Hypogaea

Abstract: Cultivated peanut or groundnut (Arachis hypogaea L.) is the fourth most important oilseed crop in the world, grown mainly in tropical, subtropical and warm temperate climates. Due to its origin through a single and recent polyploidization event, followed by successive selection during breeding efforts, cultivated groundnut has a limited genetic background. In such species, microsatellite or simple sequence repeat (SSR) markers are very informative and useful for breeding applications. The low level of polymorphism in cultivated germplasm, however, warrants a need of larger number of polymorphic microsatellite markers for cultivated groundnut. A microsatellite-enriched library was constructed from the genotype TMV2. Sequencing of 720 putative SSR-positive clones from a total of 3,072 provided 490 SSRs. 71.2% of these SSRs were perfect type, 13.1% were imperfect and 15.7% were compound. Among these SSRs, the GT/CA repeat motifs were the most common (37.6%) followed by GA/CT repeat motifs (25.9%). The primer pairs could be designed for a total of 170 SSRs and were optimized initially on two genotypes. 104 (61.2%) primer pairs yielded scorable amplicon and 46 (44.2%) primers showed polymorphism among 32 cultivated groundnut genotypes. The polymorphic SSR markers detected 2 to 5 alleles with an average of 2.44 per locus. The polymorphic information content (PIC) value for these markers varied from 0.12 to 0.75 with an average of 0.46. Based on 112 alleles obtained by 46 markers, a phenogram was constructed to understand the relationships among the 32 genotypes. Majority of the genotypes representing subspecies hypogaea were grouped together in one cluster, while the genotypes belonging to subspecies fastigiata were grouped mainly under two clusters. Newly developed set of 104 markers extends the repertoire of SSR markers for cultivated groundnut. These markers showed a good level of PIC value in cultivated germplasm and therefore would be very useful for germplasm analysis, linkage mapping, diversity studies and phylogenetic relationships in cultivated groundnut as well as related Arachis species.

Abstract: The 5S and the 18S-25S rRNA genes were physically mapped by fluorescent in situ hybridization (FISH) in all botanical varieties of cultivated peanut Arachis hypogaea (2n = 4x = 40), in the wild tetraploid A. monticola, and in seven wild diploid species considered as putative ancestors of the tetraploids. A detailed karyotype analysis including the FISH signals and the heterochromatic bands was carried out. Molecular cytogenetic landmarks are provided for the construction of a FISH-based karyotype in Arachis species. The size, number, and chromosome position of FISH signals and heterochromatic bands are similar in all A. hypogaea varieties and A. monticola, but vary among the diploid species. Genome constitution of the species is discussed and several chromosome homeologies are established. The bulk of the chromosome markers mapped, together with data on geographical distribution of the taxa, suggest that peanut originated upon domestication of A. monticola and evidence that the diploids A. duranensis and A. ipaensis are the most probable ancestors of both tetraploid species. Allopolyploidy could have arisen by a single event or, if by multiple events, always from the same diploid species.

Abstract: 'Tifguard' (Reg. No. CV-101, PI 651853) is a runner-type peanut (Arachis hypogaea L. subsp. hypogaea var. hypogaea) cultivar released by the USDA-ARS and the Georgia Agricultural Experiment Stations in 2007. Tifguard was developed at the University of Georgia Coastal Plain Experiment Station, Tifton, GA. Peanut cultivars are available that have high resistance to the peanut root-knot nematode [Meloidogyne arenaria (Neal) Chitwood race 1] or spotted wilt caused by tomato spotted wilt tospovirus (TSWV). However, no cultivars exist that have resistance to both pathogens. Our research objective was to combine resistance to both pathogens in a single cultivar. Breeding populations were developed by hybridizing the TSWV-resistant 'C-99R' with the nematode-resistant 'COAN'. Selection for nematode resistance was conducted using standard greenhouse screening techniques. Selection for TSWV resistance was conducted in the field with natural virus infection. A breeding line (C724-19-15) was selected that had high resistance to both pathogens. Tifguard exhibited higher resistance to TSWV and higher yield than standard check cultivars when grown in fields with little or no nematode pressure. Because of its high level of resistance to both TSWV and M. arenaria, Tifguard had significantly higher yield than all others entries when grown in two locations with high pressure from both pathogens. This cultivar should be valuable for peanut growers who have to deal with both pathogens.

Abstract: Genus Arachis L. includes 80 described species, of which 31 belong to section Arachis, including the two diploid spedes A. ipaensis Krapov. and W.C. Gregory and A. duranensis Krapov. and W.C. Gregory, considered the putative B and A genome parents of the cultivated peanut This work contributes to the study of evolution of the peanut (Arachis hypogaea L.), based on the successful hybridization between A. ipaensis and A. duranensis, chromosome doubling of the hybrid, and crosses between the synthetic amphidiploid and representatives of the diversity of the crop. Diploid hybrids between A. ipaensis and A. duranensis, confirmed by molecular markers, had pollen stains of 0.98%. Colchicine-induced tetraploids were confirmed by mitotic chromosome counts. Progeny from these amphidiploid plants had a 97.74% pollen stain and significant differences among structure sizes measured in diploid and tetraploid flowers. Hybrid individuals [A. hypogaea × (A. ipaensis x A. duranensis) 4x ] were produced from crosses involving all six botanical varieties of A. hypogaea. These hybrids indicate the evolutionary similarity between the wild species and the cultigen. The successful hybridization between diploid spedes A. ipaensis and A. duranensis and between A. hypogaea and the synthetic amphidiploid support the theory that these two diploids are the parents of the cultivated peanut. Resulting materials are of great importance to peanut breeding.