INVESTIGADORES
ORTIZ alejandra marcela
congresos y reuniones científicas
Título:
Analysis of the genome constitution of Arachis species of section Rhizomatosae by classical and molecular cytogenetic
Autor/es:
ORTIZ, ALEJANDRA MARCELA; SEIJO, JOSÉ GUILLERMO; SILVESTRI MARÍA CELESTE; LAVIA, GRACIELA INÉS
Lugar:
Brasilia
Reunión:
Conferencia; VIII ENCUENTRO LATINOAMERICANO DE ESPECIALISTAS EN ARACHIS; 2013
Institución organizadora:
Embrapa Recursos Genéticos y Biotecnología
Resumen:
The Rhizomatosae section of the genus Arachis is defined exclusively by morphological features, mainly because all of its species have rhizomes as mean of vegetative propagation. This section includes three tetraploid species (2n=4x=40) -A. glabrata, A. pseudovillosa y A. nitida- and only one diploid entity (2n=2x=20) -A. burkartii-. From a phylogenetic point of view, this section constitutes an interesting biological model, since it has the highest frequency of polyploid species of the genus and only one diploid taxon. Several research carried out in the context of the genus, have shown a high degree of genetic isolation between the only diploid species included in this section and the tetraploid entities; however, some genetic similarity between the tetraploids and diploids belonging to the sections Arachis, Erectoides and Procumbentes has been suggested. This fact has led to question the origin of the tetraploids from A. burkartii and, as consequence, to propose the hypothesis of multiple origin of these tetraploids including species from other sections. On the other hand, this section has a particular interest due to the presence of a species with high forage value, A. glabrata. Several cultivars have been released to the market; however, their massive diffusion is limited by their low to null seed production. Based on the above background, in the current thesis different studies were carried out: 1) the meiotic, karyotypic and genomic characterization of the four species included in the Rhizomatosae section by classical and molecular cytogenetic techniques, 2) the comparative karyotipic analyses between the rhizomatous tetraploid species and close diploid entities of the sections Rhizomatosae, Erectoides and Procumbentes and, 3) the analysis of genomic affinities between the tetraploid A. glabrata and several diploid entities through genomic in situ hybridization; with the objectives of analyzing the correlation between meiotic irregularities and low seed production in the rhizomatous tetraploids, the polyploidy nature of these tetraploids, and the genomic affinities between the tetraploids and diploids to clarify the relationships within the Rhizomatosae section and to infer the genetic origin of tetraploids. Detailed meiotic analysis from metaphase I to the sporad stage and the assessment of the morphology and pollen viability in polyploid species of Rhizomatosae section allowed to detect the presence of several irregularities along the meiotic process, to infer the possible causes of their origin and to propose what meiotic mechanisms would be involved in the reduction of pollen viability and morphological variation of pollen grains in the analyzed species. However, meiotic irregularities only represented an average less than 6% of the PMCs analyzed. Accordingly, the meiotic indexes were over 94% and pollen viabilities of three tetraploids ranged from 81.44 to 97.56%. Therefore, results obtained in this thesis indicate that meiotic abnormalities found in the microsporogenesis do not contribute significantly to the reduction of seed production. Comparative analyses of the chromosome markers revealed by fluorescence banding and FISH with the analyses of meiotic associations in diacinesis-metaphase I allowed making inferences about the nature of the tetraploids of the Rhizomatosae section. The results found in the current thesis evidenced that A. nitida probably resulted from hybridization between two entities having different number of 45S rDNA loci. Nevertheless, the finding of up to four multivalents (trivalents and tetravalents) in meiosis and the similar pattern of C-DAPI+ heterochromatic found among the four chromosomal sets indicate that the genomes of A. nitida have partial homology. Therefore, this species may be considered as a segmental allopolyploid. On the other hand, A. pseudovillosa and A. glabrata have a high degree of homology between their genomes, due to the similar pattern of C-DAPI+ heterochromatin and the number and similarity of the chromosomes carrying both types of rDNA loci it may be suggested that these species are autopolyploid or segmental allopolyploid originated from species with the same genome type. Karyotypes for 10 species of Rhizomatosae, Erectoides and Procumbentes were constructed by means of the morphometric analysis of chromosomes, the localization of the different types of heterochromatin (CMA+/DAPI-, DAPI+/CMA-, C-DAPI+) and the ribosomal loci mapping through the simultaneous application of classical and molecular cytogenetic techniques. Comparative karyotypic analysis allowed establishing specific markers for each tetraploide species, which can be differentiated by the number and position of 45S rDNA loci. However, shared karyotypic features among them indicate that they constitute a natural group. Likewise, the addition of chromosome markers found permitted to characterize for the first time R genome and to determine that it is only found in the diploid entity of Rhizomatosae section, showing that the section does not constitute a monophyletic group. The distribution pattern of C-DAPI+ heterochromatin and the number of chromosomes with ribosomal loci showed that tetraploid species have greater genomic affinity with the diploid of the Erectoides and Procumbentes sections. These results, supported by molecular phylogenies, suggested that tetraploid species could have a genomic constitution EEEE and, that the progenitors of them have to be searched in these sections. In addition, the group of diploid species more close to the tetraploide was identified. Simple GISH experiments showed that there is a high degree of genomic homology between the tetraploid A. glabrata and diploid species harboring the A, E2 and E3 genome, being higher with the last species. Double GISH experiments revealed that the probes corresponding to species harboring the A, E2 and E3 genome hybridized on the whole chromosome complement of A. glabrata, although none of them hybridized uniformly on the entire length of chromosomes. Moreover, the probes corresponding to the E genome, and particularly to A. paraguariensis subsp. capibariensis, had the higher degree of hybridization, evidencing a greater degree of genome homology with A. glabrata. However, the differential hybridization pattern observed suggests the hypothesis of a common ancestral genome to the diploid species of the Arachis, Erectoides and Procumbentes sections and the rhizomatous tetraploids. From these results, some hypotheses about the genomic constitution of A. glabrata are proposed.