Frequency Of Tetrads Research Articles

Cytological mechanism of 2n pollen formation in Chinese jujube (Ziziphus jujuba Mill. ‘Linglingzao’)

In order to elucidate the cytological mechanism of 2n pollen formation in Chinese jujube, a cultivar named ‘Linglingzao’ (2n = 2x = 24) which produces relative more 2n pollens naturally was employed for microsporogenesis analysis. Chromosomes paired in 12 bivalents at diakinesis and the first meiotic division was normal, whereas, the second division was characterized by frequent abnormal spindle orientation (parallel and tripolar spindles). Perpendicular, tripolar, and parallel spindles at metaphase II accounted for 72.93, 17.22, and 9.85% respectively. Perpendicular and tripolar spindles led to tetrads and triads formation respectively. Two types of parallel spindles were observed and a significant association correlation (r = 0.84, P < 0.05) between parallel spindle II (fused spindles) and dyads was found which meant only fused spindles can form dyads, while parallel spindle I with a long distance between two sets of chromosomes seemed to lead tetrad formation. At tetrad stage, the observed frequencies of tetrads, triads and dyads were 80.99, 16.40, and 2.61% respectively. By the rule that each tetrad can form four n pollens, each triad can form one 2n pollen and two n pollens, each dyad can form two 2n pollens, the frequency of 2n pollen estimated was 5.71%. Based on the pollen diameter difference between n and 2n pollens, frequency of 2n pollen observed was 6.15%. No significant difference was observed between frequency of 2n pollen estimated and that of observed. The results indicated that, tripolar spindles and fused spindles at metaphase II followed by more triads and few dyads formation at tetrad stage were responsible for the production of 2n pollen in Chinese jujube. 2n gametes observed correspond to first division restitution. Research on the cytological mechanism of unreduced pollen will provide a platform for unreduced pollen induction and polyploidy breeding in Chinese jujube.

Mortality of pollen grains may result from errors of meiosis: study of pollen tetrads in Typha latifolia L.

In the cattail Typha latifolia the four haploid products of meiosis remain attached and form the flat tetrad of pollen grains. Gametophytic lethals arisen de novo in diploid cells of sporophyte must manifest themselves as pollen tetrads with two dead grains. This could allow to estimate the rate of recessive lethals arresting pollen grain development. We studied pollen samples collected from 44 sprouts in two populations in the vicinity of Novosibirsk. The anomalous tetrads T1, T2, T3, and T4 carrying one, two, three, and four dead grains, respectively, were detected in each sampled individual. The mean frequency of all anomalous tetrads in the two populations was 3.4% and 8.7%. The frequencies of tetrad classes varied widely among the individuals with correlation coefficient up to 0.94, but their ratios remained nearly constant. The majority of anomalous tetrads were presented by T1 and T2 classes (their sum comprising 72.7 and 74.0% in two populations), T1 being a little more abundant. The observed pattern of frequencies of tetrads with dead grains can be explained by errors of male meiosis such as chromosome non-disjunction in both meiotic divisions. The tetrads with two dead pollen grains may result mostly from non-disjunction in anaphase I, and those with one pollen grain from non-disjunction in anaphase II, thus making tetrad analysis ineffective for estimating the rate of gametophytic lethals.

Tetrad-FISH analysis reveals recombination suppression by interstitial heterochromatin sequences in rye (Secale cereale).

Tetrad analysis is a genetic method that can locate genes and centromeres on a linkage map with a high degree of precision. Despite its effectiveness and accuracy, application of this method is generally limited to fungi, algae and mosses. Here we demonstrate a new method of tetrad analysis that is applicable to other organisms. This combines tetrad analysis with fluorescence in situ hybridization (FISH), and is thus referred to as tetrad-FISH analysis. We demonstrate the effectiveness of this method using tetrads of rye, Secale cereale. The rye strain JNK contains interstitial heterochromatin in a region of Chromosome 2R. We have previously cloned the tandemly repeated sequence forming this heterochromatin in the plasmid pScJNK. We performed FISH using pScJNK as the probe on tetrads obtained from heterozygotes for the heterochromatin region. The frequency of tetrads demonstrating positive signals in two cells that are diagonally opposite one another must correspond to the frequency of recombination in the interval between the heterochromatin and the centromere. Comparison between the results of tetrad-FISH analysis and linkage maps based on RFLP markers clearly indicated that heterochromatin strongly suppresses recombination of whole chromosomal regions. We discuss the effectiveness of tetrad-FISH analysis, particularly for the localization of functional centromeres in linkage maps.

Conversion-type and restoration-type repair of DNA mismatches formed during meiotic recombination in Saccharomyces cerevisiae.

Meiotic recombination in yeast is associated with heteroduplex formation. Heteroduplexes formed between nonidentical DNA strands contain DNA mismatches, and most DNA mismatches in wild-type strains are efficiently corrected. Although some patterns of mismatch correction result in non-Mendelian segregation of the heterozygous marker (gene conversion), one predicted pattern of correction (restoration-type repair) results in normal Mendelian segregation. Using a yeast strain in which a marker leading to a well-repaired mismatch is flanked by markers that lead to poorly repaired mismatches, we present direct evidence for restoration-type repair in yeast. In addition, we find that the frequency of tetrads with conversion-type repair is higher for a marker at the 5' end of the HIS4 gene than for a marker in the middle of the gene. These results suggest that the ratio of conversion-type to restoration-type repair may be important in generating gradients of gene conversion (polarity gradients).

Pollen morphology of Zelkova sicula (Ulmaceae), a recently discovered relic species of the European Tertiary flora: description, chromosomal relevance, and palaeobotanical significance

Pollen morphological characteristics of Zelkova sicula Di Pasquale, Garfi et Quézel and Z. carpinifolia (Pallas) Koch (Ulmaceae) are described. Z. sicula is a shrub species which was recently discovered on southeastern Sicily. Z. carpinifolia is the Zelkova species to which some European Tertiary to Quaternary leaf fossils have been attributed in recent studies. The new results were compared with the pollen morphological characteristics of some other Ulmaceae species such as Z. abelicea (Lamark) Boissier, Z. serrata (Thunberg) Makino, Ulmus glabra Hudson, U. laevis Pallas, U. minor Miller, and U. procera Salisbury. It was observed that Z. sicula usually possesses 5 to 6 pores, whereas the other Zelkova species normally have fewer than 5 pores. Higher frequencies of tetrads, a Y-mark on the proximal face, and a smoother exine sculpture (compared with other Zelkova species) were noted for pollen of Z. sicula. The equatorial diameter of Z. sicula was larger than every other Ulmaceae species, with a smaller P E ratio. Some of these differences can be explained by the triploid nature of Z. sicula, which is quite exceptional for Ulmaceae. Given these remarkable characteristics, it should be possible to recognise Z. sicula pollen grains in fossil pollen assemblages. It is thus anticipated that a more comprehensive pollen analytical study will lead to a revision of the Mediterranean phytogeographical history of the genus Zelkova.

Cytological Mechanism of 2n Microscope Formation in Garden Asparagus

Microsporogenesis was studied in 42 randomly chosen Fz plants of garden asparagus (Asparagus officinalis L. cv. UC 157) (2n = 2x = 20) that had been previously screened for production of pollen of heterogeneous size. At the tetrad stage, the average frequencies of tetrads, triads, and dyads were 58.9%, 15.4%, and 25.9%, respectively. Dyads and triads originated from the lack of chromosome migration toward opposite poles at anaphase II in either one or both cells of a microsporocyte, followed by the absence of cytokinesis in telophase II. The resulting 2n microspores were, therefore, genetically equivalent to second meiotic division restitution products. The observation that all plants examined produced 2n microspores in high frequencies is taken as an indication that the modified meiosis in these plants is under genetic control.

Some Observations on the Cytology in Ochna sp(Ochnaceae).

Ochna's meiosis has been analysed in search of factors that could cause the abortion of fruits. Results showed low frequency of meiotic irregularities but high frequency of tetrads with a variable number of microcytes, besides converging spindles in meiosis II leading up to the formation of triads. Abnormal behaviour of the nucleolus was also observed in many microsporocytes. These factors may explain, at least in part, the high frequency of abortion in Ochna's fruit.

Chromosome damage induced by artificial seed aging in barley

Barley (Hordeum vulgare L. 'Himalaya') seeds were artificially aged under two storage conditions (32 °C/12% moisture content (m.c.) and 38 °C/18% m.c.) to study the behavior of induced chromosomal aberrations during plant growth. The frequencies of aberrant anaphases at first mitosis in root tips were correlated with loss of germinability. However, after 3 and 5 weeks' growth, aberration frequency declined. In plants grown from artificially aged seeds, the frequency of aberrant anaphases appeared to be stabilized at about 1% after 5 weeks' growth, in spite of the large differences in the frequencies at first mitosis. This suggests that because of their genetic imbalance, cells with chromosomal aberrations induced by seed aging were being excluded during plant growth. Meiotic chromosome configurations at MI were normal (7 II) in all plants studied, although a few precocious separations were found. Meiotic aberrations were found at AI-TI, AII-TII and the tetrad stages in the pollen mother cells of plants grown from the control and artificially aged seeds. However, there were no clear differences among the control and the two aging treatments. It was obvious that some cells with meiotic chromosomal aberrations were lost between the AI-TI and AII-TII stages, and still more between the AII-TII and tetrad stages. The frequency of tetrads with micronuclei in plants produced from artificially aged seeds was the same as in the control. The plants grown from artificially aged seeds showed high pollen fertility (95.2 to 97.0%) and seed fertility (90.1 to 97.2%) which was comparable to the control values (97.4 and 97.9%) respectively, indicating no special effects of seed aging. Anaphase cells of the first mitosis in the next (A2) generation were analyzed to study the transmission of chromosomal aberrations through mitotic and meiotic cell divisions in the A1 generation. Aberrant anaphases in the progeny from the artificially aged seeds were not higher than those of the control progeny. This indicates that the chromosomal aberrations induced by seed aging are not transmitted to the next generation.

Untersuchungen �ber den Einfluss der Temperatur auf die Rekombinationsh�ufigkeit beiSphaerocarpus

The effect of temperature on recombination frequency was investigated in the left arm of linkage group I inSphaerocarpus donnellii. 29501 tetrads from 4-factor crosses were analysed from sporophytes grown during premeiotic interphase and meiosis at different temperatures between 6° and 33°C. When temperature was plotted against recombination frequency a curve with a minimum at about 20°C was obtained (Table 1). The frequency of tetrads with double and multiple exchanges showed a minimum at the same temperature (Fig. 3). The effect of temperature is more pronounced in regions close to the centromer than in more distal regions.

Use of Loosely Linked Genes to estimate Chromatid Interference by Tetrad Analysis

STRICKLAND1 has applied to tetrad data for Aspergillus nidulans the method I have suggested2 for estimating the magnitude of chromatid interference between cross-overs when loosely linked genes are used. The principle underlying this method is as follows : Consider three linked loci X, Y and Z spaced along a chromosome in that order. By means of tetrad analysis one can distinguish a number of tetrad genotypes. Certain of these, called (a), (b) and (c), respectively, are most simply interpreted as resulting from the following cross-overs at meiosis : (a) One cross-over between X and Y and another between Y and Z involving the same chromatids, that is, a two-strand double cross-over. (b) One cross-over between X and Y and another between Y and Z, with a three-strand relationship. (c) One cross-over between X and Y and another between Y and Z involving the other two chromatids, that is, a four-strand double cross-over. With closely linked genes, the frequencies of (a), (b) and (c) provide accurate information about the frequencies of two-, three-and four-strand relationships between cross-overs. If the genes are loosely linked, these tetrad genotypes can also arise as a consequence of a three-strand double cross-over between X and Y (or Y and Z) and a third cross-over between Y and Z (or X and Y), but the strand-relationships with the third cross-over are two- or four-strand for genotype (b) and three-strand for genotypes (a) and (c). In other words, the occurrence of these triple cross-over tetrads will hinder the detection of chromatid interference by tending to obscure differences from the 1 : 2 : 1 proportion of genotypes (a), (b) and (c). This is the ratio expected with no chromatid interference. The frequency of these inconvenient triple cross-over tetrads can be estimated from the observed frequency of another tetrad genotype (d), which results from a four-strand double cross-over between X and Y (or Y and Z) and a third cross-over between Y and Z (or X and Y). In this way, allowance can be made for the effects of triple cross-overs.