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Origin of polyploids in the Rhizomatosae and Caulorrhizae sections of Arachis (Leguminosae).
Montpellier, Francia
Workshop; Workshop: ?Reference sets of food crop germplasm for international collaboration?,; 2008
Institución organizadora:
The Generation Challenge Programme.
Section Rhizomatosae is currently defined mainly because all the entities have rhizomes and includes one diploid species (A. burkartii with 2n=2x=20) and three tetraploids (A. glabrata, A. pseudovillosa and A. nitida with 2n=4x=40). Some accessions of A. glabrata have a great value as forage and several cultivars were developed and commercialized. On the other hand, section Caulorrhizae includes two diploid species (A. pintoi A. repens, both with 2n=2x=20) that can be propagated by stolons, besides of having good seed production. Arachis pintoi produces forage of high quality and several commercial cultivars have been released. Triploid plants of these species were isolated from an implanted diploid cultivar and showed higher forage potential than the diploids. In an attempt to test the genetic origin of the polyploids in these sections, classical and molecular cytogenetic studies have been carried out. Meiotic analysis in different materials of the rhizomatous A. glabrata revealed chromosome association which ranged from 20 II to 4II + 8IV. This meiotic behavior suggests an autotetraploid origin, although populations may have different diploidization degrees. Chromosome pairing in the 3x A. pintoi also showed a high proportion trivalents, indicating that it may be an autotriploid clone. Within section Rhizomatosae, tetraploid species presented centromeric DAPI+ heterochromatic bands in the whole complement, two (A. glabrata) or three (A. nitida) pairs of 45S rDNA and, two pairs of 5S rDNA loci (all in different chromosomes). However, the diploid A. burkartii showed bands in all (except one) pair of chromosomes, four pairs of 45S rDNA loci (two synthenic) and, one pair of 5S rDNA loci. The later were co-localized with one pair of 45S rDNA loci. These results showed that A. burkartii is not a good candidate as a genome donor of the tetraploid species, and that the autotetraploid nature of the species may be arguable. Concerning section Caulorrhizae, the classical cytological analysis has demonstrated that the 30 chromosomes of the triploid A. pintoi are arranged in 27m+3sm with three SAT chromosomes of the same type, reflecting three identical chromosome sets. Physical mapping of rDNA genes has revealed one cluster of 45S rDNA and two of 5S rDNA in metacentric chromosomes of median size per haploid complement. The number, position and size of the ribosomal clusters in each complement of the triploid were identical to those observed in the diploid cytotype. Therefore, the bulk of evidence demonstrated that the 3x cytotype may have originated by autopolyploidy.