Meiotic arrest compromises pollen fertility in an interspecific hybrid between Brachiaria ruziziensis x Brachiaria decumbens (Poaceae: paniceae)

Microsporogenesis was analyzed in an interspecific hybrid between an artificially tetraploidized sexual accession of Brachiaria ruziziensis (2n=4x=36) and a natural apomictic tetraploid accession of B. decumbens. Syncytes involving a large number of cells were recorded in 15.4% of meiocytes. Meiosis progressed normally in syncytes during prophase I; in metaphase I, however, several nuclei were found fusioned, showing chromosome stickiness and several chromosome fragments. Meiosis was arrested in metaphase I and pycnotic nuclei and micronuclei were formed. Abnormal cytokinesis fractionated the syncyte into abnormal meiotic products that were covered by the pollen wall. Meiocytes in leptotene were recorded in all the slides prepared for both meiotic divisions, and abnormal "pollen grains" with well-developed pollen wall but containing leptotene nuclei were recorded in 9.18% of grains analyzed. These findings suggested that the meiocytes received the signal to enter meiosis but lacked the signal to proceed beyond leptotene. Despite the absence of the meiotic process, such cells were covered by pollen grain wall. Total pollen sterility resulted from these abnormalities combined with still others observed among meiocytes.<br>A microsporogênese de um híbrido interespecífico entre um acesso sexual tetraploidizado artificialmente de Brachiaria ruziziensis (2n=4x=36) e um acesso apomítico tetraplóide natural de B. decumbens (2n=4x=36) foi analisada. Sincícios envolvendo um grande número de células foram encontrados em 15,40% dos meiócitos. A meiose progrediu normalmente nos sincícios durante a prófase I; em metáfase I, todavia, muitos núcleos fundiram-se, mostrando ainda aderências cromossômicas e inúmeros fragmentos. O processo meiótico foi interrompido na metáfase I, quando a cromatina formou núcleos picnóticos. Citocineses anormais fracionaram os sincícios em produtos meióticos anômalos que foram recobertos pela parede do grão de pólen. Meiócitos em leptóteno também foram observados durante todo o processo meiótico e grãos de pólen anormais com parede de pólen bem desenvolvida, mas contendo núcleos leptotênicos, foram observados em 9,18% dos grãos de pólen analisados. Os resultados sugerem que os meiócitos receberam o sinal para entrar em meiose, mas não receberam o sinal para prosseguir além do leptóteno. Apesar da ausência de processo meiótico completo, os meiócitos foram cobertos pela parede do grão de pólen. Estas anormalidades, combinadas com outras causadas pela poliploidia, resultaram em total esterilidade de pólen

Normal microspore production after cell fusion in Brachiaria jubata (Gramineae)

Cytogenetic studies were carried out on 22 accessions of Brachiaria jubata from the Embrapa Beef Cattle Brachiaria collection. One accession was diploid (2n = 2x = 18) and the remaining 21 were tetraploid (2n = 4x = 36). Among five tetraploid accessions, a specific and constant pattern of cell fusion involving only two microsporocytes was recorded. Meiosis proceeded normally from prophase I to the end, giving rise to an octad with normal microspores that developed into fertile pollen grains. Regular octad formation was possible because each cellular chromosome set was maintained in its proper domain, spindles were correctly positioned, and cytokinesis planes were formed in the correct places. Such behavior of meiosis in syncytes has never been reported in any other plant species

Meiotic behavior in Panicum maximum Jacq. (Poaceae: Panicoideae: Paniceae): hybrids and their genitors = Comportamento meiótico em Panicum maximum Jacq. (Poaceae: Panicoideae: Paniceae): híbridos e seus progenitores

Panicum maximum Jacq. is one of the most cultivated grasses in the world. The intraspecific hybridization breeding program underway at Embrapa Beef Cattle aims at combining several agronomic characteristics in a single plant with high seed production. Four intraspecific hybrids originated from the cross between sexual tetraploid female plants with apomictic tetraploid male plants, including the genitors, were cytologically evaluated in the meiotic process throughout conventional methodology. Hybrids and genitors showed high meiotic stability. The percentage of abnormal cells among them ranged from 6.7 to 14.2%. Abnormalities recorded were irregular chromosome segregation, chromosome stickiness, and absence of the first cytokinesis, but the latter did not compromise pollen viability. Multivalent chromosome configurations at diakinesis, ranging from one to three quadrivalents, suggest that genetic recombination and introgression of some genes can be expected in the hybrids. Based on meiotic stability, the hybrids should be selected for high seed production, and the genitors could remain in the program participating in other intraspecific crosses.Panicum maximum Jacq. é uma das gramíneas mais cultivadas no mundo. O programa de hibridização intraespecífica, desenvolvido pela Embrapa Gado de Corte objetiva combinar diversas características agronômicas em uma única planta com alta produção de sementes. Quatro híbridosintraespecíficos, originados a partir do cruzamento entre fêmeas sexuais tetraploidizadas artificialmente com plantas apomíticas, incluindo os respectivos genitores, foram citologicamente avaliados em seu processo meiótico por meio de metodologia convencional.Híbridos e genitores apresentaram alta estabilidade meiótica, e a percentagem de anormalidades meióticas entre eles variou de 6,7 a 14,2%. As anormalidades encontradas foram segregação irregular de cromossomos, aderências e ausência de citocinese, mas esta última não comprometeu a viabilidade do pólen. Configurações cromossômicas emmultivalentes na diacinese, que variam de um a quatro quadrivalentes, sugerem que a recombinação e a introgressão de alguns genes pode ser esperada em híbridos. Com base na estabilidade meiótica observada, os híbridos poderão ser selecionados para alta produção de sementes, e os genitores podem permanecer no programa de melhoramento participandoem outros cruzamentos intraespecíficos

Abnormal spindle orientation during microsporogenesis in an interspecific Brachiaria (Gramineae) hybrid

This paper reports a case of abnormal spindle orientation during microsporogenesis in an interspecific hybrid of the tropical grass Brachiaria. In the affected plant, prophase I was normal. In metaphase I, bivalents were regularly co-oriented but distantly positioned and spread over the equatorial plate. In anaphase I, chromosomes failed to converge into focused poles due to parallel spindle fibers. As a consequence, in telophase I, an elongated nucleus or several micronuclei were observed in each pole. In the second division, the behavior was the same, leading to polyads with several micronuclei. A total of 40% of meiotic products were affected. The use of this hybrid in production systems needing good-quality seeds is discussed

Cytogenetic evidence for genome elimination during microsporogenesis in interspecific hybrid between Brachiaria ruziziensis and B. brizantha (Poaceae)

Microsporogenesis was analyzed in an interspecific hybrid between an artificially tetraploidized sexual accession of Brachiaria ruziziensis (R genome) and a natural apomictic tetraploid accession of B. brizantha (B genome). Chromosomes associated predominantly as bivalents. From this phase to the end of meiosis, chromosomes presented irregular segregation and abnormal arrangement in the metaphase plate. During metaphase I, in 27.8% of meiocytes, bivalents were distributed in two metaphase plates. In anaphase I, two distinct and typical bipolar spindles were formed. In 29.7% of pollen mother cells, one genome did not divide synchronically, with chromosomes lagging behind or not segregating at all. The second division was very irregular, resulting in polyads. Based on previous results from analysis of a triploid hybrid between these species, where the R genome was eliminated by asynchrony during meiosis, it is suggested that the laggard genome in this hybrid also belongs to B. ruziziensis