(Arachis hypogaea L.)
Groundnut (Arachis hypogaea L.) after soybean is the second most important oilseed crops. Globally, it is cultivated on 26 million ha producing about 36 million tons annually. Developing countries account for about 94% of the world groundnut production, grown mostly under rainfed conditions, predominantly in Asia and Africa. India, China, Indonesia, Myanmar, and Vietnam in Asia; Nigeria, Sudan, Senegal, Chad, Congo, Burkina Fasso, Mozambique, Cameroon, Zimbabwe, and Niger in Africa; USA and Mexico in North Central America; and Argentina and Brazil in South America are the major groundnut producing countries.
Groundnut is a rich source of oil, protein, minerals (Ca, Mg, P, and K) and vitamins (E, K, and B1). There is a growing demand of groundnut as a food (in terms of peanut confectionery products) in Asia, Latin America, and Caribbean. The cake remaining after oil extraction is used in human food or incorporated into animal feeds. Groundnut halm is excellent forage for cattle as it is rich in protein and more palatable than many other fodders. Wild Arachis are used in pasture improvement in the Americas and Australia.
The genus Arachis is of South American origin and contains about 81 known species with natural distributions restricted to Brazil, Bolivia, Paraguay, Argentina and Uruguay. The wild species are divided into nine taxonomical sections, based upon morphology and sexual compatibilities.
Arachis hypogaea, the cultivated groundnut originated through the hybridization of two diploid species with distinct genomes (A. duranensis the AA genome donor while A. ipaensis the BB genome donor) giving rise to a sterile hybrid. A spontaneous duplication of chromosomes restored fertility, but left the plant reproductively isolated from its wild relatives. A. hypogaea is divided into two subspecies, hypogaea and fastigiata, and six botanical varieties, hypogaea, hirsuta fastigiata, vulgaris, aequatoriana, and peruviana.
Germplasm in CGIAR and NARS genebanks
World’s largest collection of peanut germplasm consisting 14 966 accessions of cultivated and 453 accessions of 44 wild Arachis species from 93 countries is housed in the RS Paroda Genebank of International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, India. This collection represents all the six botanical varieties: var. hypoagaea (6838 accessions, 45.8%), var. vulgaris (5493 accessions, 36.6%), var. fastigiata (2351 accessions, 15.7%), var. aequitoriana (14 accessions, 0.10%), var. peruviana (251 accessions, 1.7%), and var. hirsuta (19 accessions, 0.13%). Other major holders are the National Research Center for Groundnut, Junagadh, India (7935 accessions), and USDA Southern Regional Plant Introduction Station, Griffin, GA, USA (9027 accessions). About half of the holdings of cultivated accessions at ICRISAT and in Griffin are landraces.
Peanut genetic resources at ICRISAT have been characterized for various morphological and agronomic traits using a series of descriptors (IBPGR and ICRISAT 1992). Peanut germplasm showed a large range of variation for qualitative and quantitative traits. Evaluation of these germplasm identified a large number of accessions possessing tolerance or resistance to abiotic and biotic stresses.
Assessing genetic diversity for phenotypic traits
To enhance the use of germplasm and to determine the important descriptors in the evaluation of groundnut, diversity analysis was performed on 13,342 accessions (origin 92 countries) using 16 morphological and 10 agronomic descriptors. The analysis revealed a vast diversity for pod size and shape and the seeds. The accessions consisted of predominantly var. fastigiata (2121, 15.90%), var. vulgaris (4743, 35.43%), and var. hypogaea (6194, 46.42%). The number of var. peruviana (249, 1.87%), var. aequatoriana (15, 0.11%), and var. hirsuta (20, 0.15%) accessions was very low. The 92 countries were grouped into 14 regions. PCA using 38 traits and clustering on first seven PC scores delineated all the regions into three clusters; consisting North America, Middle East, and East Asia in the first Cluster, South America in the second cluster, and West Africa, Europe, Central Africa, South Asia, Oceania, Southern Africa, Eastern Africa, Southeast Asia, Central, and Caribbean in the third cluster. The means for different agronomic traits differed significantly among regions. The variances for all the traits among regions were heterogeneous. South America, which showed 100% range variation for 12 of the 16 morphological traits, also revealed highest range variation. South America among regions, primary seed color among morphological traits, leaflet length among agronomic traits showed highest pooled H`. Three of the six botanical varieties, aequatoriana, hirsuta, and peruviana were poorly represented and need to be collected.
Similarly, to determine the usefulness of a core collection (10% of entire collection) in crop improvement programs, phenotypic diversity was assessed in core collection of groundnut. One thousand seven hundred and four accessions of groundnut core collection consisting of 910 accessions belonging to subspecies fastigiata (var fastigiata, vulgaris, aequatoriana, peruviana) and 794 accessions to subsp. hypogaea (var. hypogaea and, hirsuta) were evaluated for sixteen morphological and 15 agronomic characters during the rainy and postrainy seasons and for two quality characters only in postrainy season at ICRISAT Center. Significant variation for morphological and agronomic characters was observed in the groundnut core collection. The phenotypic correlations depended upon the subspecies group. The average phenotypic diversity index was higher in the fastigiata group (0.146) than the hypogaea group (0.141), but the maximum phenotypic diversity (0.453) was observed in ICG 13723 and ICG 15419 of hypogaea group. The two groups differed significantly for all the traits except, leaflet surface and oil content. The hypogaea group showed significantly greater mean pod length, pod width, seed length, seed width, yield per plant, and 100-seed weight than the fastigiata group in both seasons where as it is opposite for plant height, leaflet length, leaflet width and shelling percentage with fastigiata group showing significantly greater means. There was significant relationship for the phenotypic correlations between rainy and postrainy season for various characteristics. Four of these, days to 50% flowering (r = 0.752), leaflet length ( r = 0743), pod length (r = 0.758), and seed length (r = 0.759) in the rainy explained more than 50% variation in the postrainy season. Principal coordinate and principal component analysis showed that 12 morphological descriptors (growth habit, stem pigmentation, stem surface, branching pattern, leaflet color, standard petal color, peg pigmentation, and pod beak, constriction and reticulation, primary seed color, and seeds per pod) and 15 agronomic traits (Days to emergence and 50% flowering, number of primary branches, plant height, leaflet length and width, pod length and width, seed length, pods per plant yield per plant and per plot, shelling percentage, 100-seed weight, and oil content) were important in explaining multivariate polymorphism. Leaflet shape & surface, color of standard petal markings, seed color pattern, seed width, and protein content did not significantly account for variation in the first five principal coordinates or components of fastigiata and hypogaea types as well as for the entire core collection, indicating their relatively low importance as groundnut descriptors. The average phenotypic diversity index was similar in both subspecies groups. The Shannon-Weaver diversity index varied among traits between the two groups, and the diversity within a group depended upon the season and traits recorded.
Development of core and mini core and identification of trait specific germplasm for use in peanut improvement
A very small proportion of peanut germplasm had been used in breeding programs. From the 14966 accessions of the cultivated peanut and 453 accessions of wild Arachis, available at ICRISAT, only 132 cultivated germplasm and 10 wild species accessions have been used in developing 8, 279 breeding lines in 17 years from 1986 to 2002. Some accessions have been extensively used in breeding programs: Chico (ICG 476), 1180 times and Robut 33-1 or Kadiri 3 (ICG 799) 3096 times). Such derivatives, if released for large-scale cultivation, may trigger epidemics and consequent wide scale destruction and huge economic loses. Understanding the range of diversity and the genetic structure of gene pools is critical for the effective management and use of germplasm resources. Continuous progress in plant breeding depends on the discovery of new sources of genetic variation, accurate identification of lines with beneficial traits, and their judicious use in such a way that a combination of alleles when brought together produces progenies with superior performance. To achieve this goal, core collection of groundnut consisting 1704 accessions was established. Development of the core collection in peanut is expected facilitate easier access to peanut genetic resources, improved efficiency of genebank management, more cost-effective and speedy evaluations, and hastens the progress in both conventional and molecular breeding programs. However, the size of core (1704 accessions) looks unwieldy for its extensive evaluation to identify parents by the breeders. To overcome this, peanut mini core (10% of core collection, 1% of entire collection) collection consisting of only 184 (from 1704 core accessions) was developed. The evaluation of peanut global core, Asia region core and mini core collections, led to identification of new sources for drought tolerance (18 accessions), early-maturity (21 accessions), and tolerance to low temperature (158 accessions) from global core collection, and for high yield, meat content (or shelling percentage), and 100 seed weight (15 fastigiata, 20 vulgaris, 25 hypogaea) from Asia region core. These new accessions have trait-specific characteristics similar to the best control cultivars, for example Chico for early maturity, but were agronoically superior or similar and genetically diverse. The use of these diverse sources would help in bringing diversity and broaden the genetic base besides improving the traits. Similarly, evaluation of USDA core has resulted in identification of resistant to root-knot nematode (Meloidogyne arenaria (Neal) Chitwood race 1, tomato spotted wilt virus, cylindrocladeum black rot [Cylindrocladium crotalaria (Loos) Bell and Sobers], early leaf spot (Cercospora arachidicola Hori), preharvest aflatoxin contamination, rhizoctonia limb rot (Rhizoctonia solani Kuhn), sclerotinia blight (Sclerotinia minor Jagger and S. sclerotium (Lib) de Berry) pepper spot [Leptosphaerulina crassiasca (Sechet) Jackson and Bell] .The newly identified accessions would be good source for dissecting genetic variation and allelic relationships and enhance the level of expression by pyramiding non-allelic genes.
PCR-based markers, genotypic diversity and genetic maps
ICRISAT and EMBRAPA jointly developed a global composite collection, consisting of 1000 diverse groundnut accessions, which included 184 groundnut mini core subset, another 184 mini core comparator, 110 accessions from Asia core and mini core, 408 elite germplasm/cultivars and trait-specific (resistance to biotic and abiotic stresses, early maturity and/or fresh seed dormancy, large-seed, high shelling percentage, high oil and/or protein content, and interspecific derivatives) accessions, and 114 wild Arachis accessions. This composite collection has been molecularly profiled using 21 SSRs in high throughput assay (ABI3700).
Twenty-one SSR loci data on 852 accessions were analyzed. This composite collection showed rich allelic diversity (490 alleles, 23.3 alleles per locus, 246 common alleles and 244 rare alleles at 1%), group-specific unique alleles, and common alleles sharing between subspecies and geographical groups. Gene diversity ranged from 0.559 to 0.926, with an average of 0.819.
Unique alleles are those detected in a group of accessions but absent in other groups. Group-specific unique alleles were 101 in wild Arachis, 50 in subsp. fastigiata, and only 11 in subsp. hypogaea. Accessions from America’s revealed highest number of unique alleles (109) while Africa and Asia, respectively, had only six and nine unique alleles. The two subsp. hypogaea and fastigiata shared 70 alleles. The wild Arachis in contrast shared only 15 alleles with hypogaea and 32 alleles with fastigiata. A tree-diagram using DARwin 5.0 separated majority of the hypogaea from fastigiata accessions while wild Arachis accessions clustered with hypogaea.