RESUMEN
Paspalum is a noteworthy grass genus due to the forage quality of most species, with approximately 330 species, and the high proportion of those that reproduce via apomixis. Harnessing apomictic reproduction and widening knowledge about the cytogenetic relationships among species are essential tools for plant breeding. We conducted cytogenetic analyses of inter- and intraspecific hybridisations involving a sexual, colchicine-induced autotetraploid plant of P. plicatulum Michx. and five indigenous apomictic tetraploid (2n = 40) species: P. compressifolium Swallen, P. lenticulare Kunth, two accessions of P. nicorae Parodi, P. rojasii Hack. and two accessions of P. plicatulum. Fertility of the hybrids was investigated and their reproductive system was analysed considering the relative embryo:endosperm DNA content from flow cytometry. Morphological, nomenclatural and taxonomic issues were also analysed. Cytogenetic analysis suggested that all indigenous tetraploid accessions of five apomictic species are autotetraploid or segmental allotetraploid. If segmental allotetraploids, they probably originated through autoploidy followed by diploidisation processes. Autosyndetic male chromosome pairing observed in all hybrid families supported this assertion. Allosyndetic chromosome associations were also observed in all hybrid families. In the hybrids, the proportion of male parent chromosomes involved in allosyndesis per pollen mother cell varied from 5.5% to 35.0% and the maximum was between 25% and 60%. The apomictic condition of the indigenous male parents segregated in the hybrids. These results confirm a strong association between autoploidy and apomixis in Paspalum, and the existence of cytogenetic relationships between different species of the Plicatula group. Allosyndetic chromosome pairing and seed fertility of the hybrids suggest the feasibility of gene transfer among species.
Asunto(s)
Hibridación Genética/genética , Paspalum/genética , Poliploidía , Tetraploidía , Cromosomas de las Plantas/genética , Citogenética , Meiosis , Paspalum/anatomía & histología , Paspalum/clasificación , Filogeografía , FitomejoramientoRESUMEN
Trichloris crinita is a perennial forage grass species native to arid regions of the American continent. Due to its extensive area of distribution, good forage quality and resistance to drought and grazing, this species is widely utilised as forage and for revegetation purposes in environments with low water availability. Despite its importance, genetic improvement of T. crinita has been very limited, partly as consequence of the lack of knowledge on its mode of reproduction. In the present work, we studied the reproductive biology of T. crinita by means of embryological analyses, flow cytometric seed screen (FCSS), self-compatibility tests and progeny testing with morphological and molecular markers. Cytological analyses revealed embryo sacs with eight nuclei and of Polygonum type for all T. crinita accessions analysed. FCSS histograms exhibited two clear peaks corresponding to 2C and 3C DNA content, indicating embryo sacs of sexual origin. Controlled pollination experiments designed to evaluate seed set (%) demonstrated that T. crinita is self-compatible, whereas results from morphological and simple sequence repeat (SSR) marker analysis of progeny revealed lack of outcrossing. Together, these results indicate that T. crinita reproduces sexually. It is a self-compatible and autogamous species. It is expected that these data will have a positive impact in the genetics and breeding of this species, and therefore contribute to its proper utilisation in arid regions.
Asunto(s)
Poaceae/fisiología , Semillas/citología , Animales , Citometría de Flujo , Flores/fisiología , Heterocigoto , Endogamia , Repeticiones de Microsatélite , Poaceae/genética , Polinización , Semillas/fisiología , AutofecundaciónRESUMEN
It is generally accepted that polyploids have downsized basic genomes rather than additive values with respect to their related diploids. Changes in genome size have been reported in correlation with several biological characteristics. About 75 % of around 350 species recognized for Paspalum (Poaceae) are polyploid and most polyploids are apomictic. Multiploid species are common with most of them bearing sexual diploid and apomictic tetraploid or other ploidy levels. DNA content in the embryo and the endosperm was measured by flow cytometry in a seed-by-seed analysis of 47 species including 77 different entities. The relative DNA content of the embryo informed the genome size of the accession while the embryo:endosperm ratio of DNA content revealed its reproductive mode. The genome sizes (2C-value) varied from 0.5 to 6.5 pg and for 29 species were measured for the first time. Flow cytometry provided new information on the reproductive mode for 12 species and one botanical variety and supplied new data for 10 species concerning cytotypes reported for the first time. There was no significant difference between the mean basic genome sizes (1Cx-values) of 32 sexual and 45 apomictic entities. Seventeen entities were diploid and 60 were polyploids with different degrees. There were no clear patterns of changes in 1Cx-values due to polyploidy or reproductive systems, and the existing variations are in concordance with subgeneric taxonomical grouping.
Asunto(s)
ADN de Plantas/genética , Diploidia , Paspalum/clasificación , Paspalum/fisiología , Poliploidía , Cromosomas de las Plantas/genética , Citometría de Flujo , Paspalum/genética , Reproducción/genética , Especificidad de la EspecieRESUMEN
The objective of this work was to determine the type of inheritance (disomic/polysomic) in tetraploid (2 n=4 x=40) Paspalum notatum and investigate the transmission pattern of the chromosome region associated with apospory. An F(1) family segregating for the reproductive mode (aposporous vs non-aposporous) was generated by crossing a tetraploid sexual plant as female parent with an apomictic individual as pollen donor. Pollen mother cells from both parental plants were examined to ascertain chromosome-pairing behavior at meiosis. The high rate of quadrivalent chromosome associations indicated an autotetraploid origin of the species, although bivalent pairing and occasional univalents were detected. The observation of a lagging bivalent, a bridge of chromatin, or two aligned laggards in the aposporous parent suggested a chromosome inversion in this strain. Segregation ratios of AFLP markers and the proportion of linkages in repulsion versus coupling phase denoted tetrasomic inheritance, but markers displaying disomic ratios were also observed. Preferential chromosome pairing (disomic inheritance) in the chromosome segment related to apospory was detected. The possible relationship between a chromosome rearrangement and the inheritance of apospory is discussed.
Asunto(s)
Emparejamiento Cromosómico/genética , Segregación Cromosómica/genética , Patrón de Herencia/genética , Paspalum/genética , Ploidias , Polen/genética , Alelos , Mapeo Cromosómico , Cruzamientos Genéticos , Marcadores Genéticos , Meiosis/genética , Técnica del ADN Polimorfo Amplificado Aleatorio , Reproducción/genéticaRESUMEN
Progeny tests employing molecular markers allow the identification of individuals originated by sexual means among the offspring of a facultative apomict. The objective of this work was to evaluate the effect of the pollination timing on the proportion of sexually formed individuals in progenies of a facultative apomictic Paspalum notatum genotype. Progeny families of approx. 30 plants each were generated at five different pollination times: 1-3 d pre-anthesis; at anthesis; and 2, 4 and 6 d post-anthesis. Cytoembryological analyses indicated that approx. 17% of the ovules carried a meiotic cytologically reduced embryo sac in florets formed simultaneously with those used for crosses. The parental plants and the five F1 families were analysed using RAPD molecular markers. Ninety-five oligonucleotides were assayed on the progenitors in order to search for male-specific bands. Eight primers presenting clear polymorphic bands were selected for use in the progeny tests. The proportion of sexually produced progeny reached 3.4% before anthesis and 20 % at anthesis, while pollination after anthesis generated only maternal plants. A second progeny of 97 plants obtained from pollination at anthesis produced 16 off-type plants (16.5%), of which only one was a B(III) hybrid (2n + n). Our results indicate that pollination at anthesis allows the greatest potential for sexuality to be expressed in this facultative apomictic genotype. When pollination is delayed as soon as 2 d after anthesis, only the aposporous sacs develop endosperm through pseudogamy to set seed.
Asunto(s)
Poaceae/genética , Polen/fisiología , Técnica del ADN Polimorfo Amplificado Aleatorio/métodos , Cruzamientos Genéticos , Dermatoglifia del ADN/métodos , Marcadores Genéticos , Genotipo , Meiosis/genética , Meiosis/fisiología , Mitosis/genética , Mitosis/fisiología , Tallos de la Planta/genética , Tallos de la Planta/fisiología , Poaceae/fisiología , Poliploidía , Reproducción/fisiología , Semillas/citología , Semillas/embriología , Factores de TiempoRESUMEN
Paspalum notatum is a subtropical forage grass, which reproduces by either sexuality or aposporous gametophytic apomixis. The objective of this work was to identify and isolate mRNA transcripts differentially expressed during the development of the megagametophyte from spikelets of apomictic and sexual P. notatum. Crossing of a sexual mother plant with an apomictic pollen donor generated a progeny family segregating for reproductive mode. Individuals from this F1 family were cytoembryologically classified as sexual or apomictic. Spikelet mRNA compositions from both groups of plants were compared by differential display using an RNA-bulked procedure. Fifty primer combinations were assayed to generate nearly 2,500 total bands in the fingerprints. Three transcripts expressed at higher levels in apomictic plants (apo417, apo398, and apo396) were identified, isolated and cloned. Sequencing showed a high level of homology among the isolated clones. Analysis by RT-PCR Southern blots followed by densitometric studies confirmed that expression reached a level around 30 times higher in apomictic than in sexual individuals and was probably induced at early stages of the megagametophyte development. Genomic DNA from the parental and the F1 progeny plants showed 4-5 bands when hybridised with apo417 in Southern blots. Comparisons to the sequence data banks revealed no identities to genes of known function. However, a putative deduced 3' protein fragment showed homology to the well-characterised KSP multi-phosphorylation domain previously detected in several cdc2-regulated cytoskeletal proteins.
Asunto(s)
ADN Complementario/aislamiento & purificación , Regulación de la Expresión Génica de las Plantas , Poaceae/genética , ARN Mensajero/genética , ARN de Planta/genética , Secuencia de Aminoácidos , Secuencia de Bases , Northern Blotting , Southern Blotting , Cruzamientos Genéticos , ADN Complementario/genética , Perfilación de la Expresión Génica , Datos de Secuencia Molecular , Proteínas de Plantas/genética , Ploidias , Poaceae/embriología , Poaceae/fisiología , ARN Mensajero/biosíntesis , ARN de Planta/biosíntesis , Reproducción , Reacción en Cadena de la Polimerasa de Transcriptasa Inversa , Semillas , Alineación de Secuencia , Homología de Secuencia , Transcripción GenéticaRESUMEN
Previous studies on the inheritance of aposporous apomixis in bahiagrass showed a wide range of segregation ratios in crosses involving sexual and aposporous apomictic plants. The F1 progenies were classified through a visual progeny test carried out on few F2 plants. The number of sexual F1s highly exceeded the apomictics leading to the conclusion that apomixis was controlled by a few recessive genes. The present study examines the inheritance of apospory in bahiagrass. A sexual plant was self-pollinated and crossed with an aposporous apomictic plant as pollen donor. Backcross and F2 progenies were obtained in several combinations. All self-pollinated sexual plants or sexual x sexual crosses produced progenies free of apospory. All crosses involving a sexual and an apomictic plant produced approximately three times more apospory-free plants than plants with apospory. Bahiagrass is of autotetraploid origin and hence is expected to display tetrasomic inheritance. The most widely accepted genetic model for inheritance of apospory in tropical grasses is a single dominant gene with tetrasomic inheritance. In the present experiments none of the apospory-free F1s segregated for the apospory trait indicating that it is most likely a dominant character. However, the observed results fit better a modified model: tetrasomic inheritance of a single dominant gene with pleiotropic effect and incomplete penetrance. The excess of apospory-free plants in the F1 progeny could be ascribed to some distortion in the segregation pattern due to a pleiotropic lethal effect of the dominant A allele with incomplete penetrance. Alternatively, partial lethality of factors linked to aposporous gene may account for segregation distortion against apospory.