Cacilda Borges do Valle

Chromosome numbers and microsporogenesis in Brachiaria brizantha (Gramineae)

The genus Brachiaria, native to the African tropical savannas, has achieved significance as a pasture grass in many tropical and subtropical countries, including Brazil. Many species and accessions are polyploid and apomictic, which complicates the improvement of breeding stocks through hybridization. In support of breeding programs, cytogenetic characterization, including chromosome counts and evaluation of the meiotic behavior in the accessions of the Brachiaria has been undertaken at the Embrapa Beef Cattle Center. In this study, 22 accessions of B. brizantha were analyzed of which one was found to be diploid (2n = 2x = 18), 18 were tetraploid (2n = 4x = 36) and three were hexaploid (2n = 6x = 54). The meiotic chromosome behavior was slightly irregular in the diploid and in some tetraploid accessions, and highly irregular in most tetra- and hexaploid accessions. Meiotic abnormalities were those common to polyploidy, i.e., multivalent chromosome association at diakinesis and irregular chromosome segregation leading to micronuclei formation in the tetrad stage. Low frequencies of multivalent chromosome associations among polyploids suggest that they may be segmental allopolyploids.

Brazilian scientific progress in pasture research during the first decade of XXI century

This paper aims to discuss the scientific progress obtained in the past ten years in genetics and plant breeding, soil fertility and plant nutrition and the importance of target sward conditions for planning, controlling and recommending management grazing of tropical grasses. In addition, progress in crop-livestock integration systems and management alternatives for mitigation of greenhouse gas emission by beef cattle production systems, two very important areas related to sustainable production systems, will also be discussed.

Valor nutritivo da forragem e produção animal em pastagens de Brachiaria brizantha

The objectives of this work were to evaluate animal production and its relationship with pasture characteristics of Brachiaria brizantha cultivars Marandu, Xaraes and Piata. The experiment had a randomized complete block design, with three treatments and two replicates. Two-ha paddocks were divided into two and submitted to alternated grazing, with 28 days of grazing and 28 days of rest. Three tester steers were kept in each paddock; additional steers were placed in each paddock by the put and take technique, to assure post grazing residues of about 3 Mg ha-1 of dry matter. The pastures were sampled monthly to estimate the nutritive value of the forage. The animals were weighted, and the stocking rate was adjusted twice a week. Despite the lower average daily gain (ADG), in the cv. Xaraes pasture, the stocking rate was greater, which resulted in greater productivity, in comparison with cvs. Marandu and Piata. The pasture with cv. Piata had the highest ADG. These results indicate that cvs. Xaraes and Piata are new alternatives for Cerrado pastures. The choice of the forage cultivar should fit the animal and production system targets.

Chromosome number and meiotic behaviour in Brachiaria jubata (Gramineae)

The production of new genetic variability through hybridization has been undertaken in Brachiaria, an important forage grass genus of African origin widely used in pastures in the Brazilian tropics. Although the genus comprises about 100 species, only a few of them present favourable agronomic attributes and are explored. In the genus Brachiaria, the majority of species and accessions are polyploid and apomictic, which makes breeding through hybridization more complex. Sexuality has been found in diploids and eventually among tetraploids with normal meiosis. Brachiaria jubata presents good forage value and adaptation to riverbanks and lake margins, and is therefore relevant to pasture systems i n the tropics. Furthermore, the most interesting form of resistance to spittlebugs—antibiosis—was identified in one a ccession of B. jubata. Thus its interest in the Brazilian breeding programme lies in the possibility of using hybridization to transfer high resistance to this insect to susceptible ac cessions. This paper reports chromosome numbers and meiotic behaviour in 21 accessions of this species as a tool in selecting promising ones for crossing. Only one accession was found to be diploid; the others were tetraploid. Microsporogenesis was normal in the diploid accession, while among the tetraploid ones, irregular chromosome segregation leading to unbalanced gamete formation was common but variable. Pollen fertility among tetraploid accessions was hig h. Cytogenetic studies were carried out on accessions of Brachiaria jubatafrom the Brachiaria germplasm collection at Embrapa Beef Cattle kept in the field in Campo Grande (state of Mato Grosso do Sul, Brazil), which comprises 475 accessions of 15 species collected in Africa in the mid-1980s by CIAT (Colombia). Inflorescences were collected and

Multiple spindles and cellularization during microsporogenesis in an artificially induced tetraploid accession of Brachiaria ruziziensis (Gramineae)

The genus Brachiaria is characterized by a majority of polyploid accessions—mainly tetraploid—and apomictic reproduction. Sexuality is found among diploids. To overcome incompatibility barriers, accessions with the same ploidy level are necessarily used in hybridization. Thus, sexual diploid accessions were tetraploidized to be used as female genitors. This paper reports microsporogenesis in an artificially induced tetraploid accession of Brachiaria ruziziensis. Chromosome pairing at diakinesis ranged from univalents to tetravalents, with predominance of bivalents. Irregular chromosome segregation was frequent in both meiotic divisions. During the first division, multiple spindles showing different arrangements were recorded. The spindle position determined the plane of first cytokinesis and the number of chromosomes determined the size of the cell. Meiotic products were characterized by polyads with spores of different sizes. Pollen sterility was estimated at 61.38%. The limitations of using this accession in the breeding program are discussed.

Meiotic behavior in interspecific hybrids between Brachiaria ruziziensis and Brachiaria brizantha (Poaceae)

The meiotic behavior of two half-sib interspecific tetraploid (2n = 4x = 36) promising hybrids, a sexual and an apomictic one, from crosses B. ruziziensis and B. brizantha, was evaluated. Although chromosome paired predominantly as bivalents, a few tri- and quadrivalents were recorded. Results suggest that B. brizantha and B. ruziziensis are closely related and genetic recombination is expected in hybrids. Introgression of specific target genes from B. ruziziensis into B. brizantha and vice-versa may be foreseen. However, abnormalities such as irregular chromosome segregation, chromosome stickiness and abnormal cytokinesis reported in these hybrids affect pollen fertility. More than 65% of pollen grains are sterile. Since the distinctive cytological feature of these hybrids is abnormal cytokinesis, this fact suggests that both parental genomes are unable to coordinate their activities with regard to this cytological phenomenon. Deployment of such hybrids in the process of developing varieties is discussed.

The value of improved pastures to Brazilian beef production

Abstract. Brazil is an agricultural country, with 190 Mha of pastures sustaining 209 million cattle. Fewer than 10% of the cattle are fattened in feedlots, whereas cattle reared on pastures have a competitive advantage for export, eliminating the risks presented by the mad cow disease (bovine spongiform encephalopathy) and considerations related to animal welfare. Brazil has been the world’s largest exporter of beef since 2004 and has the largest commercial herd in the world. In 2011, 16.5% of its production was exported, and the livestock sector contributed 30.4% of the gross national product from agribusiness and 6.73% of the total GNP. Many forage breeding programs, mainly at Embrapa, have contributed to the development of improved pastures, and cultivars of Brachiaria brizantha, B. decumbens, B. humidicola and Panicum maximum are the main pastures used in the country. All have apomictic reproduction, which means there are few cultivars occupying very large, continuous areas, thus suggesting a risk to the productive system. Such is the case of B. brizantha cv. Marandu, which occupies around 50 Mha. The Brazilian tropical forage seed industry is also important, and Brazil is the main seed exporter, supplying all Latin American countries. Due to pasture degradation, around 8 Mha is renovated or recovered each year. Forages are also used and planted each year in integrated crop–livestock and integrated crop–livestock–forest systems. Nowadays, these systems occupy 4 Mha. Improved pastures are thus a major asset in Brazil not only for the beef production chain but also for the dairy industry.

Abnormal timing of cytokinesis in microsporogenesis in Brachiaria humidicola (Poaceae: Paniceae)

Meiosis is controlled by a large number of genes, generally dominant, whose expression is stage-specific, site-specific and time-specific (Gottschalk and Kaul 1974, 1980a,b; Baker et al. 1976; Golubovskaya 1979, 1989). Among genes acting in the meiotic process, those responsible for the partitioning of the cytoplasm after nuclear division play a very important role in the formation of viable gametes. After two rounds of chromosome segregation (karyokinesis) and one simultaneous or two successive cytoplasmic divisions (cytokinesis), the final product of male meiosis in flowering plants emerges as a tetrad of haploid microspores enclosed in a callose wall. The timing of cytokinesis varies among angiosperms. In most monocot plants, cytokinesis is successive, i.e. one partitioning of the cytoplasm occurs after telophase I and another after telophase II, so that there is a distinct dyad stage. However, in most dicots, it is simultaneous and occurs after telophase II (Peirson et al. 1996). Many meiosis mutations affecting the pattern of cytokinesis have been reported in higher plants (Peirson et al. 1996). In Brachiaria, a promising African genus of forage grass for the Brazilian savannas, absence of cytokinesis leading to 2n microspores and binucleated or tetranucleated microspores have been reported in B. brizantha (Risso-Pascotto et al. 2003) and B. nigropedata (Utsunomiya et al. 2005). The present study details meiotic and postmeiotic abnormalities related to cytokinesis observed in one accession of B. humidicola.

Non-radioactive mRNA fingerprinting to visualise gene expression in mature ovaries of Brachiaria hybrids derived from B. brizantha, an apomictic tropical forage

Apomixis is an asexual mode of reproduction by seed, which is found in a number of families of higher plants. During apomictic processes, both meiosis and egg cell fertilisation are omitted making offspring exact genetic replicas of the mother plant. To gain an insight into the molecular basis of apomictic reproduction, gene expression in mature ovaries containing reduced and unreduced embryo sacs was compared in Brachiaria, an apomictic tropical forage grass. Experimental systems for the molecular study of apomixis and sexuality have limitations mainly resulting from numerous differences in the genetic make-up of the plant material. When unrelated to the reproductive behaviour, these may dramatically affect the results if a conventional differential display strategy is applied (comparison made on a one-to-one plant basis). To overcome these limitations, we examined mRNA populations extracted from ovaries collected from pooled samples of various apomictic and sexual plants using the polymerase chain reaction (PCR) differential display and non radioactive cDNA detection procedures. Using this approach, two mRNAs whose expression appeared to be related to the late stages of apomictic reproduction were identified. These results indicate that gene expression in pooled mRNA analysis is a promising tool to elucidate the molecular basis of apomixis.

Estimation of genetic parameters and selection of Brachiaria humidicola progenies using a selection index

Objetivou-se obter parâmetros geneticos para caracteres agronomicos e de valor nutritivo da folha e do colmo de progenies de Brachiaria humidicola e comparar a selecao das melhores por um indice de selecao aditivo considerando-se apenas os caracteres agronomicos e em conjunto com os principais de valor nutritivo. Avaliaram-se 50 progenies do cruzamento cv. BRS Tupi x H31. O delineamento foi em blocos ao acaso com oito repeticoes. Foram realizados sete cortes em intervalos de 30 a 35 dias no periodo de chuvas e dois com intervalos de 60 dias no periodo seco. Observou-se variabilidade genetica entre progenies para os caracteres. As herdabilidades medias de progenies variaram de 49,14% a 75,56% para caracteres agronomicos e de 19,59% a 71,11%, para os de valor nutritivo. Houve coincidencia de nove das dez melhores progenies quando se usou o indice para somente os caracteres agronomicos ou conjuntamente com os principais caracteres de valor nutritivo.