Cytogenetic studies carried out in the tetraploid accession BRA001068 of Brachiaria decumbens, also known as cv. Basilisk, revealed an unusual pattern of microsporogenesis. The spindle in metaphase I and anaphase I became heavily stained with propionic carmine. In telophase I, the interzonal microtubules continued to be intensely stained, and during the phragmoplast formation the fibers were pushed to the cell wall, persisting until prophase II, even after cytokinesis. Due to its tetraploid condition, the accession presented many cells with precocious chromosome migration to the poles in metaphase I and laggards in anaphase I that gave rise to micronuclei in telophase I. While in other polyploid accessions of Brachiaria micronuclei remained in this condition until the second cytokinesis, the micronuclei in this accession organized their own spindle in the second division. In several microsporocytes, the micronuclei with their minispindle were divided further into microcytes by additional cytokinesis. Some curious planes of cytokinesis were found in some cells, with partitioning of cytoplasm into cells of irregular shape. The result consisted of a high frequency of abnormal products of meiosis. Quadrivalents were observed in diakinesis at low frequency, which suggests a segmental allotetraploid and the inability of both genomes to co-ordinate their activities, leading to multiple spindle and precocious cellularization. In spite of abnormal meiotic products reducing pollen fertility, seed production was normal. Enough normal pollen was available to fertilize the central-cell nucleus of the embryo sac and produce normal endosperm in this pseudogamous aposporous apomictic accession.

Among 25 accessions of Brachiaria brizantha in the Embrapa Beef Cattle collection, one accession (GC 1113/95) presented a severe case of chromosome stickiness in meiosis, impairing normal chromosome segregation. Accession was tetraploid (2n =4x=36) with chromosomes pairing in bivalents and few quadrivalents at diakinesis. Stages of prophase I were normal and chromosome stickiness became evident from metaphase I persisting to microspore stage. Bridges of different thickness were formed in anaphase I and II by chromosomes that did not separate. Some of them even persisted until telophase stages. The precise causes of chromosome stickiness could not be ascertained, but genetic factors might be controlling the phenomenon, since only this accession cultivated on Brazilian savannas under the same environmental conditions of the 24 other accessions presented the abnormality.

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Meiotic division and male gametophyte development were analyzed in one tetraploid (2n = 4x = 36) accession of Brachiaria decumbens cv. Basilisk that showed some pollen sterility. Meiotic process was typical of polyploids in that it consisted of multiple chromosome associations. Precocious chromosome migration to the poles, laggards, and micronucleus formation were abundant in both meiosis I and II and resulted in tetrads with micronuclei. After callose dissolution, microspores were released into the anther locule and had the semblance of being normal. Although each microspore initiated its differentiation by pollen mitosis, in 43.24% of the microspores, nuclear polarization was not observed and the typical hemispherical cell plate was not detected. Division was symmetric and microspores lacked differentiation between the vegetative and the generative cell. Both nuclei were of equal size, presented equal chromatin condensation, and had a spherical shape. After the first pollen mitosis and cytokinesis, each cell underwent a new symmetric mitosis without nuclear polarization. At the end of the second pollen mitosis, four equal nuclei were observed in each pollen grain. After the second cytokinesis, the cells gave rise to four equal-sized pollen grains with a similar tetrad configuration that initially remained together. Sterile pollen grains resulted from abnormal pollen mitosis. This anomaly may be explained by a mutation, probably affecting microtubule cytoskeleton formation. The importance of this male-sterile mutation for Brachiaria breeding programs is discussed.

The genus Centrosema includes several species both potentially and currently useful as pasture and fodder crops in tropical and subtropical areas. Genetic variability, population genetic structure, and the mating systems of three species of this genus, C. acutifolium, C. braslllanum, and C. pubescens have been studied using isozyme (17 loci) and DNA markers (random amplified polymorphic DNA and rDNA intergenlc spacer length polymorphism). Both kinds of markers discriminate populations belonging to different species. Outcrossing rates and other experimental parameters have been determined in populations of an active germplasm collection outside the species native habitat. Populations of all three species showed relatively high polymorphism and heterozygosity, and total genetic diversity was higher within than between populations. The mating system parameters indicated that outcrossing was higher than expected in the three species, previously considered as being predominantly selfers. The multllocus analysis carried out In C. acutifolium provided estimations of the outcrossing rate ranging from 0.27 to 0.41 among populations; estimations carried out from the fixation Index In the other two species also suggested relatively high outcrossing rates. These species should be considered species with mixed mating systems, and this fact should be taken Into consideration In further breeding programs and germplasm conservation.

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Isozyme patterns and their genetic control in three Centrosema species are described. Seven isozymatic systems (aspartate aminotransferase, glucose-6-phosphate isomerase, phosphoglucomutase, anodal peroxidase, malate dehydrogenase, 6-phosphogluconate dehydrogenase, and isocitrate dehydrogenase) were studied in 18 populations and several breeding lines of C. acutifolium, C. brasilianum and C. pubescens, using starch gel electrophoresis techniques. All systems, except glucose-6-phosphate isomerase, are described for the first time in these species. A total of 17 isozyme loci were scored; this represents the largest set of Mendelian loci known up to now in Centrosema species. Isozyme polymorphism and variability within and between populations and species were relatively high and allowed discrimination among species
Padrões isoenzimáticos e seus respectivos controles genéticos foram descritos para três espécies de Centrosema, C. acutifolium, C. brasilianum e C. pubescens. As avaliações foram feitas utilizando-se a técnica de eletroforese horizontal em gel de amido e envolveram 18 populações em estado natural e várias outras selecionadas. Os sete sistemas isoenzimáticos analisados foram: aspartato aminotransferase, fosfoglicose isomerase, fosfoglicomutase, peroxidase anódica, malato desidrogenase, 6-fosfogliconato desidrogenase e isocitrato desidrogenase. Todos os sistemas, exceto fosfoglicose isomerase, sao descritos pela primeira vez em Centrosema. Detectou-se um total de 17 loci isoenzimáticos, representando o maior conjunto de loci mendelianos conhecidos, até o momento, para o gênero Centrosema. O polimorfismo isoenzimático e a variabilidade genética observados entre e dentro de populações e espécies foram relativamente altos. Os padrões isoenzimáticos descritos são muito eficientes na discriminação das espécies estudada

Com o objetivo de identificar espécies forrageiras adaptadas a condições de má drenagem, foram avaliados seis acessos de Paspalum e quatro de leguminosas. O experimento foi conduzido por três anos consecutivos na Fazenda Modelo, município de Terenos (MS), em área de campo cerrado, recém-desmatado, sobre um Plintosolo mal drenado. As produções de matéria verde e seca foram avaliadas por cortes, efetuados às três, seis, nove e 12 semanas posteriores à uniformização das parcelas, em épocas representativas das estações de seca e águas de cada ano experimental. As leguminosas não sobreviveram às condições do experimento e, entre as gramíneas, destacou-se o acesso BRA 010707 de Paspalum guenoarum, que é indicado para estudos posteriores visando ao lançamento comercial.