Germplasm resources and conservation source's of germplasm for plant breeding

Germplasm

1. Germplasm

2. Germplasm Resources and Conservation Plant germplasm is the genetic source material used by plant breeders to develop new cultivars. Germplasm may include seeds or other plant propagules such as a leaf, stem, pollen, or cultured cells that could be grown into mature plants. Seeds may be from new or old cultivars, landraces, special breeding lines or populations developed by the breeder, or special genetic stocks such as mutant lines. Plant germplasm is one of our most important natural resources and must be managed properly if breeders are to continue to develop improved cultivars. Survival of the human race depends upon the proper conservation of these valuable plant germplasm resources.

3. Sources of germplasm for plant breeding Germplasm may be classified into five major types – advanced (elite) germplasm , improved germplasm , landraces , wild or weedy relatives , and genetic stocks . The major sources of variability for plant breeders may also be categorized into three broad groups – domesticated plants , undomesticated plants , and other species or genera

4. Undomesticated plants When desired genes are not found in domesticated cultivars, plant breeders may seek them from wild populations. When wild plants are used in crosses, they may introduce wild traits that have an advantage for survival in the wild (e.g., hard seed coat, shattering, indeterminacy) but are undesirable in modern cultivation. These undesirable traits have been selected against through the process of domestication. Wild germplasms have been used as donors of several important disease- and insectresistance genes and genes for adaptation to stressful environments. The cultivated tomato has benefited from such introgression by crossing with a variety of wild Licopersicon species. Other species such as potato, sunflower, and rice have benefited from wide crosses. In horticulture, various wild relatives of cultivated plants may be used as rootstock in grafting (e.g., citrus, grape) to allow cultivation of the plant in various adverse soil and climatic conditions.

5. Other species and genera Gene transfer via crossing requires that the parents be cross-compatible or cross-fertile. As previously stated crossing involving parents from within a species is usually successful and unproblematic. However, as the parents become more genetically divergent, crossing (wide crosses) is less successful, often requiring special techniques (e.g., embryo rescue) for intervening in the process in order to obtain a viable plant. Sometimes, related species may be crossed with little difficulty

6. 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 were defined, primary, secondary, and tertiary gene pools: 1 Primary gene pool (GP1) . GP1 consists of biological species that can be intercrossed easily (interfertile) 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 . 2 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, but more difficulty with success. 3 Tertiary gene pool (GP3) . GP3 involves the outer limits of potential genetic resources. Gene transfer by hybridization between GP1 and GP3 is very prob lematic, 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 nurture an embryo from a wide cross to a full plant and to obtain fertile plants

7. Crop gene pools. Harlan proposed the crop gene pools to guide the germplasm use by plant breeders. The number of species in each of the pools that plant breeders use varies among crops. Harlan suggested that breeders first utilize the germplasm in GP1 and proceed outwards.

8. A classification of dry bean and rice is presented in Figure 6.1 for an illustration of this concept. In assembling germplasm for a plant breeding project, the general rule is to start by searching the domesticated germplasm collection first, before considering other sources, for reasons previously stated. However, there are times when the gene of interest occurs in undomesticated germplasm, or even outside the species. Genetransfer techniques enable breeders to transfer genes beyond the tertiary gene pool. Whereas all crop plants have a primary gene pool that includes the cultivated forms, all crops do not have wild forms in their GP1 (e.g., broad bean, cassava, and onions whose wild types are yet to be identified). Also, occasionally, the GP1 may contain taxa of other crop plants (e.g., almond belongs to the primary gene pool of peach). Most crop plants have a GP2, which consists primarily of species of the same genus. Some crop plants have no secondary gene pools (e.g., barley, soybean, onion, broad bean).

9. Germplasm Conservation Crop plants have been domesticated for a very long period of time, but the germplasm resources accumulated in the process are being eroded very rapidly. Farmer selected cultivated forms, called landraces , originally evolved from wild plant populations. The landraces that did not acquire broad genetic diversity during this evolutionary period eventually succumbed to the ravages of disease, drought, cold, competition with weeds, or other unfavorable environmental stresses. The landraces that survived became modern crop cultivars in the countries of origin and progenitors to modern cultivars in other countries. Unfortunately, progress in breeding, often by selection and purification of these heterogeneous landraces, inevitably led to more uniformity and less genetic variability within the improved cultivars than was present in the original landraces .

10. International Germplasm Centers International Board for Plant Genetic Resources (IBPGR) with headquarters in the Food and Agriculture Organization (FAO) of the United Nations in Rome. The functions of the IBPGR are to develop an overview of the worldwide state of genetic resource conservation, to promote and coordinate efforts among countries to conserve germplasm of the major field crops, and to assist appropriate institutions in extending their capabilities for storing and evaluating germplasm collections International Rice Research Institute (IRRI), Los Baños , Philippines, which has collected more than 70,000 accessions of rice. International Maize and Wheat Improvement Center (CIMMYT), Mexico: corn, wheat, and triticale; International Crops Research Institute for the Semi­Arid Tropics (ICRISAT), Patancheru , A.P., India: sorghum, pearl millet, chickpea, pigeonpea , and peanut (groundnut); International Center of Tropical Agriculture (CIAT), Cali, Colombia: dry bean, cassava, tropical forages; Asian Vegetable Research and Development Center (AVRDC), Shanhua , Taiwan: mungbean , soybean, tomato, chinese cabbage, pepper; International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria: cowpea, cassava, sweet potato, yam; International Potato Center (CIP), Lima, Peru: potato, sweet potato; and International Center for Agricultural Research in Dry Areas (ICARDA), Aleppo, Syria: wheat, barley, broad bean, lentils. CCRI cotton NARC