Time course study of the chromosome-type breakage-fusion-bridge cycle in maize.

The B chromosome of maize has been used in a study of dicentric chromosomes. TB-9Sb is a translocation between the B and chromosome 9. The B-9 of TB-9Sb carries 60% of the short arm of 9. For construction of dicentrics, a modified B-9 chromosome was used, B-9-Dp9. It consists of the B-9 chromosome plus a duplicated 9S region attached to the distal end. In meiosis, fold-back pairing and crossing over in the duplicated region gives a chromatid-type dicentric B-9 that subsequently initiates a chromatid-type breakage-fusion-bridge cycle. In the male, it forms a single bridge in anaphase II of meiosis and at the first pollen mitosis. However, the cycle is interrupted by nondisjunction of the B centromere at the second pollen mitosis, which sends the B-9 dicentric to one pole and converts it from a chromatid dicentric to a chromosome dicentric. As expected, the new dicentric undergoes the chromosome-type breakage-fusion-bridge cycle and produces double bridges. A large number of plants with chromosome dicentrics were produced in this way. The presence of double bridges in the root cells of plants with a chromosome dicentric was studied during the first 10 wk of development. It was found that the number of plants and cells showing double bridges declined steadily over the 10-wk period. Several lines of evidence indicate that there was no specific developmental time for dicentric loss. "Healing" of broken chromosomes produced by dicentric breakage accounted for much of the dicentric loss. Healing produced a wide range of derived B-9 chromosomes, some large and some small. A group of minichromosomes found in these experiments probably represents the small end of the scale for B-9 derivatives.     

Locating a site on the maize B chromosome that controls preferential fertilization.

In maize, the B chromosome can undergo nondisjunction at the second pollen mitosis, producing sperm with two B chromosomes and sperm with zero B chromosomes. Preferential fertilization is the ability of the sperm carrying two B chromosomes to transmit more frequently to the embryo of a kernel than the sperm lacking the B chromosome. A translocation involving the B chromosome and chromosome 9, TB-9Sb, has been used to study preferential fertilization. The B-9 chromosome has the same properties of nondisjunction and preferential fertilization as the standard B chromosome. Deletion derivatives of B-9, which lack the centric heterochromatin and possibly some adjacent euchromatin, were tested for their ability to induce preferential fertilization. They were found to lack the capacity for preferential fertilization.     

TRANSMISSION AND SIGNIFICANCE OF B CHROMOSOMES IN ANTHURIUM WAROCQUEANUM

Somatic and meiotic chromosomes of one plant of Anthurium warocqueanum J. Moore and its selfed offspring were analyzed. The parent showed 2n = 30 + 3B in both somatic cells and pollen mother cells. The B chromosomes divided normally in somatic cells, but meiotic associations of Bs varied. Three configurations of three B chromosomes were observed at metaphase I of parent meiosis: one trivalent, one bivalent and one univalent, or three univalents. The number of B chromosomes in offspring ranged from 0 to 6, indicating their transmission from both male and female gametes. Offspring with two B chromosomes appeared in greatest frequency. It was hypothesized that both male and female gametes of the 3 B parent frequently contained one B chromosome through the normal distribution of the bivalent Bs at meiosis and the elimination of the univalent B chromosome due to lagging. Examination of pollen mother cells of offspring also revealed irregular behavior of B chromosomes. With a high number of B chromosomes, normal A chromosome bivalent formation seemed to be reduced. No phenotypic effects of B chromosomes were observed.     

Aneuploid model of meiotic transformation mechanisms in wheat. 1. Mechanism of chromosome shift from mitosis to meiosis

A model of chromosome shift from mitosis to meiosis based on analysis of univalent behavior in meiotic anaphase I in the complete series of monosomic lines of Milturum 553 wheat cultivar is proposed. According to the model, chromosomes of homologous pairs change their pole orientation by two different mechanisms. Such co-orientation is achieved through segregation of initial pole zone in one haploid set and through synapsis of homologues in the other.     

Segregation of the amphitelically attached univalent X chromosome in the spittlebug <Emphasis Type="Italic">Philaenus spumarius</Emphasis>

In meiosis I, homologous chromosomes combine to form bivalents, which align on the metaphase plate. Homologous chromosomes then separate in anaphase I. Univalent sex chromosomes, on the other hand, are unable to segregate in the same way as homologous chromosomes of bivalents due to their lack of a homologous pairing partner in meiosis I. Here, we studied univalent segregation in a Hemipteran insect: the spittlebug Philaenus spumarius. We determined the chromosome number and sex determination mechanism in our population of P. spumarius and showed that, in male meiosis I, there is a univalent X chromosome. We discovered that the univalent X chromosome in primary spermatocytes forms an amphitelic attachment to the spindle and aligns on the metaphase plate with the autosomes. Interestingly, the X chromosome remains at spindle midzone long after the autosomes have separated. In late anaphase I, the X chromosome initiates movement towards one spindle pole. This movement appears to be correlated with a loss of microtubule connections between the kinetochore of one chromatid and its associated spindle pole.     

Effect of a rye B chromosome and its segments on homoeologous pairing in hybrids between common wheat and Aegilops variabilis

Rye B chromosomes, which are supernumerary chromosomes dispensable for the host but increase in number by non-disjunction after meiosis, have been reported to affect meiotic homoeologous pairing in wheat-rye hybrids. The effect of a rye B chromosome (B) and its segments (B-9 and B-10) on homoeologous pairing was studied in hybrids between common wheat (2n=42) and Aegilops variabilis (2n=28), with reference to the Ph1 gene located on wheat chromosome 5B. The B-9 and B-10 chromosomes are derived from reciprocal translocations between a wheat and the B chromosomes, and the former had the B pericentromeric segment and the latter had the B distal segment. Both the B and B-9 chromosomes suppressed homoeologous pairing when chromosome 5B was absent. On the other hand, the B-9 and B-10 chromosomes promoted homoeologous pairing when 5B was present. On pairing suppression, B-9 had a greater effect in one dose than in two doses, and B-9 had a greater effect than B-10 had in one dose. These results suggested that the effect of the B chromosomes on homoeologous pairing was not confined to a specific region and that the intensity of the effect varied depending on the presence or absence of 5B and also on the segment and dose of the B chromosome. The mean chiasma frequency (10.23) in a hybrid (2n=36) possessing 5B and one B-9 was considerably higher than that (2.78) of a hybrid (2n=35) possessing 5B alone, and was comparable with that (14.09) of a hybrid (2n=34) lacking 5B. This fact suggested that the B chromosome or its segment can be used in introducing alien genes into wheat by inducing homoeologous pairing between wheat and alien chromosome.     

Unstable inheritance of maize B-type chromosomes that lack centric heterochromatin.

The B chromosome of maize undergoes frequent non-disjunction at the second pollen mitosis. In B-A translocations, the B-A chromosome retains the capacity for non-disjunction. We have collected deletion-derivative TB-9Sb stocks. One derivative, the "type 1 telocentric", has a B-9 chromosome that lacks centric heterochromatin. It produces few recessive (non-disjunctional) phenotypes in pollen parent testcrosses of the translocation heterozygote, 9 9-B telo B-9. The finding helped demonstrate the role of centric heterochromatin in non-disjunction. An isochromo some derivative of the type 1 telocentric was also recovered. It was tested in the 9-B 9-B iso B-9 constitution. This is equivalent to 9 9-B telo B-9 in terms of chromosome 9 dosage. Surprisingly, crosses with the isochromosome gave significant levels of recessive phenotypes. In addition, high levels of variegated phenotypes were found. Recently, a circumstance was found that makes inheritance of the type 1 telocentric chromosome somewhat similar to that of the isochromosome. Crosses with hypoploid 9-B 9-B telo B-9 plants showed significant levels of recessive and variegated phenotypes. These crosses were investigated to help explain the source(s) of the phenotypes. Cytological and genetic studies were performed. Centric misdivision was found to account for the variegated phenotypes. A mixture of conventional B non-disjunction and centric misdivision produced the recessive phenotypes. The significance of conventional non-disjunction in the absence of centric heterochromatin is discussed.     

Genetic regulation of the centromere division in rye and wheat univalent chromosomes in dimonosomics during meiotic anaphase I

Cytogenetic analysis of meiosis in the wheat--rye dimonosomics 1Rv-1A, 1Ron-1A, 2R-2D, 5R-5A, and 6R-6A was conducted. C-banding was used to study the segregation pattern of each of two univalent chromosomes during the first meiotic division. It has been shown that the division frequency of the centromeric regions of all rye chromosomes in the pair studied is significantly higher than in the wheat chromosomes. The ANOVA performed suggest that the plant genotype contributes significantly (at P = 0.05) to the behavior pattern of univalent chromosomes in meiosis. The data obtained demonstrate that the rye and wheat chromosomes studied are involved in genetic regulation of centromere division in meiotic anaphase I (AI). The presence of rye chromosome 2R and wheat chromosome 2D suppresses the division of centromeres of the sister chromatids in AI. Rye chromosomes 1Rv, 1Ron, 5R, and 6R induce equational division; however, rye chromosome 1Rv increases to a greater degree the frequency of equational division of wheat chromosome 1A as compared with chromosome 1Ron.     

Genetic regulation of the centromere division in rye and wheat univalent chromosomes in dimonosomics during the meiotic anaphase I

Cytogenetic analysis of meiosis in the wheat-rye dimonosomics 1Rv-1A, 1Ron-1A, 2R-2D, 5R-5A, and 6R-6A was conducted. C-banding was used to study the segregation pattern of each of two univalent chromosomes during the first meiotic division. It has been shown that the division frequency of the centromeric regions of all rye chromosomes in the pair studied is significantly higher than in the wheat chromosomes. The ANOVA performed suggest that the plant genotype contributes significantly (at P = 0.05) to the behavior pattern of univalent chromosomes in meiosis. The data obtained demonstrate that the rye and wheat chromosomes studied are involved in genetic regulation of centromere division in meiotic anaphase I (AI). The presence of rye chromosome 2R and wheat chromosome 2D suppresses the division of centromeres of the sister chromatids in AI. Rye chromosomes 1Rv, 1Ron, 5R, and 6R induce equational division; however, rye chromosome 1Rv increases to a greater degree the frequency of equational division of wheat chromosome 1A as compared with chromosome 1Ron.     

The reduction of chromosome number in meiosis is determined by properties built into the chromosomes

In meiosis I, two chromatids move to each spindle pole. Then, in meiosis II, the two are distributed, one to each future gamete. This requires that meiosis I chromosomes attach to the spindle differently than meiosis II chromosomes and that they regulate chromosome cohesion differently. We investigated whether the information that dictates the division type of the chromosome comes from the whole cell, the spindle, or the chromosome itself. Also, we determined when chromosomes can switch from meiosis I behavior to meiosis II behavior. We used a micromanipulation needle to fuse grasshopper spermatocytes in meiosis I to spermatocytes in meiosis II, and to move chromosomes from one spindle to the other. Chromosomes placed on spindles of a different meiotic division always behaved as they would have on their native spindle; e.g., a meiosis I chromosome attached to a meiosis II spindle in its normal fashion and sister chromatids moved together to the same spindle pole. We also showed that meiosis I chromosomes become competent meiosis II chromosomes in anaphase of meiosis I, but not before. The patterns for attachment to the spindle and regulation of cohesion are built into the chromosome itself. These results suggest that regulation of chromosome cohesion may be linked to differences in the arrangement of kinetochores in the two meiotic divisions.     

普通小麦栽培品种丰抗13的4B—1D染色体易位的鉴定

通过细胞学观察,在普通小麦栽培品种“丰抗13”和“京红1号”的杂交后代中,发现有多价体出现,这就表明有染色体易位发生。为进一步弄清究竟是哪条染色体发生了易位,我们采用单体测交方法,观察鉴定所有各单体系F_1的花粉母细胞第一次减数分裂中期Ⅰ(以下简称PMCs中Ⅰ)染色体构型。从鉴定结果发现,凡2n=42的F_1 PMCs中Ⅰ出现19~Ⅱ 1~Ⅳ,而2n=41的F_1PMCs中Ⅰ的染色体构型不同,单体与易位有关的两个单体系4B和1D F_1 PMCs中的Ⅰ构型中有部分呈现为19个二价体加1个三价体,即19~Ⅱ 1~Ⅲ,没有单价体,而其余各单体系F_1 PMCs中Ⅰ构型则表现为18个二价体,1个四价体和1个单价体,即18~Ⅱ 1~Ⅰ 1~Ⅳ。因此,可以肯定“丰抗13”存在1个染色体易位,其有关染色体就是4B和1D。     

Unpaired chromosomes at meiosis: cause or effect of gametogenic insufficiency?

Pairing failure at meiosis has been postulated as a cause of gametogenic arrest in both heterozygous translocation carriers and males whose spermatocytes exhibit univalent X and Y chromosomes. The present investigation is a survey of pachytene translocation configurations, at the electron microscopic level, in six stocks of mice, comprising a total of 464 spermatocytes and 343 oocytes. Univalence of the X and Y chromosomes was studied in the same stocks, as well as in three additional homozygous translocation stocks. Fully paired as well as asynaptic configurations were found in all translocation stocks, and the proportions of each configuration differed considerably between spermatocytes and oocytes of mice carrying the same translocation. In both spermatocytes and oocytes, other pairing anomalies were more frequent in cells with asynapsed than with fully synapsed configurations, and spermatocytes with univalent sex chromosomes had a higher proportion of autosomal anomalies than did spermatocytes with XY bivalents. It is concluded that pairing failure at meiosis is primarily a symptom, rather than a cause, of gametogenic arrest, and that chromosome rearrangements, even if they appear to be balanced, may affect the rate of atresia by interfering with the normal rate of meiotic progression. Once pairing failure is established, it could secondarily increase the probability of gametogenic failure.     

The Origin of the Achiasmatic XY Sex Chromosome System in Cacopsylla peregrina (Frst.) (Psylloidea, Homoptera)

The status of an extra univalent, if it is a B chromosome or an achiasmatic Y chromosome, associating with the X chromosome in male meiosis of Cacopsylla peregrina (Frst.) (Homoptera, Psylloidea) was analysed. One extra univalent was present in all males collected from three geographically well separated populations, it was mitotically stable, and showed precise segregation from the X chromosome. These findings led us to propose that the univalent represents in fact a Y chromosome. The behaviour of the X and Y chromosomes during meiotic prophase suggested that their regular segregation was based on an achiasmatic segregation mechanism characterised by a 'touch and go' pairing of segregating chromosomes at metaphase I. To explain the formation of the achiasmatic Y within an insect group with X0 sex chromosome system, it was suggested that the Y chromosome has evolved from a mitotically stable B chromosome that was first integrated into an achiasmatic segregation system with the X chromosome, and has later become fixed in the karyotype as a Y chromosome.     

A Brief Report on the Chromosome Number of Neodiplogaster pinicola and Panagrellus redivivoides (Nematoda: Rhabditida)

Both Neodiplogaster pinicola and Panagrellus redivivoides reproduce amphimictically, with XO type of sex determination. In N. pinicola, primary spermatocytes have six bivalent chromosomes and one univalent; after two meiotic divisions, sperm are produced with either six or seven chromosomes. In primary oocytes, with seven bivalents, meiosis is initiated by entrance of a sperm. After two meiotic divisions, three polar nuclei are produced, and egg and sperm pronuclei fuse. Cleavage begins after the egg is laid. Males have a 2n number of 13 chromosomes; females, 14. In P. redivivoides, primary spermatocytes have four bivalents and one univalent. After two meiotic divisions, spermatids are produced with either four or five well separated chromosomes. In primary oocytes, the first maturation division is initiated after penetration of a sperm; after two meiotic divisions, each egg has five chromosomes. Cleavage begins immediately after fusion of egg and sperm pronuclei, and embryonic development and hatching occur within the uterus. Males have a 2n chromosome number of 9; females, 10.     

木立芦荟小孢子母细胞减数分裂与花粉育性关系的初步研究

文章从减数分裂过程、小孢子发育两方面,探讨了木立芦荟（Aloe arboresens Mill.）花粉败育的原因。木立芦荟花粉母细胞染色体数目为2n=14,由四对长染色体和三对短染色体组成,属二型性核型。其减数分裂异常,发现存在单价体和多价体、染色体桥、落后染色体、不均等分离、微核等。同时观察到木立芦荟染色体具有极度的粘质性,使减数分裂各阶段的染色体不易散开。统计各种异常现象出现的频率并分析了这些异常现象形成的可能机制及对正常小孢子形成的影响,推测染色体间的丝状粘连可能是木立芦荟小孢子母细胞减数分裂异常并导致败育花粉产生的主要因素。成熟花粉粒中90%以上为败育花粉,属碘败型。     

Role of rye chromosome 2R from wheat-rye substitution line 2R(2D)1 (Triticum aestivum L. cv. Saratovskaya 29-Secale cereale L. cv. Onokhoiskaya) in genetic regulation of meiotic restitution in wheat-rye polyhaploids

A study was made of the role of rye chromosome 2R from the wheat-rye substitution line 2R(2D)1 (Triticum aestivum L. cv. Saratovskaya 29-Secale cereale L. cv. Onokhoiskaya) in genetic regulation of meiotic restitution in wheat-rye polyhaploids 2R(2D)1 x S. cereale L. cv. Onokhoiskaya. Rye chromosome 2R proved to affect the completeness of the meiotic program, suppressing the formation of restitution gametes. This was evident from the reductional division of univalent chromosomes in AI and the occurrence of the second meiotic division. The interrelationships between the type of chromosome division in AI and the two-step character of meiosis are discussed. The structural and functional organization of the centromeric regions of chromosomes undergoing reductional division is assumed to determine the two-step character of division.     

A procedure for localizing genetic factors controlling mitotic nondisjunction in the B chromosome of maize

The B chromosome of maize undergoes nondisjunction at the second pollen mitosis at rates as high as 98% (Roman, 1948; Carlson, 1969a). Nondis-junction is controlled by at least two separable regions on the B chromosome (Roman, 1949; Longley, 1956; Carlson, 1969b; Ward, 1972). A procedure for identifying and localizing the chromosomal sites required for nondisjunction is reported here. A translocation between the B and chromosome 9 (TB-9b) was utilized. Plants carrying TB-9b were screened for mutants of nondisjunction, i.e. translocations in which nondisjunction does not occur. Two such translocations were identified in a small screening. While the mutant translocations have not been analyzed in pachytene, they are most likely deletions or rearrangements of regions on the B chromosome vital to nondisjunction. Diminutive and rearranged B chromosomes are known to arise spontaneously in small populations (Randolph, 1941; Longley, 1956). — Also reported here are the nondisjunctional properties of the B⁹ isochromosome (Carlson, 1970) and several telocentric (or subtelocentric) derivatives of this chromosome. Some derivatives of the isochromosome are virtually incapable of nondisjunction, and should provide information on the role of the centromere in nondisjunction.     

An uncommon H3/Ser10 phosphorylation pattern in Cestrum strigilatum (Solanaceae), a species with B chromosomes

Cestrum strigilatum (Solanaceae) is a South American shrub with B chromosomes. Bs show a univalent behavior when a single B is present, have non-Mendelian segregation, and are poor in genes and rich in repetitive DNA. In this study, the histone H3 at serine 10 (H3/Ser10) phosphorylation pattern was investigated during mitosis and meiosis of C. strigilatum collected from the wild and was compared in A and B chromosomes. The results revealed that H3/Ser10 phosphorylation of A chromosomes occurred only in the pericentromeric region in both mitosis and meiosis, whereas in the B univalent, phosphorylation appeared in almost the whole extent of the chromosome, except in the terminal portion of the long arm. In meiosis II, the phosphorylation of A chromosomes was similar to that in the first division of meiosis, but the Bs did not show H3/Ser10 phosphorylation. Our results suggest that phosphorylation at the pericentromeric region may be associated with chromosome motility during cell divisions and with the cohesion of B chromatids in a univalent structure in meiosis I.     

B Chromosome Behavior in Maize Pollen as Determined by a Molecular Probe

The B chromosomes of maize typically undergo nondisjunction during the second microspore division (generative cell division). When the microspore nucleus contains only one B chromosome, two kinds of sperm result, one with two B chromosomes and one with no B chromosomes. The sperm with the B chromosomes preferentially fertilizes the egg cell. Previous studies of these phenomena have been limited to genetic analysis and chromosome spreads. In this study we show that a B chromosome-specific probe can be used with fluorescence in situ hybridization (FISH) analysis to detect the presence, location, and frequency of B chromosomes in intact interphase nuclei within mature pollen of maize. Using genetic line TB-10L18, our results indicate that nondisjunction of the B centromere occurs at an average frequency of 56.6%, based on four plants and 1306 pollen grains analyzed. This is consistent with the results of genetic studies using the same B-A translocation. In addition, our results suggest that B chromosome nondisjunction can occur during the first microspore division. Spatial distribution of the B chromosome-specific probe appears to be largely confined to one tip of the sperm nucleus, and a DNA fragment found outside the pollen nuclei often hybridizes to the B chromosome-specific probe.     

The teflon gene is required for maintenance of autosomal homolog pairing at meiosis I in male Drosophila melanogaster

In recombination-proficient organisms, chiasmata appear to mediate associations between homologs at metaphase of meiosis I. It is less clear how homolog associations are maintained in organisms that lack recombination, such as male Drosophila. In lieu of chiasmata and synaptonemal complexes, there must be molecules that balance poleward forces exerted across homologous centromeres. Here we describe the genetic and cytological characterization of four EMS-induced mutations in teflon (tef), a gene involved in this process in Drosophila melanogaster. All four alleles are male specific and cause meiosis I-specific nondisjunction of the autosomes. They do not measurably perturb sex chromosome segregation, suggesting that there are differences in the genetic control of autosome and sex chromosome segregation in males. Meiotic transmission of univalent chromosomes is unaffected in tef mutants, implicating the tef product in a pairing-dependent process. The segregation of translocations between sex chromosomes and autosomes is altered in tef mutants in a manner that supports this hypothesis. Consistent with these genetic observations, cytological examination of meiotic chromosomes suggests a role of tef in regulating or mediating pairing of autosomal bivalents at meiosis I. We discuss implications of this finding in regard to the evolution of heteromorphic sex chromosomes and the mechanisms that ensure chromosome disjunction in the absence of recombination.