Development of Core Set of Wheat (Triticum spp.) Germplasm Conserved in the National Genebank in India
Abstract Plant genetic resources, the source of genetic diversity provides a broad genetic foundation for plant breeding and genetic research, however, large germplasm resources are difficult to preserve, evaluate and use. Construction of core and mini core collections is an efficient method for managing genetic resources and undertaking intensive surveys of natural variation, including the phenotyping of complex traits and genotyping of DNA polymorphisms allowing more efficient utilization of genetic resources. A mega characterization and evaluation programme of the entire cultivated gene pool of wheat conserved in the National Genebank, India was undertaken. Wheat accessions with limited seed quantity, were multiplied in the off-season nursery at IARI Regional Station, Wellington during rainy season 2011 and the entire set of 22,469 wheat accessions were characterized and evaluated at CCS HAU, Hisar, Haryana during winter season 2011–12 for 34 characters including 22 highly heritable qualitative, and 12 quantitative parameters. The core sets were developed using PowerCore Software with stepwise approach and grouping method and validated using Shannon-Diversity Index and summary statistics. Based on Shannon-Diversity index, PowerCore with stepwise approach was found better than PowerCore with grouping. The core set included 2,208 accessions comprising 1,770 T. aestivum, 386 T. durum, and 52 T. dicoccum accessions as a representative of the total diversity recorded in the wheat germplasm. The core set developed will be further validated at different agro-climatic conditions and will be utilized for development of mini core set to enhance the utilization by wheat researchers and development of climate resilient improved varieties.
Keywords Characterization • Core collection • Genebank • Germplasm • PowerCore • Triticum • Wheat
Wheat (Triticum aestivum L.) is the most widely cultivated food crop worldwide with an area of 220.39 million hectares and production of 704.08 million tonnes reported during 2011–12 (FAOSTAT 2012). In India, it is the second most important staple food crop after rice, grown in an area of 29.90 million hectares with a total production of 94.88 million tonnes and productivity of 3,140 kg/ha in 2011–12 (DWR Annual Report 2013). The Indo-Gangetic plains comprising the states of Punjab, Haryana, Uttar Pradesh and Rajasthan together account for nearly 85 % of total wheat production in the country. India is probably one of the few countries in the world where three wheat types namely T. aestivum, T. durum Desf., and cT. dicoccum Schuebl. are grown although the major area (90 %) is under bread wheat (T. aestivum). Bread wheat is grown in all the wheat growing areas while durum wheat is largely grown in Central and Peninsular India mostly under rainfed conditions. In recent years, semidwarf durum wheat varieties have also become popular in Northern India, particularly in Punjab and Haryana. The dicoccum wheat is grown in Maharashtra and Karnataka on an area of about 0.5 million hectares.
Wheat originated in the Fertile Crescent area of south-western Asia among the first domesticated food crops around 8,000 years ago. The north-western end of Indian subcontinent, the fold between Hindukush and Himalaya is regarded as the secondary centre of origin of hexaploid wheat (Vavilov 1926). Archaeological records from many parts of India also revealed cultivation of wheat since the Harappan period (2300–1750 B.C.).
Abundant plant germplasm resources, a rich source of genetic diversity provides a broad genetic foundation for plant breeding and genetic research. However, large germplasm resources are also difficult to preserve, evaluate and use (Holden 1984). Therefore, establishing a core collection (CC) is a favored approach for the efficient exploration and utilization of novel variation in genetic resources (Hodgkin et al. 1995; Zhang et al. 2011). The concept of a CC was first proposed by Frankel (1984) and later developed by Brown (1989a, b). Frankel (1984) defined a core collection as a limited set of accessions representing, maximum diversity with minimum repetitiveness, the genetic diversity of a crop species and its wild relatives. The core collection could serve as a working collection which could be extensively evaluated. It involves the selection of a subset from the whole germplasm by certain methods in order to capture the maximum genetic diversity of the whole collection while minimizing accessions and redundancy. Frankel and Brown (1984) and Brown (1989a, b) developed this proposal further and described methods to select a core subset using information on the origin and characteristics of the accessions. In developing the core collection, the first issue was its size, second, the grouping of accessions in the entire collection and third, the number of accessions to be selected from a group and fourth the sampling theory. Brown (1989a) using sampling theory of selectively neutral alleles, argued that the entries in a core subset should be ~10 % of the total collection with a ceiling of 3,000 per species. This level of sampling is effective in retaining 70 % of the alleles of the entire collection. The hierarchy of grouping begins with the classification suggested by taxonomy (species, subspecies, and races) followed by assigning accessions to major geographic groups (country, state), climate, or agro-ecological regions. The clustering within the broad geographic group could be done to sort accessions into clusters. A germplasm collection with abundant discriminating data would require multivariate clustering to form groups of similar accessions (Zeuli and Qualset 1993). The number of accessions selected from each class will depend on the sampling strategy used. A good core set should capture maximum genetic diversity with a minimal number of genotypically redundant entries and should be small. Brown (1989a) proposed three procedures based on groups sizes, constant (C), proportional (P) and logarithmic strategies (L). Subsequently, Franco et al. (2005, 2006) proposed that efficiency of sampling for allocation of accessions to different groups could be improved by using diversitydependent (G) strategy. Of the four strategies, strategy G was reported superior to P strategy (Hodgkin et al. 1999; Yonezawa et al. 1995). Since the original concept of Frankel (1984), core collections have been established in many crop species.
The National Genebank of India currently conserves 31,007 accessions of wheat germplasm comprising 19,116 indigenous and 11,891 accessions of exotic origin. However, the available diversity has not been adequately evaluated and extensively used in wheat improvement due to the large size of germplasm collection. Proper evaluation is feasible only for the traits which can be scored easily and do not show genotype by environment (G x E) interactions. Recognizing this, the present study was aimed to develop the core collection of cultivated wheat germplasm conserved in the National Genebank (NGB) based on characterization and preliminary evaluation data at one representative site with a view to reduce the genebank collection to a manageable level for facilitating utilization of germplasm in applied research.