Chromosome contribution to andean polyploid species of Senecio (Asteraceae), from Argentina

Fil: López, Mariana G.. Lab. de Citogenética y Evol.; Depto. de Ciencias Biológicas; Universidad de Buenos AiresFil: Wulff, Arturo F.. Lab. de Citogenética y Evol.; Depto. de Ciencias Biológicas; Universidad de Buenos AiresFil: Xifreda, Cecilia Carmen. Laboratorio de Etnobotánica y Botánica Aplicada (LEBA); Facultad de Ciencias Naturales y Museo; Universidad Nacional de La Plat

Deep cytogenetics analysis reveals meiotic recombination depletion in species of Senecio (Asteraceae)

Background: Senecio is the largest genus in the Asteraceae family growing in all environments around the world. It displays taxonomic and systematical difficulties. Cytogenetic knowledge of this genus is ancient, scarce and mainly restricted to chromosome number records. Results: In this study we analyzed chromosome number, meiotic configuration, bivalent morphology, meiotic behavior and pollen grain stainability on 100 accessions of 27 different polyploid Senecio L. sect Senecio entities. Median, standard deviation and mode were calculated for number and position of chiasmata and meiotic recombination was statistically evaluated. Although high frequency of multivalents and associated meiotic irregularities are expected in high polyploids, bivalents predominance and, consequently, regular meiosis were observed, with normal sporogenesis and high pollen grain stainability. Conclusion: Depletion in the total chiasmata was significant only in some species but the terminal position was preferential in all the entities analyzed, indicating significant reduction in recombination. The regular meiosis observed suggest that intra and intergenomic reorganization process occur quickly and efficiently in this genus. Mechanisms of diploidization, common to all polyploids, are reinforced by the strong reduction in crossing-over rushing polyploids stabilization.Facultad de Ciencias Naturales y Muse

South African fireweed Senecio madagascariensis (Asteraceae) in Argentina: Relevance of chromosome studies to its systematics

The systematic identity of Senecio madagascariensis is ratified against the opinion that it is conspecific with Senecio inaequidens. Both species are native to South Africa and have been merged in the 'Senecio inaequidens complex', a group of entities difficult to distinguish from each other. Senecio madagascariensis is widespread in South America and Australia, where it is an invasive weed. Mitotic and meiotic studies were conducted on Argentinian material; chromosome counts solved the chromosome number controversy, validating 2n = 20. The karyotype was symmetrical, composed of ten pairs of metacentric chromosomes varying from 1.62 to 2.38 μm in length. The most frequent number of satellited chromosomes was three, but their position was difficult to assign. Meiosis was regular, with a configuration of ten predominantly open bivalents. Univalents and quadrivalents were rarely observed. High frequencies of secondary associations of bivalents, chromosome asynchrony and bivalent grouping were documented, reinforcing the hypothesis that x = 5 is the basic chromosome number. Pollen stainability ranged from 94 to 99%. The relevance of chromosomal studies in the circumscription of S. madagascariensis is discussed. Hybridization and polyploidy, as principal evolutionary forces in this genus, explain the systematic difficulties.Facultad de Ciencias Naturales y Muse

South African fireweed Senecio madagascariensis (Asteraceae) in Argentina: Relevance of chromosome studies to its systematics

The systematic identity of Senecio madagascariensis is ratified against the opinion that it is conspecific with Senecio inaequidens. Both species are native to South Africa and have been merged in the 'Senecio inaequidens complex', a group of entities difficult to distinguish from each other. Senecio madagascariensis is widespread in South America and Australia, where it is an invasive weed. Mitotic and meiotic studies were conducted on Argentinian material; chromosome counts solved the chromosome number controversy, validating 2n = 20. The karyotype was symmetrical, composed of ten pairs of metacentric chromosomes varying from 1.62 to 2.38 μm in length. The most frequent number of satellited chromosomes was three, but their position was difficult to assign. Meiosis was regular, with a configuration of ten predominantly open bivalents. Univalents and quadrivalents were rarely observed. High frequencies of secondary associations of bivalents, chromosome asynchrony and bivalent grouping were documented, reinforcing the hypothesis that x = 5 is the basic chromosome number. Pollen stainability ranged from 94 to 99%. The relevance of chromosomal studies in the circumscription of S. madagascariensis is discussed. Hybridization and polyploidy, as principal evolutionary forces in this genus, explain the systematic difficulties.Facultad de Ciencias Naturales y Muse

Chromosome contribution to Andean Polyploid Species of Senecio (Asteraceae), from Argentina

Meiotic chromosome numbers were determined and observations of meiotic behavior were made in six species and two hybrid specimens of Senecio L. (Asteraceae). Two different numbers were found: 2n=4x=40 in S. pogonias Cabr., S. ragonesei Cabr., S. sectilis Griseb. and S. viridis Phil. var. radiatus R. E. Fr. and 2n=8x=80 in S. subulatus D. Don. var. subulatus and in S. uspallatensis Hook. et Arn.; the latter showed the same chromosome number previously reported. The chromosome number of the hybrid individuals was 2n=4x=40, being their putative parents S. ragonesei and S. sectilis. B chromosomes were found in some individuals of S. pogonias, S. sectilis, S. subulatus var. subulatus and S. uspallatensis. Many meiotic irregularities such as bridges, fragments, laggard chromosomes, micronuclei and cellular fusion were observed. The results are analyzed in relation to the persistence and success of these polyploids, and the speciation mechanisms operating within this group are discussed.Facultad de Ciencias Naturales y Muse

Primeros estudios cariotípicos en Mimosa farinosa y M. velloziana (Leguminosae, Mimosoideae)

The first karyotype studies in Mimosa farinosaand M. vellozianaare presented. The chromosome number of M. farinosa, 2n = 2x = 26 (Mimosasection Batocaulonseries Farinosae) is mentioned for the first time, while the chromosome number of M. vellozianavar. velloziana, 2n = 4x = 52 (Mimosasection Mimosaseries Mimosasubseries Mimosa) is confirmed. The formula and parameters of karyotype (chromosome length and A1 and A2 asymmetry indexes) were analyzed. The karyotype formula of M. farinosa(16m+10sm) and M. vellozianavar. velloziana (48m+4sm) were calculated. Mimosa farinosahas a more asymmetric karyotype and larger chromosome size than M. vellozianavar. velloziana. A1 index, total chromosome length, and chromosome length per haploid genome showed significant differences between the two taxa. These results agree with differences previously found among other members of series Farinosae and subseries Mimosa, suggesting that certain karyotype parameters could have cytotaxonomic interest in some groups of species of this genusSe presentan los primeros estudios de cariotipo en Mimosa farinosa y M. velloziana. Se cita por primera vez el número cromosómico de M. farinosa, 2n = 2x = 26 (Mimosa sección Batocaulon serie Farinosae) y se confirma el número cromosómico de M. velloziana var. velloziana: 2n = 4x = 52 (Mimosa sección Mimosa serie Mimosa subserie Mimosa). Se analizó la fórmula y los parámetros del cariotipo (longitud cromosómica e índices de asimetría A1y A2). Se determinaron las fórmulas cariotípicas de M. farinosa (16m+10sm) y de M. velloziana var. velloziana (48m+4sm). M. farinosa posee un cariotipo más asimétrico y mayor tamaño cromosómico que M. velloziana var. velloziana. El índice A1, el largo cromosómico total y el largo cromosómico por genoma haploide exhibieron diferencias significativas entre ambos taxones. Estos resultados concuerdan con diferencias halladas previamente entre otros representantes de la serie Farinosae y la serie Mimosa subserie Mimosa, lo que sugiere que algunos parámetros evaluados tienen interés citotaxonómico en grupos de especies de este género

South African fireweed Senecio madagascariensis (Asteraceae) in Argentina: Relevance of chromosome studies to its systematics

The systematic identity of Senecio madagascariensis is ratified against the opinion that it is conspecific with Senecio inaequidens. Both species are native to South Africa and have been merged in the 'Senecio inaequidens complex', a group of entities difficult to distinguish from each other. Senecio madagascariensis is widespread in South America and Australia, where it is an invasive weed. Mitotic and meiotic studies were conducted on Argentinian material; chromosome counts solved the chromosome number controversy, validating 2n = 20. The karyotype was symmetrical, composed of ten pairs of metacentric chromosomes varying from 1.62 to 2.38 μm in length. The most frequent number of satellited chromosomes was three, but their position was difficult to assign. Meiosis was regular, with a configuration of ten predominantly open bivalents. Univalents and quadrivalents were rarely observed. High frequencies of secondary associations of bivalents, chromosome asynchrony and bivalent grouping were documented, reinforcing the hypothesis that x = 5 is the basic chromosome number. Pollen stainability ranged from 94 to 99%. The relevance of chromosomal studies in the circumscription of S. madagascariensis is discussed. Hybridization and polyploidy, as principal evolutionary forces in this genus, explain the systematic difficulties.Facultad de Ciencias Naturales y Muse

Deep cytogenetics analysis reveals meiotic recombination depletion in species of Senecio (Asteraceae)

Background: Senecio is the largest genus in the Asteraceae family growing in all environments around the world. It displays taxonomic and systematical difficulties. Cytogenetic knowledge of this genus is ancient, scarce and mainly restricted to chromosome number records. Results: In this study we analyzed chromosome number, meiotic configuration, bivalent morphology, meiotic behavior and pollen grain stainability on 100 accessions of 27 different polyploid Senecio L. sect Senecio entities. Median, standard deviation and mode were calculated for number and position of chiasmata and meiotic recombination was statistically evaluated. Although high frequency of multivalents and associated meiotic irregularities are expected in high polyploids, bivalents predominance and, consequently, regular meiosis were observed, with normal sporogenesis and high pollen grain stainability. Conclusion: Depletion in the total chiasmata was significant only in some species but the terminal position was preferential in all the entities analyzed, indicating significant reduction in recombination. The regular meiosis observed suggest that intra and intergenomic reorganization process occur quickly and efficiently in this genus. Mechanisms of diploidization, common to all polyploids, are reinforced by the strong reduction in crossing-over rushing polyploids stabilization.Facultad de Ciencias Naturales y Muse

Chromosome numbers and meiotic studies in species of Senecio (Asteraceae) from Argentina

Meiotic chromosome counts, chromosomal behaviour and meiotic configurations of ten taxa of Senecio from Argentina were examined. Most counts are original: S. crepidifolius DC., S. francisci Phil. and S. octolepis Griseb. var. saltensis (Hicken) Cabrera & Zardini have 2n = 40, and S. chrysolepis Phil., 2n = 80 + 8B. We confirmed previous reports for S. deferens Griseb. (2n = 40 + 4B), S. filaginoides DC. var. filaginoides (2n = 40), S. hieronymi Griseb. (2n = 40 + 7B), S. pampeanus Cabrera (2n = 40) and S. rudbeckiifolius Meyen & Walp. (2n = 40 + 7B). In S. bracteolatus Hook. & Arn. var. bracteolatus, we found a new number (2n = 40) that differs from the one cited previously. In four species, numerical polymorphisms for B-chromosomes were observed. Several of the analysed species presented secondary bivalent association. This phenomenon, together with other evidence, supports x = 5 as the basic chromosome number. The number of chiasmata and their positions were also surveyed, with the finding that open bivalents were the most frequent meiotic figures and terminal chiasmata the preferential position. These features are related to recombination rate, and the success and persistence of these polyploids. We discuss some systematic and evolutionary aspects in the light of cytogenetic data and conclude that polyploidy is the major force modelling the chromosome evolution within this genus. Laboratorio de Etnobotánica y Botánica Aplicad