Meiotic Arrest Compromises Pollen Fertility in an Interspecific Hybrid Between \u3cem\u3eBrachiaria Ruziziensis\u3c/em\u3e X \u3cem\u3eBrachiaria Decumbens\u3c/em\u3e (Gramineae)

Disruptions in meiosis, development of the free microspores, microspore mitosis, pollen differentiation or anthesis can result in male-sterile plants (Glover et al., 1998). An understanding of the meiotic process is pivotal to work on reproduction, fertility, genetics and breeding in plants, with serious implications in crop production (Armstrong & Jones, 2003). Some African species of Brachiaria are the most important for pastures in the American tropics due to good adaptation and production. Artificial hybridization is underway in Embrapa to improve production, quality and insect resistance (Valle & Miles, 2001). For a cultivar to be successfully adopted good seed production and pollen viability are required. This paper reports on meiotic abnormalities impairing pollen fertility in a hybrid between B. ruziziensis x B. decumbens

Analysis of Genomic Affinity Between \u3cem\u3eBrachiaria Ruziziensis\u3c/em\u3e and \u3cem\u3eB. Brizantha\u3c/em\u3e Through Meiotic Behaviour

Genetic divergence between polyploid hybrids is displayed in chromosome pairing and in the rate of chromosome elimination due to differences in cell cycle between the two combined genomes (Sundberg et al. 1991). In Brachiaria, a genus of African grasses reaching continental proportions as a tropical pasture in Latin America, genome analysis has never been performed. The majority of accessions in this genus is polyploid and apomictic, which restricts breeding. The relative ease of obtaining fertile interspecific hybrids once ploidy barriers are overcome (Pereira et al. 2001) confirms the phylogenetic proximity among B. ruziziensis, B. decumbens and B. brizantha. Hybrids were synthesised using sexual artificial 4x as the female genitor and natural apomictic 4x as the pollen donors. Genome affinity is a pre-requisite for chosen genitors to produce fertile hybrids and plenty of viable seed to assure adoption of the new cultivar. Microsporogenesis of a hybrid between B. ruziziensis and B. brizantha is described in this paper, focusing on the behaviour of both genomes

Comportamento citológico atípico durante a microsporogênese em Brachiaria ruziziensis e B. decumbens.

Em gramíneas forrageiras do gênero Brachiaria, populações poliplóides naturais são abundantes e geralmente apomíticas, enquanto as diploídes, que se reproduzem sexualmente, são raras. Considerando-se a estreita correlação existente entre poloploidia e apomixia neste gênero, e a necessidade de identificação de genótipos sexuais compatíveis para o sucesso de hibridação, conhecimentos de características citológicas básicas como modo de reprodução, comportamento meiótico e nível de ploidia tornam-se essenciais para direcionar os programas de melhoramento. Durante análises desenvolvidas na Universidade Estadual de Maringá, com acessos de várias espécies de uma coleção de germoplasma, mantida na Embrapa Gado de Corte, três acessos, sendo um B. ruziziensis(BRA005568) e dois de B. decumbens (BRA007722 E BRA001058) apresentaram anormalidades meióticas incomuns e foram analisados com especial atenção. Inflorescências em estágio ideal para estudos meióticos foram coletadas, fixadas em uma mistura de etanol 95%, clorofórmio e ácido propiônico (6:3:2) durante 24 horas e armazenadas sob refrigeração. Os microsporócitos foram comportamento citológico atípico durante a microsporogênese em Brachiaria ruziziensis e B. decumbens. Preparados pela técnica de esmagamento e corados com carmim proiônico 0,5%. O acesso de B. ruziziensis (2n=2x=18) apresentou freqüência de produtos meióticos anormais não esperada, com base em sua condição diplóide. A meiose I apresentou baixa freqüência de anormalidades, enquanto na meiose II, a freqüência de anormalidade aumentou de forma inesperada. Alguns cromossomos não conseguiram alcançar a placa equatorial. As células com cromosossomos não alinhados na placa não receberam o sinal para prosseguir em divisão. Tais cromossomos deram origem a núcleos telofásticos de diferentes tamanhos, levando à formação de produtos meióticos anômalos. As observações sugerem a presença de um erro que afeta o "ponto de checagem do fuso", interrompendo a segunda divisão meiótica. Ciclo nucleolar anormal relacionado ao processo de desorganização do nucléolo e nuclologênese foi observado no acesso BRA007722 de B. decumbens (12n=4x=36). O acesso BRA001058de B. decumbens cv Basilisk (2n=4x=36) apresentou duas formas distintas de anormalidades quando coletado em diferentes épocas. Em uma das épocas, os micronúcleos formados na primeira divisão organizaram fusos na segunda divisão e sofreram citocineses adicionais que levaram à formação de micrócitos de diferentes tamanhos. Na outra época de coleta, algumas plantas apresentaram uma anormalidade que afetou o desenvolvimento do gametófito masculino. As mioses do grão de pólen não seguiram o padrão normal, comprometendo a formação das células vegetativa e generativa. As anormalidades encontradas no gênero Brachiaria representam um material biológico ímpar para o entendimento do controle genético da meiose. Algumas delas podem representar, também, ferramentas úteis para programa de melhoramento.bitstream/item/104572/1/Comportamento-citologico-atipico.pd

Microsporogenesis in Brachiaria bovonei (Chiov.) Robyns and B. subulifolia (Mez) Clayton (Poaceae)

Some African species of Brachiaria have been introduced into the Americas and became the most important forage for pastures in the tropics. New cultivars can be obtained either from direct selections from the natural existing variability in the germplasm collections or from interspecific hybridizations. Polyploidy is predominant in the genus Brachiaria and correlated with apomixis which complicates hybridization. The objective of cytological studies underway on the Brachiaria germplasm collection at Embrapa Beef Cattle is to determine the chromosome number and meiotic behavior of accessions. For the breeding of Brachiaria species, compatible sexual and apomictic accessions need to be identified. Microsporogenesis was evaluated in two accessions of Brachiaria bovonei (BO01 and BO05) and one accession of B. subulifolia (SU01). BO01 is pentaploid (2n = 5x = 45), BO05 is tetraploid (2n = 4x = 36), and SU01 is hexaploid (2n = 6x = 54), derived from x = 9. Meiotic abnormalities typical of polyploids, characterized by precocious chromosome migration to the poles in metaphases, laggard chromosomes in anaphases, and micronuclei in telophases and tetrads, were recorded in high frequency in all the accessions generating unbalanced gametes. Both accessions of B. bovonei presented chromosome stickiness. The results are discussed in the view of the Brachiaria breeding program objectives

Symmetric pollen mitosis I and suppression of pollen mitosis II prevent pollen development in Brachiaria jubata (Gramineae)

Microsporogenesis and pollen development were analyzed in a tetraploid (2n = 4x = 36) accession of the forage grass Brachiaria jubata (BRA 007820) from the Embrapa Beef Cattle Brachiaria collection that showed partial male sterility. Microsporocytes and pollen grains were prepared by squashing and staining with 0.5% propionic carmine. The meiotic process was typical of polyploids, with precocious chromosome migration to the poles and laggards in both meiosis I and II, resulting in tetrads with micronuclei in some microspores. After callose dissolution, microspores were released into the anther locule and appeared to be normal. Although each microspore initiated its differentiation into a pollen grain, in 11.1% of them nucleus polarization was not observed, i.e., pollen mitosis I was symmetric and the typical hemispherical cell plate was not detected. After a central cytokinesis, two equal-sized cells showing equal chromatin condensation and the same nuclear shape and size were formed. Generative cells and vegetative cells could not be distinguished. These cells did not undergo the second pollen mitosis and after completion of pollen wall synthesis each gave rise to a sterile and uninucleate pollen grain. The frequency of abnormal pollen mitosis varied among flowers and also among inflorescences. All plants were equally affected. The absence of fertile sperm cells in a considerable amount of pollen grains in this accession of B. jubata may compromise its use in breeding and could explain, at least in part, why seed production is low when compared with the amount of flowers per raceme

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