Graciela I. Lavia

Physical mapping of the 5S and 18S–25S rRNA genes by FISH as evidence that Arachis duranensis and A. ipaensis are the wild diploid progenitors of A. hypogaea (Leguminosae)

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.

Genomic relationships between the cultivated peanut (Arachis hypogaea, Leguminosae) and its close relatives revealed by double GISH.

Arachis hypogaea is a natural, well-established allotetraploid (AABB) with 2n = 40. However, researchers disagree on the diploid genome donor species and on whether peanut originated by a single or multiple events of polyploidization. Here we provide evidence on the genetic origin of peanut and on the involved wild relatives using double GISH (genomic in situ hybridization). Seven wild diploid species (2n = 20), harboring either the A or B genome, were tested. Of all genomic DNA probe combinations assayed, A. duranensis (A genome) and A. ipaensis (B genome) appeared to be the best candidates for the genome donors because they yielded the most intense and uniform hybridization pattern when tested against the corresponding chromosome subsets of A. hypogaea. A similar GISH pattern was observed for all varieties of the cultigen and also for A. monticola. These results suggest that all presently known subspecies and varieties of A. hypogaea have arisen from a unique allotetraploid plant population, or alternatively, from different allotetraploid populations that originated from the same two diploid species. Furthermore, the bulk of the data demonstrated a close genomic relationship between both tetraploids and strongly supports the hypothesis that A. monticola is the immediate wild antecessor of A. hypogaea.

Genome re-assignment of Arachis trinitensis (Sect. Arachis, Leguminosae) and its implications for the genetic origin of cultivated peanut

The karyotype structure of Arachis trinitensis was studied by conventional Feulgen staining, CMA/DAPI banding and rDNA loci detection by fluorescence in situ hybridization (FISH) in order to establish its genome status and test the hypothesis that this species is a genome donor of cultivated peanut. Conventional staining revealed that the karyotype lacked the small “A chromosomes” characteristic of the A genome. In agreement with this, chromosomal banding showed that none of the chromosomes had the large centromeric bands expected for A chromosomes. FISH revealed one pair each of 5S and 45S rDNA loci, located in different medium-sized metacentric chromosomes. Collectively, these results suggest that A. trinitensis should be removed from the A genome and be considered as a B or non-A genome species. The pattern of heterochromatic bands and rDNA loci of A. trinitensis differ markedly from any of the complements of A. hypogaea, suggesting that the former species is unlikely to be one of the wild diploid progenitors of the latter.

Chromosomal characterization of germplasm of wild species of Arachis L. belonging to sections Trierectoides, Erectoides and Procumbentes

Abstract Karyotypes of five species of genus Arachis belonging to three sections are described for the first time. Section Trierectoides: A. tuberosa Bong. ex Benth. (2n=2x=20m), section Erectoides: A. douradiana Krapov. and W.C. Gregory (2n=2x=18m+2sm), section Procumbentes: A. subcoriacea Krapov. and W.C. Gregory (2n=2x=18m+2sm), A. appressipila Krapov. and W.C. Gregory (2.=2x=14m+6sm) and A. Vallsii Krapov. and W.C. Gregory (2n=2x=18m+2sm). Satellited chromosomes are analysed for all species. “A” chromosomes were not found in none of them. Chromosome and exomorphology data suggest that A. Vallsii merit to be exclude from section Procumbentes.

Chromosome doubling inTurnera ulmifolia (Turneraceae) induced by regeneration of plants from in vitro cultured leaf explants

Plants were regenerated from cultured excised leaf segments ofTurnera ulmifolia (2n = 6x = 30). Cytological studies have demonstrated that chromosome doubling occurred in 100% of the regenerated plants. Probably it was produced by endomitosis, induced by excess of auxins in relation to cytokinins. High bivalent and low quadrivalent frequency, and univalents to octovalents were observed in metaphase I; lagging chromosomes were also found. Probably, the presence of multivalents may be due to the pairing among homoeologous chromosomes because the mother plant is a segmental allohexaploid. The high bivalent frequency may have been caused by preferential pairing of identical chromosomes against the homologous.

Cytological Features of Peanut Genome

This chapter aims to update the chromosomal features evaluated by classical and molecular cytogenetic techniques. Karyotype variability detected within and among species was very useful to unravel the taxonomy of the genus and to establish relationships among species. This chapter includes analyses of chromosome morphology, heterochromatin, rDNA loci, as well as dispersed and clustered repetitive sequences. A critical review of the genome sizes of Arachis species is also provided. The usefulness of chromosome data is presented in three examples. The first one deals with the origin of the cultivated peanut. Molecular cytogenetics evidenced that the varieties of A. hypogaea may have had a single genetic origin, that A. monticola is a direct tetraploid ancestor of peanut, and that A. duranensis (A genome) and A. ipaensis (B genome) are the diploid progenitors of the AABB tetraploids. The second one pointed to the analysis of the origin of the rhizomatous tetraploids and their relation to the unique diploid species (A. burkartii) of section Rhizomatosae. The cytogenetic data suggest that A. burkartii has to be discarded as a genome donor of the tetraploids, and that the latter may have had independent origins involving different species. The third one concerns the species of section Arachis, and how the chromosome data aided in the establishment of the genome groups (A, B, D, F, G, and K).

Corrigenda: Genomic characterisation of Arachis porphyrocalyx (Valls & C.E. Simpson, 2005) (Leguminosae): multiple origin of Arachis species with x = 9. Comparative Cytogenetics 11(1): 29–43. doi: 10.3897/CompCytogen.v11i1.10339

[This corrects the article DOI: 10.3897/CompCytogen.v11i1.10339.].

Genomic characterisation of Arachis porphyrocalyx (Valls & C.E. Simpson, 2005) (Leguminosae): multiple origin of Arachis species with x = 9

Abstract The genus Arachis Linnaeus, 1753 comprises four species with x = 9, three belong to the section Arachis: Arachis praecox (Krapov. W.C. Greg. & Valls, 1994), Arachis palustris (Krapov. W.C. Greg. & Valls, 1994) and Arachis decora (Krapov. W.C. Greg. & Valls, 1994) and only one belongs to the section Erectoides: Arachis porphyrocalyx (Valls & C.E. Simpson, 2005). Recently, the x = 9 species of section Arachis have been assigned to G genome, the latest described so far. The genomic relationship of Arachis porphyrocalyx with these species is controversial. In the present work, we carried out a karyotypic characterisation of Arachis porphyrocalyx to evaluate its genomic structure and analyse the origin of all x = 9 Arachis species. Arachis porphyrocalyx showed a karyotype formula of 14m+4st, one pair of A chromosomes, satellited chromosomes type 8, one pair of 45S rDNA sites in the SAT chromosomes, one pair of 5S rDNA sites and pericentromeric C-DAPI+ bands in all chromosomes. Karyotype structure indicates that Arachis porphyrocalyx does not share the same genome type with the other three x = 9 species and neither with the remaining Erectoides species. Taking into account the geographic distribution, morphological and cytogenetic features, the origin of species with x = 9 of the genus Arachis cannot be unique; instead, they originated at least twice in the evolutionary history of the genus.