plant Genetic Resources, Institute (IPGRI), and Rome, Italy.
(iv) Method of Collection:
Germplasm collections are made in four principal ways: viz.:
(a) Through expeditions to the areas or regions of genetic diversity,
(b) By personal visit to gene bank centre,
(c) Through correspondence, and
(d) Through exchange of material.
(v) Method of Sampling:
There are two sampling methods for collection of germplasm from the regions of diversity, viz.:
(a) Random sampling, and
(b) Biased sampling.
Random sampling is effective in capturing of alleles for biotic and abiotic stresses, whereas nonrandom or biased sampling is useful in collection of morphologically distinct geneotypes. Hence, it is advised that both random as well as biased sampling procedures should be adopted to tap the maximum genetic diversity of a crop species.
(vi) Sample Size:
The sample size should be such that 95% of the total genetic diversity can be captured from the area of collection. To achieve this goal, 50 to 100 individuals should be collected per site with 50 seeds per plant. As wide as possible range of habitats, should be sampled for capturing maximum genetic diversity.
Merits and Demerits:
There are several merits and demerits of exploration and collection of germplasm, some of which are as discussed below:
Merits:
i. Collection helps in tapping crop genetic diversity and assembling the same at one place. It reduces the loss of genetic diversity due to genetic erosion.
ii. Sometimes, we get material of special interest during exploration trips.
iii. Sometimes, we come across a new plant species during the process of collection.
iv. Collection also helps in saving certain genotypes from extinction. Once the material is collected, it can be maintained further in the germplasm.
Demerits:
i. Collection of germplasm especially from other countries, sometimes leads to entry of new diseases, new insects and new weeds.
ii. Collection is a tedious job. The collection has to be made generally from uncultivated areas like hills, mountains, river valleys and forests, where the collector faces problems of boarding, lodging and transportation.
iii. In the remote areas, the collector sometimes has encounter with wild animals like elephants, rhinos, tigers, lions and snakes which involves risk of life.
iv. Transportation of huge collections also poses difficulties in the exploration and collection.
(II) Conservation:
Conservation refers to protection of genetic diversity of crop plants from genetic erosion.
There are two important methods of germplasm conservation or preservation, viz.:
A. In-situ conservation, and
B. Ex-situ conservation.
These are described below:
A. In-Situ Conservation:
Conservation of germplasm under natural habitat is referred to as in situ conservation. It requires establishment of natural or biosphere reserves, national parks or protection of endangered areas or species. In this method of conservation, the wild species and the complete natural or semi natural ecosystems are preserved together.
This method of preservation has following main disadvantages:
a. Each protected area will cover only very small portion of total diversity of a crop species, hence several areas will have to be conserved for a single species.
b. The management of such areas also poses several problems.
c. This is a costly method of germplasm conservation.
B. Ex-Situ Conservation:
It refers to preservation of germplasm in gene banks. This is the most practical method of germplasm conservation.
This method has following three advantages:
a. It is possible to preserve entire genetic diversity of a crop species at one place.
b. Handling of germplasm is also easy.
c. This is a cheap method of germplasm conservation.
The germplasm is conserved either – (1) in the form of seed, or (2) in the form of meristem cultures. Preservation in the form of seed is the most common and easy method. Seed conservation is relatively safe, requires minimum space (except coconut, etc.) and easy to maintain. Glass, tin or plastic containers are used for preservation and storage of seeds. The seeds can be conserved under long term (50 to 100 years), medium term (10 to 15 years) and short term (3-5 years) storage conditions.
Roberts (1973) has classified seeds into two groups for storage purpose; viz.:
(1) Orthodox and
(2) Recalcitrant.
(1) Orthodox:
Seeds which can be dried to low moisture content and stored at low temperature without losing their viability are known as orthodox seeds. This group includes seeds of corn, wheat, rice, carrot, beets, papaya, pepper, chickpea, lentil, soybean, cotton, sunflower, various beans, eggplant and all the brassicas. These seeds can be dried and stored at low temperatures for long periods of time.
(2) Recalcitrant:
Seeds which show very drastic loss in viability with a decrease in moisture content below 12 to 13% are known as recalcitrant seeds. This group includes cocoa, coconut, mango, tea, coffee, rubber, jackfruit and oil palm seeds. Such seeds cannot be conserved in seed banks and, therefore, require in situ conservation. Crop species with recalcitrant seeds are conserved in field gene banks which are simply areas of land in which collection of growing plants are assembled. For conservation of meristem cultures, meristem or shoot tip banks are established.
Conservation of genetic stocks by meristem cultures has several advantages as given below:
(i) Exact genotypes can be conserved indefinitely free from virus or other pathogens and without loss of genetic integrity.
(ii) It is advantageous for vegetatively propagated crops like potato, sweet potato, cassava, etc., because seed production in these crops is poor.
(iii) Vegetatively propagated material can be saved from natural disasters or pathogen attack.
(iv) Long regeneration cycle can be envisaged from meristem cultures.
(v) Perennial plants which take 10 to 20 years to produce seeds can be preserved any time by meristem cultures.
(vi) Regeneration of meristems is extremely easy.
(vii) Plant species having recalcitrant seeds can be easily conserved by meristem cultures.
(III) Evaluation:
Evaluation refers to screening of germplasm in respect of morphological, genetical, economic, biochemical, physiological, pathological and entomological attributes.
Evaluation of germplasm is essential from following angles:
(i) To identify gene sources for resistance to biotic and abiotic stresses, earliness, dwarfness, productivity and quality characters.
(ii) To classify the germplasm into various groups.
(iii) To get a clear picture about the significance of individual germplasm line.
Evaluation requires a team of specialists from the disciplines of plant breeding, physiology, biochemistry, pathology and entomology. First of all a list of descriptors (characters) for which evaluation has to be done is prepared. This task is completed by a team of experts from IPGRI, Rome, Italy. The descriptors are ready for various crops. The material is evaluated at several locations to get meaningful results. Moreover, evaluation is done in a phased manner.
The variation for polygenic characters is assessed by three different methods as given below:
(i) By simple measures of dispersion (range, standard deviation, standard error and coefficient of variation).
(ii) By metroglyph analysis of Anderson (1957), and
(iii) By D2 statistics of P.C. Mahalanobis (1936).
The evaluation of germplasm is down in three different places, viz.:
(a) In the field,
(b) In green house, and
(c) In the laboratory.
Observations on morphological characters, productivity attributes, resistance to biotic and abiotic stresses, and some physiological parameters like photosynthetic efficiency and transpiration rate can be recorded under field conditions using portable instruments.
The resistance to biotic and abiotic stresses can be screened under greenhouse conditions. Evaluation for biochemical characters like protein, oil and amino acid contents, and technological characters is completed under laboratory conditions. Both visual observations and metric measurements are used for evaluation.
(IV) Documentation:
Documentation refers to compilation, analysis, classification, storage and dissemination of information. In plant genetic resources, documentation means dissemination of information about various activities such as collection, evaluation, conservation, storage and retrieval of data. Now the term documentation is more appropriately known as information system.
Documentation is one of the important activities of genetic resources. Information system is useful in many ways as given below:
(i) It provides information about various activities of plant genetic resources.
(ii) It provides latest information about characterization, conservation, distribution and utilization of genetic resources.
(iii) It helps explorers, evaluators and curators in the conservation of genetic resources.
(iv) It helps in making genetic resources accessible to plant breeders and other users.
Large numbers of accessions are, available in maize, rice, wheat, sorghum, potato and other major crops. About 7.3 million germplasm accessions are available in 200 crops species. Handling of such huge germplasm information is only possible through electronic computers. For uniformity of information IPGRI has designed descriptors (characters) and descriptor state for majority of crops. The entire data is put in the computer memory and the desired information can be obtained any time from the computer.
(V) Distribution:
The distribution of germplasm is one of the important activities of genetic resources centres. The specific germplasm lines are supplied to the users on demand for utilisation in the crop improvement programmes.
(i) Distribution of germplasm is the responsibility of the gene bank centres where the germplasm is maintained and conserved.
(ii) The germplasm is usually supplied to the workers who are engaged in the research work of a particular crop species.
(iii) Germplasm samples are generally supplied free of cost to avoid cumbersome work of book keeping.
(iv) The quantity of seed samples to be sent is usually small, depends on the availability of seed material and demands received for the same and several other factors.
(v) Proper records are maintained about the distribution of material. After evaluation users should send a report about important characters of the accessions to the distributor who will record the information in the germplasm register for documentation purpose.
(vi) The germplasm is usually distributed after evaluation by collecting centre for one or two crop seasons. It helps in acclimatization and purification of the material.
(vii) Without distribution to the actual users, there is no point in collecting the germplasm.
(VI) Utilization:
Utilization refers to use of germplasm in crop improvement programmes. The germplasm can be utilized in various ways.
The uses of cultivated and wild species of germplasm are briefly discussed below:
(i) Cultivated Germplasm:
The cultivated germplasm can be used in three main ways:
(a) As a variety,
(b) As a parent in the hybridization, and
(c) As a variant in the gene pool.
Some germplasm lines can be released directly as varieties after testing. If the performance of an exotic line is better than a local variety, it can be released for commercial cultivation. In some cases, new variety is developed through selection from the collection.
Some germplasm lines are not useful as such, but have some special characters, such as disease resistance, good quality of economic produce, or wider adaptability. These characters can be transferred to commercial cultivars by incorporating such germplasm lines in the hybridization programme. Transfer of desirable characters from cultivated germplasm to the commercial cultivars is very easy because of cross compatibility.
(ii) Wild Germplasm:
The wild germplasm is used to transfer resistance to biotic and abiotic stresses, wider adaptability and sometimes quality such as fibre strength in cotton.
However, utilization of wild germplasm poses three main problems, viz.:
(a) Hybrid in viability – inability of a hybrid to survive,
(b) Hybrid sterility – inability of a hybrid to produce offspring, and
(c) Linkage of undesirable characters with desirable ones.
Thus utilization of wild germplasm for crop improvement is a difficult task.
Organizations Associated with Germplasm:
There are two types of organizations, viz. International and National which are associated with germplasm. International Plant Genetic Resources Institute (IPGRI), Rome, Italy coordinates at global level. Various International Crop Research Institutes deal with germplasm of concerned crops.
In India, National Bureau of Plant Genetic Resources (NBPGR) New Delhi deals with various aspects of germplasm of agricultural and horticultural crops. Forest Research Institute, Dehradun deals with germplasm of forest species and Botanical Survey of India, Kolkata deals with germplasm of remaining plant species.
The role of IPGRI and NBPGR is briefly discussed as follows:
i. IPGRI (OLD IBPGR):
International Plant Genetic Resources Institute (IPGRI) is an autonomous international scientific organization. The functioning of IPGRI and other International Research Institutes is supported and supervised by the Consultative Group on International Agricultural Research (CGIAR).
CGIAR was constituted in 1971 by the joint efforts of Food and Agriculture Organization (FAO), the World Bank and United Nations Development Programme (UNDP) to establish International Research Institutes and assess their progress. IPGRI was established by CGIAR in 1994 and it is situated in Rome, Italy at the Food and Agriculture Organization of the United Nations.
The main function of IPGRI is to conduct research and to promote an, International Network of Plant genetic resources activities to ensure the collection, conservation, evaluation, documentation and utilization of plant germplasm. IPGRI also helps in global collection and exchange of plant genetic resources.
IPGRI has constituted crop germplasm advisory committees to help in collection, conservation, evaluation, documentation and utilization of crop germplasm. Thus IPGRI promotes and coordinates the global collection and conservation programmes of plant genetic resources. Its predecessor till December 1993 was International Board for Plant Genetic Resources (IBPGR) which was established in 1974.
ii. NBPGR:
National Bureau of Plant Genetic Resources was established by Indian Council of Agricultural Research (ICAR) in 1976 in New Delhi. In India, plant introduction started in 1946 at IARI, New Delhi in the Division of Botany. In 1961 a separate division of Plant Introduction was established under the leadership of Dr. H.B. Singh, who made remarkable contribution in the field of Plant Introduction in India.
He made huge collections of germplasm of various crop species and systematized the work. In 1976, the division of Plant Introduction was elevated to the status of independent agency known as NBPGR.
The basic function of NBPGR is to conduct research and promote collection, conservation, evaluation, documentation and utilization of crop genetic resources in India. NBPGR is assisted by various Crop Research Institutes in the collection, conservation, evaluation and documentation of crop genetic resources.
The main functions of NBPGR are briefly presented below:
1. NBPGR is the sole agency in India for import and export of plant genetic resources. Thus it helps in exchange of germplasm.
2. It promotes national genetic resources activities, viz. collection, conservation, evaluation, documentation and utilization of crop plants, and coordinates in all these activities.
3. NBPGR has five stations which are located at (i) Shimla, Himachal Pradesh, (ii) Jodhpur, Rajasthan (iii) Akola, Maharashtra (iv) Kanya Kumari, Kerala, and (v) Shillong, Meghalaya. Collections of various crops are evaluated by these centres.
4. NBPGR also organizes short term training courses on collection, conservation, evaluation, documentation and utilization of crop genetic resources.
5. National and International exploration and collection trips are also organized by NBPGR. National collection trips are organized in collaboration with the concerned Crop Research Institutes and International trips are arranged with the help of IPGRI/FAO.
6. NBPGR provides guidance about development of cold storage facilities for medium and short term conservation of germplasm.
7. NBPGR also takes decision about setting up of gene sanctuaries for endangered crop species.
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