Irregular Chromosome Segregation Research Articles

Morphological characterization and cytological studies of the green-stemmed and the red-stemmed Basella alba

Basella alba is an underutilized vegetable with ethnobotanical importance used for culinary as well as medicinal purposes in many parts of the world. Morphological characterization and chromosome studies of the mitotic and meiotic cells of the green stemmed and the red-stemmed B. alba was carried out with a view to filling the knowledge gaps that exist in their morphological characterization and also to provide insightful information on their chromosome numbers and meiotic behaviour. The B. alba accessions studied were characterized with respect to their habit, leaves, inflorescence, fruits and seeds. Mitotic and meiotic studies were carried out on the Basella accessions using standard techniques. The morphological studies revealed significant differences between the green-stemmed and red-stemmed Basella alba with respect to the green/red colour of their stems, colour of the flower bud apex, mean plant height at flower bud initiation, mean leaf length, mean leaf width, mean petiole length, mean flower per spike, mean fruit length and mean fruit diameter. The cytological study revealed a chromosome number of 2n=4x=44 for both the green-stemmed and the red-stemmed B. alba studied. It also revealed the occurrence of chromosomal aberrations such as stickiness and precocious migration of chromosomes to the poles during meiosis, which could lead to irregular chromosome segregation that could result in chromosome instability and aberrant meiotic products within the species.

Tetrasomic inheritance pattern of the pentaploid Solanum chacoense (+) S. tuberosum somatic hybrid (resistant to bacterial wilt) revealed by SSR detected alleles

Protoplast fusion may be used to introgress desirable traits from wild species into cultivated potato which are cross incompatible. However, none of commercial varieties have been developed from hyper-tetraploid somatic hybrids because of the unpredictable heredity associated with irregular chromosome segregation during meiosis. A pentaploid somatic hybrid (SH) from Solanum chacoense + S. tuberosum, 3C28-1, which carries bacterial wilt (BW) resistance from the wild species, was employed in the present research. Gamete genotypes and inheritance pattern of SH 3C28-1 were estimated by counting the chromosome numbers and detecting the parent-specific SSR alleles in a backcross population of SH 3C28-1 × E-Potato 1 (variety) which is BW susceptible. The SSR alleles retained from S. chacoense were analyzed for association with BW resistance and evaluation of wild species genetic background. Our results implied that SH 3C28-1 could produce viable gametes of n = 2x = 24 + a where a ≤ 6, which are compatible with a tetraploid pollen source to yield a high proportion of tetraploids and aneuploids with chromosome numbers between 48 and 54. Sixty-four of 83 SH 3C28-1-specific SSR alleles fit the tetrasomic or disomic/tetrasomic segregation ratio of a tetraploid, suggesting that the pentaploid mainly exhibits tetraploid inheritance pattern. Four (three positive and one negative) alleles were estimated to associate with BW resistance and backcross clones were selected for preserving less wild species genetic background and harboring the resistance. The present research elucidates the inheritance pattern of the pentaploid somatic hybrid to provide a theoretical and applied foundation for use of the pentaploid in potato breeding.

Dynamic karyotype evolution and unique sex determination systems in Leptidea wood white butterflies.

BackgroundChromosomal rearrangements have the potential to limit the rate and pattern of gene flow within and between species and thus play a direct role in promoting and maintaining speciation. Wood white butterflies of the genus Leptidea are excellent models to study the role of chromosome rearrangements in speciation because they show karyotype variability not only among but also within species. In this work, we investigated genome architecture of three cryptic Leptidea species (L. juvernica, L. sinapis and L. reali) by standard and molecular cytogenetic techniques in order to reveal causes of the karyotype variability.ResultsChromosome numbers ranged from 2n = 85 to 91 in L. juvernica and 2n = 69 to 73 in L. sinapis (both from Czech populations) to 2n = 51 to 55 in L. reali (Spanish population). We observed significant differences in chromosome numbers and localization of cytogenetic markers (rDNA and H3 histone genes) within the offspring of individual females. Using FISH with the (TTAGG)n telomeric probe we also documented the presence of multiple chromosome fusions and/or fissions and other complex rearrangements. Thus, the intraspecific karyotype variability is likely due to irregular chromosome segregation of multivalent meiotic configurations. The analysis of female meiotic chromosomes by GISH and CGH revealed multiple sex chromosomes: W1W2W3Z1Z2Z3Z4 in L. juvernica, W1W2W3Z1Z2Z3 in L. sinapis and W1W2W3W4Z1Z2Z3Z4 in L. reali.ConclusionsOur results suggest a dynamic karyotype evolution and point to the role of chromosomal rearrangements in the speciation of Leptidea butterflies. Moreover, our study revealed a curious sex determination system with 3–4 W and 3–4 Z chromosomes, which is unique in the Lepidoptera and which could also have played a role in the speciation process of the three Leptidea species.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0375-4) contains supplementary material, which is available to authorized users.

Meiotic behavior and chromosome number of Urochloa adspersa (Trin.) R. D. Webster from the Brazilian Chaco.

This is the first report of meiotic division in Uro-chloa adspersa (Trin.) collected from the Brazilian Chaco. Meiotic analyses were performed on three specimens of U. adspersa named G10, G15, and G16. Inflorescences were collected and fixed in a mixture of ethanol and acetic acid (3:1, v/v) for 24 h and then stored in 70% alcohol. Diakinesis revealed different chromosome numbers and ploidy levels. All three plants were polyploids: G10 and G15 exhibited 2n = 6x = 54 chromosomes (arranged in 27 bivalents), while G16 exhibited 2n = 4x = 36 chromosomes (18 bivalents). Meiotic behavior was mainly normal in the hexaploid G15 and the tetraploid G16 (5.3 and 6.2% of the cells were abnormal, respective-ly), revealing only a few meiotic abnormalities that are common to polyploids, i.e., those related to irregular chromosome segregation. G10 exhibited other meiotic abnormalities during meiosis II, such as chromosome stickiness, irregular spindle orientation, and irregular cytokinesis, which led to the formation of a few triads, resulting in 16.9% of the cells being abnormal. The origin of these abnormalities is discussed, and we suggest that the genes that control meiotic steps may be present in the Urochloa gene pool.

290 3-DIMENSIONAL VISUALIZATION OF BOVINE OOCYTE FERTILIZATION BY CONFOCAL LASER SCANNING MICROSCOPY

Cattle can serve as a model organism to resolve central questions in mammalian reproduction that cannot be clarified in the mouse model due to notable species-specific peculiarities of early mouse embryogenesis. As part of a project on structural, molecular, and functional deficiencies of bovine oocytes, we started to systematically investigate fertilization and the onset of embryo development in vitro by 3-dimensional multicolor fluorescence microscopy. We are using 3D visualisation as key approach to clarify the multiple parallel and sequential processes and events of fertilization as well as to identify and classify errors and failures. Moreover, we aim to gain insights into the mechanisms of aberrations by linking processes at the cellular and the molecular level. We studied class I and II oocytes collected from slaughterhouse ovaries and matured for 23 h in vitro. Oocytes were fixed at different times from 4 to 12 h postinsemination with formaldehyde in a microtubule-stabilising buffer containing taxol in such a way that the 3-dimensional cell architecture was maintained, and were stained for DNA, microtubules, and f-actin microfilaments. In addition, serine 10-phosphorylated histone H3 was used as a marker for chromosome condensation and the spindle midbody. For 3-dimensional imaging of the oocytes in toto, confocal serial sections were captured at 1-µm intervals using a 40× objective (NA = 1.3). For imaging details, we used a high spatial sampling density (pixel size: 50 × 50 nm, z-step size: 200 nm) close to the Nyquist criterion and image restoration by maximum likelihood estimation (MLE) deconvolution. A series of more than 500 three-dimensional snapshots of fertilized oocytes at different points in time gives a first detailed view on the spatial and temporal course of the sperm entry, the formation of the paternal pronucleus and the sperm aster, completion of oocyte meiosis and the formation of the maternal pronucleus, as well as dynamic changes of the cytoskeleton. Moreover, we can document a spectrum of abnormalities including spontaneous parthenogenetic oocyte activation, polyspermy, and aberrations of meiosis I and II. The latter include irregular spindle formation and chromosome segregation, the occurrence of chromatin bridges and abnormal spindle positioning and rotation (e.g. leading to nonextrusion of a first or a second polar body or the extrusion of two second polar bodies). Our microscopic investigation in the bovine system contributes to unraveling the origins of irregular cleavage, aneuploidy, and mosaicism in mammals. Three-dimensional high-speed microscopy of oocytes and zygotes in affordable timeframes could be of great value in improving the differential diagnosis of oocyte and sperm dysfunction, as well as in identifying and dissecting problems, limitations, and potential risks of reproductive technologies (ART).This work is supported by the Deutsche Forschungsgemeinschaft (DFG FOR 1041).

104 3-D VISUALIZATION OF THE FIRST CLEAVAGE AND ABERRATIONS OF BOVINE ZYGOTES BY CONFOCAL LASER SCANNING MICROSCOPY

A high proportion of bovine oocytes fertilized in vitro fail to develop beyond the first 4 cleavage cycles. The first mitotic division of the zygote and proper segregation of chromosomes and cytoplasmic components seems to be a particularly delicate task. Notably, zygotes cleaving with a delay of only a few hours seem to have a very low chance of developing to the blastocyst stage. But what exactly goes wrong, how often, and why? To answer such questions we have to visualize in greater detail basic structures and processes such as the sperm aster, DNA replication, migration and apposition of the 2 pronuclei, synchronous chromosome condensation and breakdown of the nuclear envelopes, assembly of the first mitotic spindle and chromosome congression, anaphase, and cytokinesis. Oocytes fertilized in vitro were fixed at different time points around the first cleavage and stained for DNA, Ser10-phosphorylated histone H3, microtubules, and microfilaments. Zygotes were imaged in toto by recording confocal serial sections at 1-µm intervals using a 40× objective (NA = 1.3). Details were recorded with high spatial sampling densities (pixel size 50 × 50 nm, z-step size of 200 nm) close to the Nyquist criterion and restored by maximum likelihood estimation deconvolution using the real point spread function. We present a series of 3-D confocal images captured at different stages of the first cleavage. The images reveal new insights into the formation, structure, and function of the first mitotic spindle and the occurrence of spindle aberrations, irregular chromosome segregation, and abnormal cytokinesis. The microscopic findings guide us to candidate proteins for localization analyses and functional studies based on 3-D fluorescence live-cell imaging of zygotes and early embryos. This work is supported by the Deutsche Forschungsgemeinschaft (DFG FOR 1041).

Meiotic behavior in apomictic Brachiaria ruziziensis × B. brizantha (Poaceae) progenies

Hybrids combining desirable traits from divergent parents are the main objective of some Brachiaria (Syn. Urochloa P. Beauv.) breeding programs. There is great interest in the development of apomictic hybrid cultivars that combine desirable genes such as resistance to spittlebugs, high nutritive value, and tolerance to acid soils. Microsporogenesis of six apomictic progenies resulting from a tetraploid (2n = 4x = 36) cross between B. ruziziensis × B. brizantha was evaluated under light microscopy. Genetic recombination, ensured by multivalent chromosome association and crossing-over at prophase occurred in low frequency among progenies, and in one, recombination was almost nonexistent. The percentage of meiocytes with meiotic abnormalities among progenies ranged from 16.6 % to 85.6 %. Besides an observed irregular chromosome segregation typical of polyploid hybrids in these five progenies, putative meiotic mutations characterized as desynapsis and divergent spindle organization occurred in three progenies. These anomalies caused frequent fractionation of the genome into several microspores of different sizes. In Brachiaria, new cultivars must be apomictic to fix the genotype. However, Brachiaria is a pseudogamous apomict, and viable gametes are necessary to produce viable seeds. Considering meiotic behavior, only two progenies are promising for advancement in the breeding program.

Meiotic stability in two valuable interspecific hybrids of Brachiaria (Poaceae)

With 2 figures and 2 tablesAbstractIn an attempt to increase genetic variability in the genus Brachiaria, crosses were made between sexual tetraploidized accessions of B. ruziziensis and apomictic tetraploid accessions of B. brizantha (2n = 4x = 36). Among the F1 progenies, two hybrids (Hb331, apomictic; and Hb336, sexual) displayed several agronomically desirable traits. These hybrids were evaluated for several years, and the apomictic one is now being tested under grazing to be released as a new cultivar, whereas the sexual one is in a reciprocal recurrent selection programme. The possibility of gene exchange through recombination of genitors and their meiotic behaviour was investigated. Both hybrids were evaluated for meiotic stability during three consecutive years. Analyses of meiocytes at diakinesis revealed multivalent chromosome associations among the parental genomes, indicating that their gene pools are interchangeable. Irregular chromosome segregation, chromosome stickiness and a putative meiotic mutation were recorded. The frequency of abnormalities varied among years and also between the hybrids. The sexual hybrid Hb336 had a more stable meiotic behaviour, but pollen fertility was higher in apomictic Hb331, ensuring more viable seed formation.

Microsporogenesis in Brachiaria brizantha (Poaceae) as a selection tool for breeding

The genus Brachiaria comprises more than 100 species and is the single most important genus of forage grass in the tropics. Brachiaria brizantha, widely used in Brazilian pastures for beef and dairy production, is native to tropical Africa. As a subsidy to the breeding program underway in Brazil, cytological studies were employed to determine the chromosome number and to evaluate microsporogenesis in 46 accessions of this species available at Embrapa Beef Cattle (Brazil). Thirty-four accessions presented 2n = 36; seven had 2n = 45, and five had 2n = 54 chromosomes. Based on the higher level of chromosome association observed in diakinesis, in tetra-, penta-, and hexavalents, respectively, it was concluded that they are derived from x = 9; consequently, these accessions are tetra- (2n = 4x = 36), penta- (2n = 5x = 45), and hexaploids (2n = 6x = 54). The most common meiotic abnormalities were irregular chromosome segregation due to polyploidy. Chromosome stickiness, abnormal cytokinesis, non-congressed bivalents in metaphase I and chromosomes in metaphase II, and chromosome elimination were recorded at varying frequencies in several accessions. The mean percentage of meiotic abnormalities ranged from 0.36 to 95.76%. All the abnormalities had the potential to affect pollen viability by generating unbalanced gametes. Among the accessions, only the tetraploid ones with less than 40% of abnormalities are suitable as pollen donors in intra- and interspecific crosses. Currently, accessions with a high level of ploidy (5 and 6n) cannot be used as male genitors in crosses because of the lack of sexual female genitors with the same levels of ploidy.

Chromosome numbers and meiotic analysis in the pre-breeding of Brachiaria decumbens (Poaceae)

A total of 44 accessions of Brachiaria decumbens were analysed for chromosome count and meiotic behaviour in order to identify potential progenitors for crosses. Among them, 15 accessions presented 2n = 18; 27 accessions, 2n = 36; and 2 accessions, 2n = 45 chromosomes. Among the diploid accessions, the rate of meiotic abnormalities was low, ranging from 0.82% to 7.93%. In the 27 tetraploid accessions, the rate of meiotic abnormalities ranged from 18.41% to 65.83%. The most common meiotic abnormalities were related to irregular chromosome segregation, but chromosome stickiness and abnormal cytokinesis were observed in low frequency. All abnormalities can compromise pollen viability by generating unbalanced gametes. Based on the chromosome number and meiotic stability, the present study indicates the apomictic tetraploid accessions that can act as male genitor to produce interspecific hybrids with B. ruziziensis or intraspecific hybrids with recently artificially tetraploidized accessions.

Meiotic behavior in Brachiaria humidicola (Poaceae) hybrids

In the genus Brachiaria, genetic variation can be exploited directly from germplasm collections or released using sexual reproduction in normally apomictic polyploids. The discovery of a natural sexual polyploid accession H031 of Brachiaria humidicola collected in Africa, opened new opportunities to exploit the genetic variation in this species. This accession was crossed with an apomictic cultivar BRS Tupi with the same chromosome number (2n = 36) and 361 F1 hybrids were obtained. Following visual selection for leafiness, vigor, growth habit, and the mode of reproduction, 50 hybrids were selected for further agronomic evaluation. The parents and 45 of the 50 selected hybrids were evaluated for the regularity of meiosis. In the female parent (H031), meiosis was somewhat irregular, with 16.3% of abnormal tetrads, whereas the male (cv. BRS Tupi) meiosis was very regular, with only 3.1% of abnormal tetrads. Among hybrids (sexual and apomictic), the percentage of abnormal tetrads ranged from 15.8 to 98.3%. The abnormalities included irregular chromosome segregation, chromosome stickiness and the absence of cytokinesis. Considering that apomixis in the genus Brachiaria is pseudogamic, and that meiotic aberrations can compromise pollen viability, the results of this study present another parameter to aid selection for more stable microsporogenesis. Apomictic derivatives with stable meiosis are candidates for new cultivars whereas sexual hybrids can be retained in breeding for another round of recombination.

Meiotic behavior as a selection tool in the breeding of Brachiaria humidicola (Poaceae)

Brachiaria humidicola is a tropical grass that grows in seasonally swampy grasslands in Africa. In Brazil, two apomictic cultivars (2n = 54) of this species are widely used as pastures in poorly drained soils. The recent discovery of a sexual polyploid accession (2n = 36) in the germplasm collection at the Embrapa Beef Cattle Research Center allowed intraspecific hybridization with the objective of broadening the genetic variability and selection of superior genotypes in this species. Hybridization, however, depends on accessions with the same ploidy level. Cytological analyses of 55 accessions revealed that 19 apomictic accessions also presented 2n = 36 chromosomes. Chromosome pairing in hexavalent association at diakinesis and metaphase I suggested that the basic chromosome number for this species is x = 6. Cytological analysis revealed abnormalities in variable frequencies in the meiosis of these hexaploid (2n = 6x = 36) accessions. The most common were those related to irregular chromosome segregation which led to unbalanced gamete formation, but chromosome stickiness was also recorded. These results clearly demonstrate the value of cytogenetics in the choice of genitors and for superior hybrids to be obtained in the breeding of this species. For that both the ploidy level and the frequency of abnormalities need to be considered, besides other favorable agronomic characteristics.

Meiotic analysis in induced tetraploids of Brachiaria decumbens Stapf

The meiotic behavior of three tetraploid plants (2n=4x=36) originated from somatic chromosome duplication of sexually reproducing diploid plants of Brachiaria decumbens was evaluated. All the analyzed plants presented abnormalities related to polyploidy, such as irregular chromosome segregation, leading to precocious chromosome migration to the poles and micronuclei during both meiotic divisions. However, the abnormalities observed did not compromise the meiotic products which were characterized by regular tetrads and satisfactory pollen fertility varying from 61.36 to 64.86%. Chromosomes paired mostly as bivalents in diakinesis but univalents to tetravalents were also observed. These studies contributed to the choice of compatible fertile sexual genitors to be crossed to natural tetraploid apomicts in the B. decumbens by identifying abnormalities and verifying pollen fertility. Intraespecific crosses should reduce sterility in the hybrids produced in the breeding program of Brachiaria, a problem observed with the interspecific hybrids produced so far.

Meiotic behavior in nonaploid accessions of Brachiaria humidicola (Poaceae) and implications for breeding

Brachiaria humidicola is a grass adapted to seasonally swampy grasslands in Africa; two cultivars, 'common' and Llanero, are widely used in Brazilian pastures. New cultivars are in great demand in order to diversify current production systems to achieve improved quality and yield. Cytological analyses of 55 accessions of this species available from the Embrapa Beef Cattle germplasm collection revealed that 27 are apomictic and have 2n = 54 chromosomes. Chromosome pairing as bi- to nonavalent associations at diakinesis indicated a basic chromosome number in this species of x = 6, as found in other closely related Brachiaria species. Thus, these 27 accessions are nonaploid (2n = 9x = 54). Abnormalities were found in the meiosis of these accessions, at variable frequencies. The most common abnormalities were those related to irregular chromosome segregation, which led to unbalanced gamete formation; but chromosome stickiness, cell fusion, and absence of cytokinesis were also recorded. Although some accessions have a low frequency of meiotic abnormalities, ensuring potentially good pollen viability, these cannot be used in hybridization due to a lack of sexual accessions with the same ploidy level.

Meiotic behavior in Panicum maximum Jacq. (Poaceae: Panicoideae: Paniceae): hybrids and their genitors

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.

Genetic loss through heavy metal induced chromosomal stickiness in Grass pea

Abstract Cytogenetic studies were carried out to evaluate the genotoxic effect of two heavy metals viz. lead (Pb) and zinc (Zn) on the gametic cells of grasspea (Lathyrus sativus). Our study presented a severe case of chromosome stickiness in meiosis impairing normal chromosome segregation. This phenomenon persisted upto microspore stage. The manifestation of stickiness ranged from mild to intense followed by chromosome degeneration. Stickiness taken together with abnormal spindles, irregular chromosome segregation, pycnotic nuclei were considered the most probable causes of low pollen fertility found in both metal treatments at higher doses.

Meiotic behaviour in the first interspecific hybrids between Brachiaria brizantha and Brachiaria decumbens

With 2 figures and 1 table Abstract Brachiaria brizantha and Brachiaria decumbens constitute the two most widely used tropical forage species for cultivated pastures and support both beef and dairy cattle production in the tropics. Two apomictic cultivars –B. brizantha cv. ‘Marandu’ and B. decumbens cv. ‘Basilisk’– cover more than 100 million hectares of cultivated pastures throughout Latin America and Southeast Asia. This paper discusses the cytogenetic data of the first successful attempt in obtaining interspecific hybrids between these two species. Artificial hybridization between two diploid (2n = 2x = 18) sexual accessions (B. brizantha, B105 × B. decumbens, D004) was performed in the greenhouse. Only three hybrids were recovered. One of them was treated with colchicine in tissue culture and two amphidiploid plants were obtained. Meiosis in the diploid genitors was quite normal, while in the diploid hybrid parental genome separation was observed together with some irregular chromosome segregation. Genome separation was also observed in several cells in the two 4n amphidiploid hybrids. In these, a high number of meiocytes with chromosomes chaotically spread in the cytoplasm was recorded during meiosis. The abnormalities observed compromised the meiotic products characterized by polyads. Chromosomes associated generally as bivalents but some association as quadrivalents in diakinesis in one of the amphidiploid hybrids indicates taxonomic proximity between these two species. Moreover, it indicates that introgression of desirable genes may be feasible between these two most important agronomic species of the genus.

Meiotic behavior of interspecific hybrids between artificially tetraploidized sexual Brachiaria ruziziensis and tetraploid apomictic B. brizantha (Poaceae)

The meiotic behavior of four interspecific promising hybrids was evaluated by conventional cytological methods. The female genitors were two artificially tetraploidized sexual accessions of B. ruziziensis (R41 and R44, 2n = 4χ = 36), which were crossed to an agronomically superior natural tetraploid apomictic genotype of B. brizantha (B140 - BRA003395). Three of them (HBGC313, HBGC 315, and HBGC324) were sexual and one (HBGC325) apomictic. Analyses of some cells in diakinesis revealed multivalent chromosome configurations, suggesting that genetic recombination and introgression of some genes could be present. The four hybrids had different types of meiotic abnormalities at various frequencies. Abnormalities related to irregular chromosome segregation due to polyploidy were common among these hybrids, and characterized by precocious chromosome migration to the poles, laggard chromosomes, both generating micronuclei in telophases and tetrads and, as a consequence, unbalanced gametes. One abnormality genotype-specific, related to spindle orientation (a putative divergent spindle mutation), was recorded for the first time in two of the hybrids, HBGC313 and HBGC325. The sexual hybrid HBGC324 had the lower rate of abnormalities, and it could be used as a female genitor in future crosses in the breeding program. The abnormalities present in these hybrids may impact fertility and affect seed production. Based on the results, HBGC324 is the single hybrid recommended to the breeding program. Hybrids must produce a good amount of viable seeds, besides good overall dry matter production and nutritive value, in order to be widely utilized and adopted in production systems. Due to pseudogamy, the desirable superior apomictic hybrids need viable pollen grains to fertilize the secondary nucleus of the embryo sac and thus ensure normal and vigorous endosperm development and plenty of seed set.

Analysis of meiotic behavior in selecting potential genitors among diploid and artificially induced tetraploid accessions of Brachiaria ruziziensis (Poaceae)

Sexuality is correlated with diploidy and apomixis with polyploidy in the Brachiaria genus. Brachiaria ruziziensis is a key species in Brachiaria breeding due to its obligate sexuality and intrinsic agronomic qualities. Interspecific crosses in the genus became feasible only when a few diploid accessions of B. ruziziensis were artificially tetraploidized and remained sexual. Hybridization has been done since, using natural tetraploid apomictic accessions of B. brizantha or B. decumbens as pollen donors. Twenty two accessions of B.ruziziensis from the Embrapa Beef Cattle germplasm collection (Campo Grande, MS, Brazil) were cytologically analyzed: 16 are natural diploids (2n = 2x = 18) and six are artificially induced tetraploids (2n = 4x = 36). The meiotic behavior in the 16 diploid accessions varied. The mean of meiotic abnormalities per accession ranged from zero to 24.46%. Meiotic behavior in the induced tetraploid accessions also varied with the mean of meiotic abnormalities ranging from 5.20% to 54.71%. The most common abnormalities observed in both the diploid and the tetraploid accessions, were those related to irregular chromosome segregation. In one tetraploid accession, with a high frequency of those, other irregularities involving chromosome orientation at metaphase plate and chromosome convergence to the poles, a meiotic mutation known as divergent spindle, were recorded. Meiotic behavior should be considered in selecting potential genitors for breeding.

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.