Relationship between pachytene synapsis, metaphase I associations, and transmission of 2B and 4B chromosomes in rye.

2B rye plants selected for high (H) or low (L) B transmission rate were studied at pachytene and metaphase I of meiosis to determine the relationship between synapsis, bivalents at metaphase I, and B transmission rate. The results show that the 2 B chromosomes (Bs) form bivalents at pachytene in both the H and L lines, whereas the frequency of bivalents at metaphase I is much higher in the H than in the L line. This demonstrates that B transmission is mainly related to the proper association of Bs at metaphase I, as well as that synapsis of the 2 Bs in the L line is normal, but the bivalent is not consolidated by a chiasma in most cases. Crosses were made between 2B plants of the H and L lines in all combinations (H x H, H x L, L x H, and L x L) to obtain 4B plants. Similarly, bivalent formation at pachytene and metaphase I was studied. The results show that 4B plants of the H x H and L x L classes differ significantly at pachytene and metaphase I since the former forms more bivalents. The heterozygous 4 Bs of the H x L and L x H classes show intermediate values. The relation H x H > H x L > L x H > L x L was consistently found for the variables transmission rate, bivalents at pachytene, bivalents at metaphase I, and B mean chiasma frequency. A maternal effect was also found. Our data suggest that there are two separate mechanisms acting upon synapsis and chiasma formation in H and L B chromosomes: (i) there is variable efficiency of the control of synapsis at early stages of meiosis; and (ii) there is variable efficiency of the control of the number of chiasmata.     

Ultrastructure of meiotic pairing in B chromosomes of Crepis capillaris

The meiotic pairing behaviour of four B isochromosomes of Crepis capillaris was studied by synaptonemal complex (SC) surface spreading of pollen mother cells. The four B chromosomes form a tightly associated group, separate from the standard chromosomes, throughout zygotene and pachytene. All four B chromosomes are also folded around their axis of symmetry, the centromere, and the eight homologous arms are closely aligned from the earliest prophase I stages. A high frequency of multivalent pairing of the four B chromosomes is observed at pachytene, in excess of 90%, mirroring the situation observed at metaphase I but exceeding the frequency expected (76.2%) on the assumption of random pairing among the eight B isochromosome arms with a single distal pairing initiation site per arm. The higher than expected frequency of multivalents is due to the occurrence of multiple pairing initiations along the B isochromosome arms, resulting in high frequencies of pairing partner switches. Pairing of the standard chromosome set is frequently incomplete in the presence of four B chromosomes, and abnormalities of SC structure such as thickening and splitting of axes and lateral elements are also frequently seen. Similarly, B chromosomes show partial pairing failure, the extent of which is correlated with pairing failure in the standard chromosome set. The B chromosomes themselves also show abnormalities of SC structure. Both standard and B chromosomes show non-homologous foldback pairing of regions that have failed to pair homologously.by D. Schweizer     

Synaptonemal complex and recombination nodules in rye (Secale cereale)

Meiotic prophase in rye was investigated by serial-section reconstruction of pollen mother cell nuclei. In the mid-late zygotene nucleus, all lateral elements were continuous from telomere to telomere, and 9–20 pairing initiation sites per bivalent were observed. Chromosome and bivalent interlockings detected during zygotene were resolved at early pachytene when pairing was completed. In the three pachytene nuclei, the relative synaptonemal complex (SC) lengths and arm ratios were found to be in good correlation with light microscopic data of pachytene bivalents. Spatial tracing of the bivalents showed that they occupy separate areas in the nucleus. Three types of recombination nodules were observed: large, ellipsoïdal and small nodules at early pachytene and irregularly shaped nodules mainly associated with chromatin at late pachytene. Their number and position along the bivalents correlated well with the number and distribution of chiasmata. The classification of the seven bivalents was based on arm ratio and heterochromatic knob distribution.     

Absence of qualitative genes controlling interspecific pairing in rye B chromosomes

Summary The meiotic behaviour of hybrids between Secale cereale carrying B chromosomes and S. vavilovii has been studied in order to estimate the effects of B chromosomes on hybrid meiotic pairing. The possible effect of Bs on the meiotic pairing of the offspring from backcrosses with S. vavilovii has been studied also. The results obtained clearly indicate that no detectable differences existed in chromosome pairing of hybrids with or without B chromosomes. The hypothetical existence of epistatic genes on cereale genome masking the effect of Bs has been rejected after the results obtained in backcrosses. Therefore, lack of qualitative genes controlling interspecific pairing on rye B chromosomes has been concluded. A quantitative effect of B chromosomes was detected only when they were in alien cytoplasm.     

Spreading synaptonemal complexes of B isochromosomes in the grasshopper Omocestus burri.

Meiotic pairing behaviour of one and two B isochromosomes (iso-Bs) in the grasshopper Omocestus burri was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I. Iso-Bs display a peripheral location in the surface-spread nuclei and early pairing relative to that of the long members of the A set. Single iso-Bs undergo foldback pairing to give symmetrical hairpin loops. Two iso-Bs may show interarm pairing, mterchromosome pairing, or combinations of the two. Pericentromeric interarm pairing can be delayed in one or both Bs and this delay is mostly observed in bivalents with pairing partner switches. The iso-B bivalent frequencies observed in the three males analysed were 64, 44, and 41%, respectively; the two latter values were significantly lower than the 66% predicted by the random-end-pairing model. There is a reduction in the frequencies of iso-ring univalents (in 1B males) and bivalents (in 2B males) from pachytene to metaphase I. Similarities and differences between the pairing behaviour of iso-Bs from different species are also discussed.     

Unequal nondisjunction frequencies of trivalent chromosomes in male mice heterozygous for two Robertsonian translocations

Robertsonian (Rb) translocation heterozygosity may cause pairing problems during prophase and segregation irregularities at anaphase of meiosis I. These stages of meiosis I were studied in male mice doubly heterozygous for the two Rb chromosomes Rb(9.19)163H and Rb(16.17)8Lub. At pachytene both Rb chromosomes similarly showed pairing irregularities like unpaired segments. However, highly different nondisjunction frequencies of chromosomes forming the respective trivalents were found. The nondisjunction frequency of the Rb8Lub trivalent chromosomes was about 40%, whereas a very low frequency of nondisjunction was found in combination with the Rb163H trivalent. Since both trivalents were together in the same cell, differences in kinetochore function are assumed to be responsible for the diverse frequency of nondisjunction.     

B chromosomes in plants

Abstract

B chromosomes (Bs) can be described as “selfish chromosomes”, a term that has been used for the repetitive DNA which comprises the bulk of the genome in large genome species, except that Bs have a life of their own as independent chromosomes. They can accumulate in number by various processes of mitotic or meiotic drive, especially in the gametophyte phase of the life cycle of flowering plants. This parasitic property of drive ensures their survival and spread in natural populations, even against a gradient of harmful effects on the host plant phenotype. B chromosomes are inhabitants of the nucleus and they are subject to control by “genes” in the A chromosome (As) complement. This interaction with the As, together with the balance between drive and harmful effects makes a dynamic system in the life of a Bs. In this review, we concentrate mainly on recent developments in the Bs of rye and maize, two of the species currently receiving most attention. We focus on their population dynamics and on the molecular basis of their structural organisation and mechanisms of drive, as well as on their mode of origin and potential applications in plant biotechnology.     

The effect of the wheat Ph1 locus on chromatin organisation and meiotic chromosome pairing analysed by genome painting

The Ph1 locus in wheat influences homo(eo)logous chromosome pairing. We have analysed its effect on the behaviour and morphology of two 5RL rye telosomes in a wheat background, by genomic in situ hybridisation (GISH), using rye genomic DNA as a probe. Our main objective was to study the effect of different alleles of the Ph1 locus on the morphology and behaviour of the rye telosomes in interphase nuclei of tapetal cells and in pollen mother cells at early stages of meiosis. The telosomes, easily detectable at all stages, showed a brightly fluorescing chromomere in the distal region and a constriction in the proximal part. These diagnostic markers enabled us to define the centromere and telomere regions of the rye telosomes. In the presence of functional copies of Ph1, the rye telosomes associated at pre-leptotene, disjoined and reorganised their shape at leptotene, and became fully homologously paired at zygotene – pachytene. In plants without functional alleles (ph1bph1b), the rye telosomes displayed an aberrant morphology, their premeiotic associations were clearly disturbed and their pairing during zygotene and pachytene was reduced and irregular. The Ph1 locus also influenced the behaviour of rye telosomes in the interphase nuclei of tapetal cells: in Ph1Ph1 plants, the rye telosomes occupied distinct, parallel-oriented domains, whereas in tapetal nuclei of ph1bph1b plants they were intermingled with wheat chromosomes and showed a heavily distorted morphology. The results shed new light on the effect of Ph1, and suggest that this locus is involved in chromosome condensation and/or scaffold organisation. Our explanation might account for various apparently contradictory and pleiotropic effects of this locus on both premeiotic associations of homologues, the regulation of meiotic homo(eo)logous chromosome pairing and synapsis, the resolution of bivalent interlockings and centromere behaviour. Received: 27 April 1998; in revised form: 5 August 1998 / Accepted: 11 August 1998     

Interaction between rye B-chromosomes and wheat genetic systems controlling homoeologous pairing

The interactive effect on homoeologous pairing of rye B-chromosomes with the absence of both pairing suppressor (3A, 3D, 5B) and promotor (3B, 5A, 5D) chromosomes of common wheat (Triticum aestivum L.) is analyzed by comparison of pairing at Metaphase I of 27-, 27+2B, 28- and 28+2B-chromosome plants. These plants were obtained from crosses between the respective wheat monosomics (2n=41) and rye plants (Secale cereale L.) carrying or not carrying two B-chromosomes (2n=14 or 14+2Bs). —The effect of rye B-chromosomes on pairing depends on the function of the wheat chromosome which is absent in the appropriate hybrids, i.e., rye B-chromosomes have a suppressor effect on pairing when the pairing suppressing wheat chromosomes 3A, 3D or 5B are absent, while they behave as promotors when the pairing promoting chromosomes 3B, 5A or 5D are absent.     

Unstable B chromosomes in Silene maritima With. (Caryophyllaceae)

Unstable B chromosomes in Silene maritima With. (Carophyllaceae). B chromosomes have been found in 7 out of 39 populations of Silene maritima With. (Carophyllaceae) studied. These 7 populations are all from the Norfolk coast. The B chromosome is small, 0.8 μm long, telocentric and appears euchromatic. Within plants the B chromosome is highly unstable with different numbers in cells of the same root or pollen mother cells (pmcs) of an anther. The transmission of the Bs is efficient and in controlled crosses progeny plants usually have higher mean B frequencies than the B-containing parents. At metaphase I the pairing behaviour of the Bs is variable and they often appear as univalents. In general the presence of Bs in pmcs results in an increase in chiasma frequency/cell and populations with Bs have higher mean chiasma frequencies than those without Bs. Cloned plants grown under stress conditions induced by the addition of NaCl to a nutrient solution showed no difference in mean B number/cell or distribution when compared with controls grown in nutrient solution only.     

B chromosomes in Brazilian rodents

B chromosomes are now known in eight Brazilian rodent species: Akodon montensis, Holochilus brasiliensis, Nectomys rattus, N. squamipes, Oligoryzomys flavescens, Oryzomys angouya, Proechimys sp. 2 and Trinomys iheringi. Typically these chromosomes are heterogeneous relative to size, morphology, banding patterns, presence/absence of NORs, and presence/absence of interstitial telomeric signals after FISH. In most cases, Bs are heterochromatic and late replicating. Active NORs were detected in two species: Akodon montensis and Oryzomys angouya. As a rule, Bs behave as uni or bivalents in meiosis, there is no pairing between Bs and autosomes or sex chromosomes and also their synaptonemal complexes are isopycnotic with those in A chromosomes.     

Chromosome pairing in tetraploid rye with monosomic-substitution wheat chromosomes

In tetraploid rye with single-substitution wheat chromosomes - 1A, 2A, 5A, 6A, 7A, 3B, 5B, 7B - chromosome pairing was analysed at metaphase I in PMCs with the C-banding method. The frequency of univalents of chromosome 1A was considerably higher than that of the other four wheat chromosomes of genome A (6A, 5A, 7A and 2A). Among chromosomes of genome B, the lowest mean frequency of univalents was observed for chromosome 5B. In monosomic lines, wheat chromosomes 1A, 2A, 5A, 6A, 7A and 5B paired with rye homoeologues most often in rod bivalents and in chain quadrivalents (also including 3B). The 47% pairing of 5B-5R chromosomes indicate that the rye genomes block the suppressor Ph1 gene activity. In monosomic plants with chromosomes 5A, 2A, 6A, 7A and 5B, a low frequency of rye univalents was observed. It was also found that the wheat chromosomes influenced the pairing of rye genome chromosomes, as well as the frequency of ring and rod bivalents and tri- and quadrivalents. However, the highest number of terminal chiasmata per chromosome occurred in the presence of chromosomes 5A and 2A, and the lowest - in the presence of chromosomes 3B and 7B. In the presence of chromosome 5B, the highest frequency of bivalents was observed. The results of the present study show that the rye genome is closer related to the wheat genome A of than to genome B. The high pairing of wheat-rye chromosomes, which occurs in tetraploid rye with substitution wheat chromosomes, indicates that there is a high probability of incorporating wheat chromosome segments into rye chromosomes.     

Pairing and recombination between individual chromosomes of wheat and rye in hybrids carrying the ph1b mutation

Wheat-rye chromosome associations at metaphase I studied by Naranjo and Fernández-Rueda (1991) in ph1b ABDR hybrids have been reanalysed to establish the frequency of pairing between individual chromosomes of wheat and rye. Wheat chromosomes, except for 2A and 2D, and their arms were identified by C-banding. Diagnostic C-bands and other cytological markers such as telocentrics or translocations were used to identify each one of the rye chromosomes and their arms. Both the amount of telomeric C-heterochromatin and the structure of the rye chromosomes relative to wheat affected the level of wheatrye pairing. The degree to which rye chromosomes paired with their wheat homoeologues varied with each of the three wheat genomes; in most groups, the B-R association was more frequent than the A-R or D-R associations. Recombination between arms 1RL and 2RL and their homoeologues of wheat possessing a different telomeric C-banding pattern was detected and quantified at anaphase I. The frequency of recombinant chromosomes obtained supports the premise that recombination between wheat and rye chromosomes may be estimated from wheat-rye pairing.     

The chromosomes of Micromys minutus (Rodentia, Murinae). II. Pairing pattern of X and Y chromosomes in meiotic prophase

Both light and electron microscopy were used to study the pairing behavior of the sex chromosomes of the harvest mouse, Micromys minutus, in surface-spread pachytene spermatocytes. The XY pairing pattern is very exceptional in that the site of synaptic initiation is located interstitially in the short arms of the X and the Y, next to their centromeric regions. From this tiny euchromatic site, synapsis proceeds unidirectionally along the homologous heterochromatic short arms of the X and the Y toward the ends of the chromosomes. After pairing of the short arm is concluded, synapsis begins between the nonhomologous long arms of the X and the Y in the immediate vicinity of the centromeres and progresses unidirectionally toward the end of the long arm of the Y. A synaptic complex develops between the constitutive heterochromatin of the long arm of the Y and the euchromatin of the long arm of the X. Analysis of C-banded and distamycin A/DAPI-stained diakineses revealed a trefoil-like XY bivalent, which was interpreted to be the result of an interstitial chiasma occurring in the paired short arms of the X and the Y. A conspicuous, electron-dense body, about 1 micron in diameter, was found closely associated with the centromeres of the X and the Y in numerous pachytene spermatocytes. A review of the literature showed that comparable XY-associated bodies have been found in only eight other mammals to date.     

Genetic differentiation in populations of the yellow-necked mouse,Apodemus flavicollis,harbouring B chromosomes in different frequencies

Two alternative models are used to explain maintenance of polymorphism of B chromosomes (Bs) in populations of a great number of species. The parasitic model suggests deleterious effects of Bs on fitness of carriers, while the heterotic model assumes that, in the absence of drive, equilibrium is produced by beneficial effects of Bs at low numbers. In order to determine the potential contribution of Bs to genetic differentiation and diversity, four populations of Apodemus flavicollis, differing in frequency of Bs (from 0.23 to 0.38) and settled in ecologically different habitats, were analyzed by 471 AFLP markers. Although numerous loci were demonstrated to be population specific, none of them was associated with individuals with Bs. AMOVA showed that the presence of Bs does not affect population differentiation, pointing to greater genetic similarity of Bs to A chromosomes. The greatest genetic diversity (0.241) was found in the population settled in optimal conditions for this species featured by the lowest frequency of animals with Bs (0.23). We found that the majority of loci marked as loci under directional selection, are characteristic of either a population with lower or one with a higher frequency of Bs. Several loci detected as outliers were associated with environmental variables that could directly and/or indirectly influence population dynamics of A. flavicollis. Thus, we suggest that the different frequency of Bs carriers in populations is related to adaptive differentiation to diverse habitats, which is in accordance with the heterotic model of Bs maintenance.     

Synaptic initiation and extension as prerequisites for crossing over

The effect on crossover frequency in maize of three hour heat treatment was studied when treatment was applied at zygotene (substantially later than the major DNA synthetic period) and at pachytene. Crossover frequency assay was based upon bridge and fragment frequency at anaphase I in heterozygotes for a short paracentric inversion. Effect of treatment was studied in three distinguishable synaptic classes: (1) overall crossover frequency within the inversion, (2) double crossover frequency where two separate events of pairing initiation are required (coincident crossovers within and proximal to the inversion) and (3) double crossover frequency within the inversion, where spreading of synapsis over a short distance from a single event of pairing initiation can provide the requisite pairing. Evidence is reported: (1) that overall crossover frequency within the inversion was very significantly increased by treatment at zygotene but not detectably affected by treatment at pachytene; (2) that double crossover frequency within the inversion was very significantly increased by treatment at pachytene and may have been somewhat increased by treatment at zygotene. Results are consistent with the model that most crossover sites may be established at, or approximately at, events of synaptic initiation but that establishment of infrequent second crossover sites near those formed first can follow or accompany the spreading of synapsis to adjoining regions.