Chromosome number and microsporogenesis of two accessions of Brachiaria dura Stapf (Poaceae) / Número de cromossomos e microsporogênese de dois acessos de Brachiaria dura Stapf (Poaceae)

The two accessions of B. dura analyzed (DU01 and DU02) are hexaploid (2n = 6x = 54), derived from x = 9. Meiotic abnormalities, such as precocious chromosome migration to the poles, laggards and micronuclei, were recorded in low frequency in both accessions. The few multivalent chromosome association at diakinesis and meiotic stability suggested that hexaploidy probably resulted from chromosome doubling. In DU02, chromosome transfer (cytomixis) among meiocytes, involving part or the entire genome was observed. The implication of these findings for the Brachiaria breeding is discussed.

Os dois acessos de B. dura analisados (DU01 e DU02) são hexaplóides (2n = 6x = 54), derivados de x = 9. Anormalidades meióticas como migração precoce de cromossomos para os polos, cromossomos retardatários e micronúcleos foram observados em baixa frequência em ambos os acessos. A presença de poucas associações cromossômicas em diacinese e a estabilidade meiótica sugere que a hexaploidia provavelmente resultou de duplicação cromossômica. No acesso DU02 observou-se transferência de cromossomos (citomixia) entre meiócitos, envolvendo parte ou todo o genoma. As implicações destes resultados para o melhoramento de Brachiaria são discutidas.

Desynapsis and precocious cytokinesis in Brachiaria humidicola (Poaceae) compromise meiotic division.

The forage grass species Brachiaria humidicola is native to African savannas. Owing to its good adaptation to poorly drained and infertile acid soils, it has achieved wide utilization for pastures in Brazilian farms. Among the 55 accessions of B. humidicola analysed from the Embrapa Beef Cattle collection, one (H022), presented desynapsis and an abnormal pattern of cytokinesis in the first meiotic division. Among 28 inflorescences analysed in this accession, 12 were affected by the anomaly. In affected meiocytes, the first cytokinesis occurred in metaphase I and was generally perpendicular to a wide-metaphase plate, dividing the genome into two parts with an equal or unequal number of chromosomes. The normal cytokinesis after telophase I did not occur, and the meiocytes entered metaphase II, progressing to the end of meiosis with the occurrence of the second cytokinesis. As the first cytokinesis occurred precociously, whereas the second was normal, tetrads were formed but with unbalanced chromosome numbers in microspores. Abnormal cytokinesis occurred only in those meiocytes that underwent desynapsis after diakinesis. The implications of this abnormality in the Brachiaria breeding programme are discussed.

Meiotic behaviour in three interspecific three-way hybrids between Brachiaria ruziziensis and B. brizantha (Poaceae: Paniceae).

The meiotic behaviour of three three-way interspecific promising hybrids (H17, H27, and H34) was evaluated. These hybrids resulted from the crosses between B. ruziziensis X B. brizantha and crossed to another B. brizantha. Two half-sib hybrids (H27 and H34) presented an aneuploid chromosome number (2n = 4x = 33), whereas hybrid H17 was a tetraploid (2n = 4x = 36), as expected. Chromosome paired predominantly as multivalents suggesting that genetic recombination and introgression of specific target genes from B. brizantha into B. ruziziensis can be expected. Arrangement of parental genomes in distinct metaphase plates was observed in H27 and H34, which have different male genitors. Hybrids H17 and H34 have the same male genitor, but did not display this abnormality. In H17, abnormalities were more frequent from anaphase II, when many laggard chromosomes appeared, suggesting that each genome presented a different genetic control for meiotic phase timing. Despite the phylogenetic proximity among these two species, these three hybrids presented a high frequency of meiotic abnormalities, mainly those related to irregular chromosome segregation typical of polyploids, H34, 69.1%; H27, 56.1% and H17, 44.9%. From the accumulated results obtained through cytological studies in Brachiaria hybrids, it is evident that cytogenetical analysis is of prime importance in determining which genotypes can continue in the process of cultivar development and which can be successfully used in the breeding. Hybrids with high frequency of meiotic abnormalities can seriously compromise seed production, a key trait in assuring adoption of a new apomictic cultivar of Brachiaria for pasture formation.

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 percent 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 percent 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.

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 por cento 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 por cento 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.

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 percent of meiocytes, bivalents were distributed in two metaphase plates. In anaphase I, two distinct and typical bipolar spindles were formed. In 29.7 percent 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.

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 percent of meiotic products were affected. The use of this hybrid in production systems needing good-quality seeds is discussed.

Chromosome numbers and meiotic behavior of some Paspalum accessions

Chromosome number and meiotic behavior were evaluated in 36 Brazilian accessions of the grass Paspalum (which had never previously been analyzed) to determinate which accessions might be useful in interspecific hybridizations. The analysis showed that one accession of Paspalum coryphaeum was diploid (2n = 2x = 20) and one accession of Paspalum conspersum hexaploid (2n = 6x = 60), the remaining 34 accessions being tetraploid (2n = 4x = 40). The pairing configuration was typical for the ploidy level i.e. in the diploid, chromosomes paired as 10 bivalents, in tetraploids as bi-, tri- and quadrivalents, and in hexaploid as 30 bivalents. A low frequency of meiotic abnormalities (less than 10%) was observed in the diploid, hexaploid and some tetraploid accessions, although the majority of tetraploid accessions showed a high frequency of meiotic irregularities. The use of accessions with a low frequency of meiotic abnormalities in breeding programs is discussed

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.

Cell fusion and chromatin degeneration in an inbred line of maize

A large number of maize genotypes are currently being evaluated to determine the effect of environmental stress caused by the aluminum found in cerrado soils on meiotic behavior. In the present study we report the occurrence of cell fusion followed or not by chromatin degeneration in meiocytes of some plants of an inbred line of maize grown on acid and corrected soil in the Brazilian central plateau. The results suggest the absence of any soil effect on the irregularities observed since the number of affected plants was higher among those grown on corrected soil. The possible causes of these irregularities and their influence on male sterility are discussed.

Número de cromossomos em plantas medicinais: I / Chromosome numbers in medicinal plants: I

The chromosome numbers of 37 species belonging to 31 genus and 14 families were analized. The following results were obtained: a) 14 species presented the same chromosome numbers already reported in the specialized literature; b) species showed chromosome numbers equal to at least one of the previous determinations; c) 1 species presented chromosome number different from the number of all previous reports; d) 5 species had their chromosome number reported for the first time