Precocious Chromosome Research Articles

Meiotic behavior of two grapevine somatic mutants with ornamental potential

‘Niagara Steck’ and ‘Niagara Variegada’ are two grapevine varieties that arose spontaneously as somatic mutants of ‘Niagara Rosada’. Berries characterize ‘Niagara Steck’ at young stages exhibiting a remarkable brown reticulated structure on the epidermis which develops into a brown-bronze russet-like structure as the berries ripe. The mature berries have strongly acidic flesh. ‘Niagara Variegada’ shows notable variegation in leaves and berries. Meiotic studies were carried out to ascertain if the mutant phenotypes could be related to any change in chromosome number, structure or other irregularity. Floral buds of both varieties at different developmental stages were collected and fixed for cytological analyses. Chromosome counts at diakinesis/metaphase I showed n=19 bivalents for both varieties. Univalent, trivalent, tetravalent chromosomes or chain configuration were not recorded ruling out the possibility of any alteration in the chromosome number or structure in both mutant varieties. However, low percentages of meiotic irregularities were recorded such as chromosome stickiness, laggards, non-oriented chromosomes, precocious chromosome segregation, tetrads with one microcyte, microspore fusion, and tetrad degeneration. The total percentage of abnormalities was higher in ‘Niagara Variegada’ (19.68%) than in ‘Niagara Steck’ (14.40%) which may have contributed to a lower percentage of pollen fertility (79.80%) when compared to ‘Steck’ (90.74%). The aforementioned varieties can be propagated by hardwood cuttings and constitute an interesting option for ornamentation of home backyards, patios, and gardens with the advantage that ‘Niagara Variegada’ bearing sweet edible berries.

Ecotoxicological Assessment of Water and Sediment in Areas of Taal Lake with Heavy Aquaculture Practices Using Allium cepa and Daphnia magna Assay

Aquaculture is one of the main livelihoods of people living near Taal Lake. However, the quality of the lake is deteriorating due to the increasing number of fish cages and fish pens. In this study, ecotoxicological assessment of water and sediment in three areas (Gonzales, Bañaga, and Balakilong) of Taal Lake were conducted using Allium cepa and Daphnia magna assay. The result of A. cepa assay showed that areas with heavy aquaculture practices have a higher percentage of chromosomal aberrations (CAs) such as anaphase bridge, precocious chromosome, sticky chromosome, C-mitosis, and micronucleus (MN). This indicates the presence of cytotoxic and genotoxic compounds in the lake water. On the other hand, assessment of sediment samples using D. magna assay showed that the population exposed to samples from areas with heavy aquaculture practice showed a significantly lower number of offsprings compared to the control area. The result of our study shows the effectiveness of the two bioassays on the evaluation of ecotoxicity of water and sediment samples.

Cytogenetic analysis of interspecific hybridization in oil-tea (Camellia oleifera)

Oil-tea (Camellia oleifera) is an important woody plant species producing edible oil in southern China, which shows complicated cytogenetics and intraspecific polyploidy. In order to explore the ploidy variation in interspecific hybridization of oil-tea Camellia, the ploidy of ‘Huashuo’, ‘Youza 2’ and 18 superior hybrids of ‘Youza 2’ × ‘Huashuo’ were identified by flow cytometry and chromosome counting. C.oleifera variety ‘Huashuo’ was hexaploid (2n = 90), and Camellia yuhsienensis superior plant ‘Youza 2’ was aneuploid based on hexaploid (2n = 91). There are not only hexaploid but also aneuploid in the ‘Huashuo’ × ‘Youza 2’ hybrids. Among the offspring plants, there are 11 plants each with chromosome number of 90, 6 plants each with 91, and 1 plants each with 92. To elucidate the causes of ploidy variation in above findings, the pollen characteristics and chromosomal behavior in meiosis of ‘Huashuo’ were examined by microscopy. The chromosome pairing configuration of meiosis terminal stage of microsporocytes of ‘Huashuo’ was 43 II + 2 IV. In all stages of meiosis, numerous abnormal behaviors were found, such as precocious chromosome migration, free chromosomes, lagging chromosomes and micronucleus. The abnormal rates of metaphase I, anaphase I, metaphase II and anaphase II were 67.5%, 88.1%, 58.4% and 78.9%, respectively. In the tetrad stage, abnormal phenomena such as dyads, triads, hexads and micronucleus were observed, with the frequency of 1.4%, 1.0%, 1.0% and 4.2%, respectively. The pollen diameter of ‘Huashuo’ varied widely (16.92–79.13 μm), suggesting a wide range of ploidy level of pollen grains. From pollen diameter distribution, we concluded that ‘Huashuo’ produced a small number of unreduced pollen grains. These findings are valuable for cytogenetics research and selection strategy of ploidy breeding in C. oleifera.

Effects of crude ethanolic extract of <i>Gloriosa superba</i> L. tuber, on root growth and root tip mitosis, of <i>Allium sativum</i> L.

Colchicine is an alkaloid produced by some members of the Liliaceae family which include Gloriosa superba L. Apart from its other medicinal uses, it is used for both cytological and plant breeding studies. Absence of pure colchicine in Nigeria has retarded research progress in the above-mentioned areas of research. In this study, the genotoxic and cytotoxic effect of ethanolic crude extract of G. superba tubers, obtained by freeze-drying method, on Allium sativum meristematic roots were examined; to explore the possibility of using it, in research, in the absence of pure colchicine. The mitotic index of the roots treated with aqueous crude extract concentrations of 500mg/L, 1000mg/L and 2000mg/L for 3, 6, 12 and 24 hours, showed that there was significant reduction in the number of dividing cells with time. There were evidences of disturbed mitotic phases, C-metaphase, precocious chromosome movement and anaphase bridges, which increased as concentration and duration of treatment increased. Distilled water and 8-hydroxyquinoline were used as negative and positive controls. A. sativum roots exposed to the crude extract concentrations for 3, 6, 12, 24, 48 and 72 hours showed significant inhibition of root growth which increased with concentration and duration of treatment. The results obtained from this study indicate that the crude colchicine contained in G. superba , is an effective tubulin binder, which can arrest cells at the metaphase stage and is therefore recommended for cytological studies in Nigeria and other underdeveloped economies, in the absence of pure colchicine. Keywords: Root growth, root tip, mitosis, Gloriosa superba , Allium sativum , colchicine

Cytological Investigation in Two Mutants of the Hybrid Grapevine ‘Niagara’

‘Niagara’ is a hybrid table grape (V. vinifera × V. labrusca). Several spontaneous somatic mutants from this grapevine have already been described. However, cytological information about these mutants is scarce. Therefore, studies in the microsporogenesis have been carried out in two of these mutants, a seedless and a large berry mutant (giant mutant). A low percentage of abnormalities such as precocious chromosome segregation, bivalent not aligned, laggard and stickiness chromosomes were seen in both mutants. A merging process of microspores was observed in several tetrads in the seedless mutant: the microspore came together by large cytoplasmic bridges giving rise to a large and amorphous single-celled tetrad, which eventually collapsed. The giant mutant showed 10.29% of tetrads with microcytes. However, these tetrad abnormalities did not hinder pollen viability. Significant differences were seen concerning stomata and pollen grain sizes, stomata frequency and anther number per flower between the mutants. The chromosome count at diakinesis (n=19) and in diploid tapetal cells (2n=38) clarified the fact that the seedless somatic mutation was not associated with any ploidy event. Conversely, the giant mutant showed n=38 at diakinesis and 2n=76 in diploid tapetal cells, pointing to a case of spontaneous chromosome duplication. Since these abnormalities did not disturb the percentage of pollen viability very much, in both mutants (higher than 86%), the seedless one may be used as a male parent in crossing, regarding the desirable seedlessness character, while the mutant giant may be employed as female parent for the obtainment of seedless triploid plants.
 
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Abnormal meiosis in an intersectional allotriploid of Populus L. and segregation of ploidy levels in 2x × 3x progeny.

Triploid plants are usually highly aborted owing to unbalanced meiotic chromosome segregation, but limited viable gametes can participate in the transition to different ploidy levels. In this study, numerous meiotic abnormalities were found with high frequency in an intersectional allotriploid poplar (Populus alba × P. berolinensis ‘Yinzhong’), including univalents, precocious chromosome migration, lagging chromosomes, chromosome bridges, micronuclei, and precocious cytokinesis, indicating high genetic imbalance in this allotriploid. Some micronuclei trigger mini-spindle formation in metaphase II and participate in cytokinesis to form polyads with microcytes. Unbalanced chromosome segregation and chromosome elimination resulted in the formation of microspores with aneuploid chromosome sets. Fusion of sister nuclei occurs in microsporocytes with precocious cytokinesis, which could form second meiotic division restitution (SDR)-type gametes. However, SDR-type gametes likely contain incomplete chromosome sets due to unbalanced segregation of homologous chromosomes during the first meiotic division in triploids. Misorientation of spindles during the second meiotic division, such as fused and tripolar spindles with low frequency, could result in the formation of first meiotic division restitution (FDR)-type unreduced gametes, which most likely contain three complete chromosome sets. Although ‘Yinzhong’ yields 88.7% stainable pollen grains with wide diameter variation from 23.9 to 61.3 μm, the pollen viability is poor (2.78% ± 0.38). A cross of ‘Yinzhong’ pollen with a diploid female clone produced progeny with extensive segregation of ploidy levels, including 29 diploids, 18 triploids, 4 tetraploids, and 48 aneuploids, suggesting the formation of viable aneuploidy and unreduced pollen in ‘Yinzhong’. Individuals with different chromosome compositions are potential to analyze chromosomal function and to integrate the chromosomal dosage variation into breeding programs of Populus.

Mitotic Checkpoint Regulators Control Insulin Signaling and Metabolic Homeostasis

Insulin signaling regulates many facets of animal physiology. Its dysregulation causes diabetes and other metabolic disorders. The spindle checkpoint proteins MAD2 and BUBR1 prevent precocious chromosome segregation and suppress aneuploidy. The MAD2 inhibitory protein p31(comet) promotes checkpoint inactivation and timely chromosome segregation. Here, we show that whole-body p31(comet) knockout mice die soon after birth and have reduced hepatic glycogen. Liver-specific ablation of p31(comet) causes insulin resistance, hyperinsulinemia, glucose intolerance, and hyperglycemia and diminishes the plasma membrane localization of the insulin receptor(IR) in hepatocytes. MAD2 directly binds to IR andfacilitates BUBR1-dependent recruitment of the clathrin adaptor AP2 to IR. p31(comet) blocks the MAD2-BUBR1 interaction and prevents spontaneous clathrin-mediated IR endocytosis. BUBR1 deficiency enhances insulin sensitivity in mice. BUBR1 depletion in hepatocytes or the expression of MAD2-binding-deficient IR suppresses the metabolic phenotypes of p31(comet) ablation. Our findings establish a major IR regulatory mechanism and linkguardians of chromosome stability to nutrient metabolism.

Unreduced gamete formation in wheat×Aegilops spp. hybrids is genotype specific and prevented by shared homologous subgenomes.

The presence of homologous subgenomes inhibited unreduced gamete formation in wheat×Aegilops interspecific hybrids. Unreduced gamete rates were under the control of the wheat nuclear genome. Production of unreduced gametes is common among interspecific hybrids, and may be affected by parental genotypes and genomic similarity. In the present study, five cultivars of Triticum aestivum and two tetraploid Aegilops species (i.e. Ae. triuncialis and Ae. cylindrica) were reciprocally crossed to produce 20 interspecific hybrid combinations. These hybrids comprised two different types: T. aestivum×Aegilops triuncialis; 2n=ABDU(t)C(t) (which lack a common subgenome) and T. aestivum×Ae. cylindrica; 2n=ABDD(c)C(c) (which share a common subgenome). The frequency of unreduced gametes in F1 hybrids was estimated in sporads from the frequency of dyads, and the frequency of viable pollen, germinated pollen and seed set were recorded. Different meiotic abnormalities recorded in the hybrids included precocious chromosome migration to the poles at metaphase I and II, laggards in anaphase I and II, micronuclei and chromosome stickiness, failure in cell wall formation, premature cytokinesis and microspore fusion. The mean frequency of restitution meiosis was 10.1%, and the mean frequency of unreduced viable pollen was 4.84% in T. aestivum×Ae. triuncialis hybrids. By contrast, in T. aestivum×Ae. cylindrica hybrids no meiotic restitution was observed, and a low rate of viable gametes (0.3%) was recorded. This study present evidence that high levels of homologous pairing between the D and D(c) subgenomes may interfere with meiotic restitution and the formation of unreduced gametes. Variation in unreduced gamete production was also observed between T. aestivum×Ae. triuncialis hybrid plants, suggesting genetic control of this trait.

Studies of Meiotic Behaviors in Some Taxa of the Nepalese Papilionaceae

This paper reports studies of meiotic behaviors of seventeen species, sub-species and varieties in seven genera of the Nepalese Papilionaceae and new chromosome numbers for three species (Crotalaria cytisoides Roxb. ex DC (n=8), Parochetus communis Buch-Ham. ex D. Don (n=8) and Piptanthus nepalensis (Hook.) D. Don. (n=9, 18) are new number reports. Meiotic behaviors are often normal in all the species). However, a few aberrations like univalents, chromosome bridges, laggards, precocious chromosomes and/ or cytomixis have been observed in most of the taxa. Pollen fertility is high (more than 80%) indicating that the abnormalities had not reached mature stages.J. Nat. Hist. Mus. Vol. 26, 2012: 68-79

English

The continous production and release of chemicals into the environment have led to the need to assess their chemical composition and genotoxic effects on cell reproduction. Two petrochemicals, air liquid and polyester resin effluents were assessed. The common onion of the purple variety was used as the test organism. The results of the chemical analysis of the wastewaters showed high concentration of some potentially mutagenic heavy metals. The effects of the wastewaters and their dilutions on root tip mitosis were mitodepressive. The mitotic index (MI%) were 14.0, 5.3 and 4.1 in control, air liquid and polyester resin effluents, respectively. The MI (%) values estimated across concentrations ranged from 15.1 in control to 5.5 in undiluted effluent. Percent abnormal dividing cells increased with increase in wastewater concentration. Abnormal dividing cells observed ranged from C-mitotic effects to precocious chromosomes and anaphase bridges. These findings indicate the cytotoxic and genotoxic effects of these wastewaters on Allium cepa chromosomes. Positive results on Allium chromosomes could serve as indicator of the deleterious effects of these wastewaters on other organisms at the point of discharge – either on land or water bodies. The need for sound sewerage system that would protect flora and fauna in the ecosystem is advocated. Key words: Chromosome, ecosystem, heavy metal, mitosis, mutation.

Pollen variation as a response to hybridisation in Populus L. section Aigeiros Duby

Interspecific hybrids often produce extensive gametic variation due to poor chromosome pairing and unbalanced chromosome segregation. To elucidate the influence of interspecific hybridisation on pollen variation in Populus L., we compared pollen size among three P. deltoides Marsh. clones, two P. nigra L. clones, and three P. deltoides × P. nigra hybrids and investigated the cytological mechanism of pollen variation by observing meiotic chromosome behaviour and microtubules. The frequency of shrunken and unstainable pollen grains was higher in the hybrids (15.54–57.28 %) than in the non-hybrid clones (0.98–4.82 %). The distribution of stained pollen size ranged more broadly in the hybrids than in the non-hybrids, suggesting that hybridisation increases pollen size variation. Precocious chromosome migration and lagging chromosomes were common in the hybrids, indicating poor chromosome pairing. Some micronuclei formed minispindles in metaphase II and emanated secondary nuclear-based radial microtubule systems in telophase II, undergoing cytokinesis to develop into microcytes in the meiotic products. Eliminated micronuclei might result in the formation of aneuploid pollen. The frequency of sporads with microcytes was higher in the hybrids (3.19–11.08 %) than in the non-hybrids (0–0.94 %). Disoriented spindles, including fused and tripolar spindles, led to the formation of dyads and triads, contributing to unreduced pollen production. These results provide new insights into the gametic variation of Populus interspecific hybrids and reveal several 2n pollen producers, which will be valuable in polyploid breeding of the section Aigeiros Duby.

Abnormal meiotic chromosome behavior and gametic variation induced by intersectional hybridization in Populus L.

Abnormal meiotic chromosome behavior and pollen variation in two intersectional hybrids between Populus simonii Carr. (section Tacamahaca Spach) and Populus euphratica Oliv. (section Turanga Bge.) were investigated by the squashed technique and indirect immunofluorescence. Both hybrids produced abnormal flower buds characterized by a primary bud with an accessary bud at frequencies of 39.0 and 17.8 %, respectively. Chromosome pairing analysis at late diakinesis showed average configurations 20.31 I + 7.04 II + 1.42 III + 0.04 IV in hybrid “Xhu-DK” and 19.18 I + 7.16 II + 1.45 III + 0.04 IV in hybrid “Xhu-TL”, indicating high divergence between their parent species. Meiotic abnormalities, including precocious chromosome migration, lagging chromosomes, chromosome bridges, and micronuclei, were observed at high frequency, resulting in imbalanced chromosome segregation and chromosome elimination and caused high gamete sterility. Some micronuclei triggered formation of minispindles in Metaphase II, which were responsible for the formation of microcytes. Fused and tripolar spindles in the second meiotic division developed into dyads and triads, respectively, leading to viable first meiotic division restitution (FDR) type unreduced pollen formation. However, Xhu-TL was completely sterile because the high frequency of precocious cytokinesis during the second division prevented unreduced pollen formation though spindle fusion. Xhu-DK produced 29.6 % viable pollen grains. The diameter of viable pollen ranged from 19.6 to 64.9 μm, suggesting that Xhu-DK might be capable of producing unreduced pollen or pollen with a higher ploidy level. The strategy of using the distant hybrids for chromosome introgression and polyploidy breeding in Populus L. was considered.

Etiology and pathogenesis of the cohesinopathies.

Cohesin is a chromosome‐associated protein complex that plays many important roles in chromosome function. Genetic screens in yeast originally identified cohesin as a key regulator of chromosome segregation. Subsequently, work by various groups has identified cohesin as critical for additional processes such as DNA damage repair, insulator function, gene regulation, and chromosome condensation. Mutations in the genes encoding cohesin and its accessory factors result in a group of developmental and intellectual impairment diseases termed ‘cohesinopathies.’ How mutations in cohesin genes cause disease is not well understood as precocious chromosome segregation is not a common feature in cells derived from patients with these syndromes. In this review, the latest findings concerning cohesin's function in the organization of chromosome structure and gene regulation are discussed. We propose that the cohesinopathies are caused by changes in gene expression that can negatively impact translation. The similarities and differences between cohesinopathies and ribosomopathies, diseases caused by defects in ribosome biogenesis, are discussed. The contribution of cohesin and its accessory proteins to gene expression programs that support translation suggests that cohesin provides a means of coupling chromosome structure with the translational output of cells. WIREs Dev Biol 2015, 4:489–504. doi: 10.1002/wdev.190This article is categorized under: 1Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics2Birth Defects > Craniofacial and Nervous System Anomalies

Cytological Analyses in ‘Niagara Branca’ Grape and in Its Somatic Mutant ‘Niagara Rosada’

An investigation in the microsporogenesis of two cultivars of grapes (Vitis L.), were carried out on ‘Niagara Branca’ and on its somatic mutant ‘Niagara Rosada’, aiming at ascertaining the normality of the microsporogenesis. Some possible changes have been checked on chromosome number in somatic cells, pollen size and fertility, stomatal size and frequency, which related to the somatic mutant ‘Niagara Rosada’, thus increasing the information already obtained regarding ‘Niagara’ grapevines. The microsporogenesis occurred normally in both cultivars and cytomixis observed only among pachytene cells. A low percentage of precocious chromosome segregation was noticed in metaphase I and II, as well as a very low percentage of laggard and stickiness chromosomes in anaphase I and II. Less than 1% of tetrads showed microspore alterations. The mean values of the meiotic index and pollen grain fertility were high. However, no association were observed between the occurrences of chromosome abnormalities and the variation in the anther number (five to seven) seen among flower buds. The cultivars showed the same chromosome number, n=19 at diakinesis and 2n=38 in somatic cells, and did not differ in regards to stomatal size and frequency. It seems that the difference between the cultivars lied at gene level. Due to the very low percentage of abnormalities and to high pollen grain fertility, the plants analyzed could be very well employed in breeding programs for the grapevines.

Characterization of the Interaction between the Cohesin Subunits Rad21 and SA1/2

The cohesin complex is responsible for the fidelity of chromosomal segregation during mitosis. It consists of four core subunits, namely Rad21/Mcd1/Scc1, Smc1, Smc3, and one of the yeast Scc3 orthologs SA1 or SA2. Sister chromatid cohesion is generated during DNA replication and maintained until the onset of anaphase. Among the many proposed models of the cohesin complex, the 'core' cohesin subunits Smc1, Smc3, and Rad21 are almost universally displayed as tripartite ring. However, other than its supportive role in the cohesin ring, little is known about the fourth core subunit SA1/SA2. To gain deeper insight into the function of SA1/SA2 in the cohesin complex, we have mapped the interactive regions of SA2 and Rad21 in vitro and ex vivo. Whereas SA2 interacts with Rad21 through a broad region (301–750 aa), Rad21 binds to SA proteins through two SA-binding motifs on Rad21, namely N-terminal (NT) and middle part (MP) SA-binding motif, located at 60–81 aa of the N-terminus and 383–392 aa of the MP of Rad21, respectively. The MP SA-binding motif is a 10 amino acid, α-helical motif. Deletion of these 10 amino acids or mutation of three conserved amino acids (L385, F389, and T390) in this α-helical motif significantly hinders Rad21 from physically interacting with SA1/2. Besides the MP SA-binding motif, the NT SA-binding motif is also important for SA1/2 interaction. Although mutations on both SA-binding motifs disrupt Rad21-SA1/2 interaction, they had no apparent effect on the Smc1-Smc3-Rad21 interaction. However, the Rad21-Rad21 dimerization was reduced by the mutations, indicating potential involvement of the two SA-binding motifs in the formation of the two-ring handcuff for chromosomal cohesion. Furthermore, mutant Rad21 proteins failed to significantly rescue precocious chromosome separation caused by depletion of endogenous Rad21 in mitotic cells, further indicating the physiological significance of the two SA-binding motifs of Rad21.

Sodium Azide Induced Complementary Effect of Chromosomal Stickiness in Brassica Campestris L.

Present investigation reports the mutagenic efficacy of sodium azide with a view to study its effects on meiotic behavior and phenotypic quantitative traits of Brassica campestris L. Seeds were treated with three concentration of sodium azide i.e. 0.3%, 0.5% and 0.7%, along with control. Morphological/phenotypic parameters studied were plant height, number of primary branches, number of secondary branches, seeds/siliqua, husk yield, seed yield and 1000-seed weight. All these traits showed the significant positive shifts in mean at 0.3% and 0.7% doses of azide as compared to control and 0.5% dose. However, cytogenetic assessment of the treated plants clearly revealed the preponderance of stickiness distributed in all the phases of meiotic division that persisted up to the tetrad stage. The phenomenon displayed mild to severe case of stickiness on the basis of number of genome involved during meiosis. Manifestation of stickiness has also been resulted in several other meiotic abnormalities such as bridge (1.88%), unorientation (0.64%), micronuclei (1.38%), asynchronous division (0.77%), and precocious chromosomes (3.89%) along with some other anomalies (laggards, univalents etc.). Frequency of these anomalies along with severe stickiness was higher at 0.5% dose of azide that must have compromised with pollen fertility resulting declined fertility rate. Though, pollen fertility at 0.3% and 0.7% doses had not been much affected and had fallen near control. Thus, from present investigation inference can be drawn that the sodium azide induced stickiness might have resulted in some meiotic mutant that has imposed some novel genetic combinations thereby positively affecting morphological traits of the crop.

Evaluation of the cytotoxicity and genotoxicity of aqueous leaf extracts of Azadirachta indica A. Juss using the Allium test

The cytotoxicity and genotoxicity of aqueous leaf extracts of Azadirachta indica A. Juss were evaluated with the aid of the Allium test. The extract was found to exhibit mitoclassic and chromatoclassic effects. The abnormalities observed included: nuclear lesions, binucleate cells, prophase accumulation otherwise called prophase poison, sticky metaphase, star metaphase, metaphase c-banding, precocious chromosomes, lagging chromosomes, anaphase bridge, star anaphase, tripolar anaphase and sticky late anaphase. No abnormal cells were observed at telophase. The implications of these anomalies were also discussed. The results lend more support to the reports of several investigators on the urgent need to standardize the dosages of medicinal plant extracts for the treatment of various ailments. Key words: Cytotoxicity, genotoxicity, leaf extract, Azadirachta indica.

Meiotic behavior in wild and domesticated species of Passiflora

The meiotic behavior of fourteen Passiflora taxa was analyzed. The species were grouped according to the n value (6, 9 and 12) for statistical studies. Some species presented tetravalent associations or univalent chromosomes in diakinesis, bivalent formation prevailing. The qui-square test revealed significant differences in the chiasma frequency among species for n = 9 and n = 6 groups. There was predominance of interstitial chiasmata in almost all studied species. The n = 12 group was the only one whose meiotic behavior was considered similar due to the quantity of chiasmata per cell, tendency of interstitial chiasma localization. Some species presented meiotic irregularities, such as laggard and precocious chromosomes in meiosis I. In telophase II the percentages of meiotic irregularities was low. Irregularities in the spindle orientation were presented in higher percentages in the end of meiosis II, and were also responsible for post-meiotic abnormal products. The irregularities observed during meiosis can have influence on the percentage of sterile pollen grains and success of interspecific crossings in Passiflora species.

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 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.