On the influence of decreased chiasma frequency on preferential MI pairing behaviour of rye chromosomes in wheat-rye derivatives

Meiotic behaviour of identical and homologous rye chromosomes was investigated in colchicine-induced duplicated meiocytes obtained from different wheat-rye derivatives. A great reduction in the amount of metaphase I (MI) associations accompanied by a strong tendency for identical over homologous nonidentical preferential MI pairing was found in all of the four rye chromosome arms analysed. Both of these features appear to be associated with a more distal chiasma localization where the presence of an interstitial C-band has allowed two distinct regions within the same chromosome arm to be studied separately. On the other hand, the MI pairing failure observed for the rye chromosomes under analysis does not seem to be an effect of telomeric or interstitial C-heterochromatin.by P.B. Moens     

Interaction between wheat chromosomes and rye telomeric heterochromatin on meiotic pairing of chromosome pair 1R of rye in wheat-rye derivatives

The effect of telomere heterochromatin on metaphase I association of chromosome pair 1R of rye was analyzed in normal diploid plants of rye (2n=14) and in wheat-rye derivatives with the chromosome constitution (0–7)A(0–7)BRR (2n=20, 21 and 22). The C-banding pattern of 1R was variable between plants. In diploid rye the presence or absence of telomeric heterochromatin in 1R does not influence its meiotic pairing. However, in wheat-rye derivatives the presence of telomeric heterochromatin decreases chiasma frequency in the 1R bivalent. This cannot be attributed to interference of heterochromatin with chiasma terminalization. This effect of heterochromatin is most pronounced in heterozygous condition. In plants heterozygous for telomeric C-bands the reduction of pairing is stronger in the short arm than in the long arm of the 1R bivalent.     

Pairing competition between identical and homologous chromosomes in autotetraploid rye heterozygous for interstitial C-bands

Pairing competition between identical and homologous non-identical chromosomes is analysed in autotetraploid metaphase I cells of rye where one pair of identical partners bears an interstitial C-band in the long arm of chromosome 6R whereas the other pair of identical partners lacks such a C-band. This makes it possible to study pairing preferences in two distinct regions of the same chromosome arm. A significant excess of associations involving homologous partners is always observed in the proximal segment (from the centromere to the C-band), whereas a good fit with the expected random ratio, or else identical pairing preferences, is found in the distal segment (from the C-band to the telomere). Differences in the processes of pairing and chiasma formation in 6RL, and/or a readjustment in the pattern of chiasma distribution due to heterozygosity for the interstitial C-band as a result of homologous nonidentical pairing, may be responsible for the different behaviour detected in the two regions of the marked arm.     

Chiasma distribution in rye chromosomes of diploid rye and in wheat/rye addition lines in relation to C-heterochromatin

Summary In five genetically different inbred lines of rye and in the seven Chinese Spring/Imperial wheatrye addition lines, chiasma distribution in rye chromosomes was studied with respect to the amount and position of constitutive heterochromatin (Giemsa C-bands). In all inbred lines, rye chromosomes with one primary terminal band were more frequently found as univalents than those with primary bands on both telomeres. These chromosomes were most probably 5R and/or 6R. In the addition lines a highly significant reduction in the number of arms bound by chiasmata was found for rye chromosomes 5R and 6R. Because of the similar chiasma distribution in the inbred lines and in the rye chromosomes of the addition lines, no effect of the wheat genome on the number of chiasmata in the rye chromosomes can be ascertained. However, a relationship between chiasma frequency and chromosome arm length seems to exist, since under reduced chiasma conditions the two shortest arms of the rye complement, those of chromosomes 5R and 6R, frequently fail to form a chiasma. No effect of the large blocks of constitutive heterochromatin in the telomeres of the rye chromosomes on the position of chiasmata within a bivalent could be established.This study was financially supported by the Deutsche Forschungsgemeinschaft     

Meiotic behaviour of single-arm tetrasomic combinations of isochromosomes,telocentrics and normal chromosomes in rye (Secale cereale L.)

The isochromosome studied was derived from the short arm of the satellite chromosome of rye (Secale cereale, 2n=14); the telocentrics represent both the short and long arms of the same chromosome. Three different combinations, tetrasomic for the short arm, have been composed and studied: I: 2 isochromosomes (short arm) + 2 telocentrics (long arm) + 6 normal pairs. II: 1 isochromosome + 2 telocentrics (short arm) + 2 telocentrics (long arm) + 6 normal pairs. III: 1 isochromosome + 1 telocentric (short arm) + 1 normal satellite chromosome + 1 telocentric (long arm) + 6 normal pairs. — Over 20,000 cells were analysed. Simple mathematical models describing the frequencies of the different types of MI configurations in terms of frequency of chiasmata in the different pairing combinations of the polysomic arms, and of the frequency of multivalent pairing of this arm, were developed. They were used to derive estimates for chiasma frequencies and multivalent pairing frequencies in the different chromosome constitutions from the observations on configuration frequencies. Variation between plants and within plants was studied, and it was concluded that much of the within plant heterogeneity was due to regulatory variation expressed independently in different chromosomal segments. There was also a significant genetic component. Analysis of the reasons for the models to fail under certain conditions led to suggestions for extension of the models.     

Unexpected behavior of an inverted rye chromosome arm in wheat

Distal location of chiasmata in chromosome arms is thought to be a consequence of the distal initiation of synapsis. Observations of meiotic behavior of a rye chromosome with an inverted arm show that patterns of chiasma distribution and frequency are also inverted; therefore, the patterns of synapsis and chiasma distribution are independent, and recombination frequency along a chromosome is position-independent and segment-specific. Since cases of random distribution of chiasmata and recombination are known in rye, a genetic mechanism must be present that licenses specific chromosome regions for recombination. Large differences in the metaphase I pairing of the inversion in various combinations of two armed and telocentric chromosomes confirm the major role of the telomere bouquet in early homologue recognition. However, occasional synapsis and chiasmate pairing of the distal regions of normal arms with the proximal regions of the inversion suggest that an alternative mechanism for juxtaposing of homologues must also be present. Synapsis in inversion heterozygotes was mostly complete but in the antiparallel orientation, hence defying homology, but non-homologues never synapsed. Instances of synapsis strictly limited to the chiasma-capable segments of the arm suggest that, in rye, both recombination-dependent and recombination-independent mechanisms for homologue recognition must be present.     

Non-random distribution of chiasmata in rye,Crotalaria and coffee

Summary The two arms of a bivalent can be bound (contain at least one chiasma) or not-bound (contain no chiasma). Formulae are presented to test the randomness of distribution of the bound arms over the chromosome complement, under the assumption that the probability of being bound is related to the arm length.Testing meiotic configurations in Secale cereale (rye), Crotalaria intermedia and several species of Coffea, non-randomness was found in all cases, resulting in an excess of bivalents with only one arm bound. The effect is slightly different in the different plants. The basis is considered to be localization of chiasmata.In rye plants, homozygous for a rather large translocation, an excess of univalents was observed to more than compensate the localization effect. This is attributed to the limited probability of the newly formed short arm of the small translocation chromosome to be bound.The section on rye was started at the Laboratory of Genetics, director Prof. Dr. R. Prakken, Agricultural University, Wageningen, Netherlands. It was completed, with the addition of the sections on Crotalaria and coffee, at the Inter-American Institute of Agricultural Sciences, Turrialba, C. R.Cytogeneticist, Nuclear Energy Program, Inter-American Institute of Agricultural Sciences, Turrialba, Costa Rica, AEC Contract AT (30-1) 2043.     

Metaphase I bonds,crossing-over frequency,and genetic length of specific chromosome arms of rye

Using a Giemsa C-banding procedure three chromosome pairs (3, 6 and 7) have been identified in meiosis of the F1 of a cross between two rye inbred lines. Two of these chromosome pairs (3 and 7) were heterozygous for a prominent telomeric heterochromatic band. The comparison between the frequencies of the different meiotic configurations at metaphase I, anaphase I and metaphase II presented by these two chromosome pairs has allowed the estimation of the chiasma frequency and the genetic length of the chromosome arms 3 short and 7 long.     

Chiasma formation in the short arm of the nucleolar chromosome of the tomato

Summary A translocation heterozygote in tomato (Lycopersicon esculentum) is shown to have a cyclical type of interchange between the long arms of chromosomes 1, 2 (nucleolar) and 3. A study of chromosome association in this plant at metaphase I has indicated that in 21% of the cells a ring of six chromosomes is present. Since an open ring hexavalent can occur only if there is chiasma formation in all the translocated segments and in all the short arms of the three chromosomes, it is concluded that there is considerable frequency of chiasma formation in the short arm of the nucleolar chromosome. This conclusion contradicts the previous observations that chiasma formation is either absent or very rare in the entirely dark staining chromatic, sometimes referred to as heterochromatic, short arm of the nucleolar chromosome.Part of this investigation was carried out at the Department of Genetics, Agricultural University, Wageningen, when the author was serving a contract between the EURATOM-I.T.A.L. and the Agricultural University.     

Nuclear gametophytic genes from chromosome arm 1RS improve regeneration of wheat microspore-derived embryos.

The effect of the 1RS chromosome arm from rye on plant regeneration from microspore-derived embryos was studied using anther culture technology with genotypes carrying the 1BL-1RS translocated chromosomes, the normal wheat chromosome 1BL-1BS, and ditelosomic lines DT 1BS and DT 1BL. A significant difference was observed in microspore-derived green plants between chromosome structure concerned with 1RS and 1BS arms. An analysis of the inheritance of the 1B-1R translocation was performed on the basis of the frequency of male gametes 1BL-1RS in the microspore-derived green plants and that of the 1B-1R translocation inherited through the pollen or the egg cell from structurally heterozygous hybrids 1BL-1BS/1BL-1RS. Both the normal 1B and the translocated 1BL-1RS chromosomes were sexually transmitted through the pollen grains with the same frequency. The 1BL-1RS chromosome is only transmitted through 45% of the egg cells. On the contrary, two-thirds of the microspore-derived green plants regenerated from the anther culture experiments possess the translocated chromosome. The involvement of the rye chromosome arm 1RS from 'Aurora' on regeneration capacity of the microspore-derived embryos has been proposed through the effect of a "gametophytic gene."     

Chiasma frequency effects of structural chromosome change

Three structural chromosome changes in the plant Hypochoeris radicata 2n = 8 have been tested for their effects on chiasma formation: (1) centric fission of chromosome 1, (2) a whole arm exchange between chromosomes 1 and 3, and (3) an interchange between the long arm of chromosome 1 and the short arm of 2 which gives an effectively three-armed pachytene multiple. Mean chiasma frequencies were compared between full-sibs in families segregating for the rearrangements. In each family the chiasma frequency was higher in heterozygotes than basic homozygotes. The size of the chiasma increase is dependant on the number of additional potentially-paired segments in the complement at pachytene. Fission heterozygotes and 1/2 interchange heterozygotes, with one extra pairing region, both form about 0.45 more chiasmata per PMC than full-sib basic homozygotes. The 1/3 exchange, with two additional pairing regions, increases chiasma frequency by twice this, about 0.85 per PMC. Individuals homozygous for the centric fission maintain the raised chiasma level. The chiasma increase appears limited to the chromosome(s) affected by structural change with no detectable interchromosomal effect.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations. IV. Interchange heterozygotes

The quadrivalent of an interchange heterozygote can be divided into six regions, each with its own probability of being bound by at least one chiasma. This gives rise to 2⁶ or 64 different possible combinations of bound and not bound regions and as many M I configurations, several of which are isomorph. Sixteen different types of configuration occur, 10 of which involve more than two chromosomes and can thus be distinguished from the non-interchange bivalents. As each combination of bound and not bound regions has a certain probability of occurring depending on the probability of each region to be bound or not, a frequency expectation value can be estimated for each configuration. Also, when the relative frequencies of the different configurations are known, the probabilities of being bound can be estimated for each of the six regions of a quadrivalent, and from these the crossing-over potentials. One translocation heterozygote was studied, involving the long arm of the satellite chromosome of rye (Secale cereale). Crossing-over was reduced, perhaps mainly by partner exchange. The terminal segments retained a relatively high chiasma frequency. The interstitial regions had low chiasma frequencies even when relatively long.     

Rye chromosome translocations in hexaploid wheat: a re-evaluation of the loss of heterochromatin from rye chromosomes

Summary Using in situ hybridization techniques, we have been able to identify the translocated chromosomes resulting from whole arm interchanges between homoeologous chromosomes of wheat and rye. This was possible because radioactive probes are available which recognize specific sites of highly repeated sequence DNA in either rye or wheat chromosomes. The translocated chromosomes analysed in detail were found in plants from a breeding programme designed to substitute chromosome 2R of rye into commercial wheat cultivars. The distribution of rye highly repeated DNA sequences showed modified chromosomes in which (a) most of the telomeric heterochromatin of the short arm and (b) all of the telomeric heterochromatin of the long arm, had disappeared. Subsequent analyses of these chromosomes assaying for wheat highly repeated DNA sequences showed that in type (a), the entire short arm of 2R had been replaced by the short arm of wheat chromosome 2B and in (b), the long arm of 2R had been replaced by the long arm of 2B. The use of these probes has also allowed us to show that rye heterochromatin has little effect on the pairing of the translocated wheat arm to its wheat homologue during meiosis. We have also characterized the chromosomes resulting from a 1B-1R translocation event.From these results, we suggest that the observed loss of telomeric heterochromatin from rye chromosomes in wheat is commonly due to wheat-rye chromosome translocations.     

Dynamics of rye chromosome 1R regions with high or low crossover frequency in homology search and synapsis development

In many organisms, homologous pairing and synapsis depend on the meiotic recombination machinery that repairs double-strand DNA breaks (DSBs) produced at the onset of meiosis. The culmination of recombination via crossover gives rise to chiasmata, which locate distally in many plant species such as rye, Secale cereale. Although, synapsis initiates close to the chromosome ends, a direct effect of regions with high crossover frequency on partner identification and synapsis initiation has not been demonstrated. Here, we analyze the dynamics of distal and proximal regions of a rye chromosome introgressed into wheat to define their role on meiotic homology search and synapsis. We have used lines with a pair of two-armed chromosome 1R of rye, or a pair of telocentrics of its long arm (1RL), which were homozygous for the standard 1RL structure, homozygous for an inversion of 1RL that changes chiasma location from distal to proximal, or heterozygous for the inversion. Physical mapping of recombination produced in the ditelocentric heterozygote (1RL/1RL(inv)) showed that 70% of crossovers in the arm were confined to a terminal segment representing 10% of the 1RL length. The dynamics of the arms 1RL and 1RL(inv) during zygotene demonstrates that crossover-rich regions are more active in recognizing the homologous partner and developing synapsis than crossover-poor regions. When the crossover-rich regions are positioned in the vicinity of chromosome ends, their association is facilitated by telomere clustering; when they are positioned centrally in one of the two-armed chromosomes and distally in the homolog, their association is probably derived from chromosome elongation. On the other hand, chromosome movements that disassemble the bouquet may facilitate chromosome pairing correction by dissolution of improper chromosome associations. Taken together, these data support that repair of DSBs via crossover is essential in both the search of the homologous partner and consolidation of homologous synapsis.     

Induction of recombination between rye chromosome 1RL and wheat chromosomes

Summary The ph1b mutant in bread wheat has been used to induce homoeologous pairing and recombination between chromosome arm 1RL of cereal rye and wheat chromosome/s. A figure of 2.87% was estimated for the maximal frequency of recombination between a rye glutelin locus tightly linked to the centromere and the heterochromatic telomere on the long arm of rye chromosome 1R in the progeny of ph1b homozygotes. This equates to a gametic recombination frequency of 1.44%. This is the first substantiated genetic evidence for homoeologous recombination between wheat and rye chromosomes. No recombinants were confirmed in control populations heterozygous for ph1b. The ph1b mutant was also observed to generate recombination between wheat homoeologues.     

Meiotic behaviour of chromosomes 1R, 2R and 5R in autotetraploid rye

The meiotic behaviour of chromosomes 1R, 2R and 5R was studied in C-banded preparations of autotetraploid rye. Analysis of pairing and chiasma formation was based on metaphase I configurations, using the model designed by Sybenga, with slight modifications. Frequencies of two modes of pairing (one quadrivalent or two bivalents) differed from those expected for random pairing. Although preferential pairing for some arm pairs of chromosome 2R was detected, this did not seem to be the cause of the increased bivalent pairing. This increase was attributed to either the spatial separation of the four homologous chromosomes in some premeiotic cells into two groups of two, or a correction of the synaptonemal complex, or both. The number of chiasmate associations showed variation between chromosomes and between arms within the same chromosome. It was closely related to arm length, but different after quadrivalent and bivalent pairing. This is suggested to be a consequence of partner exchange interfering with pairing and, consequently, with chiasma formation, and a different chiasma distribution after quadrivalent pairing. Variation between chromosomes in the frequencies of alternate and adjacent co-orientation in metaphase I quadrivalents without interstitial chiasmata suggests that the relative positions of the centromeres in the quadrivalent influence their co-orientation.     

The Development and Meiotic Behavior of Asymmetrical Isochromosomes in Wheat

To determine which segments of a chromosome arm are responsible for the initiation of chiasmate pairing in meiosis, a series of novel isochromosomes was developed in hexaploid wheat (Triticum aestivum L.). These isochromosomes are deficient for different terminal segments in the two arms. It is proposed to call them ``asymmetrical.' Meiotic metaphase I pairing of these asymmetrical isochromosomes was observed in plants with various doses of normal and deficient arms. The two arms of an asymmetrical isochromosome were bound by a chiasma in only two of the 1134 pollen mother cells analyzed. Pairing was between arms of identical length whenever such were available; otherwise, there was no pairing. However, two arms deficient for the same segment paired with a frequency similar to that of normal arms, indicating that the deficient arms retained normal capacity for pairing. Pairing of arms of different length was prevented not by the deficiency itself, but rather, by the heterozygosity for the deficiency. Whether two arms were connected via a centromere in an isochromosome or were present in two different chromosomes had no effect on pairing. This demonstrates that in the absence of homology in the distal regions of chromosome arms, even if relatively short, very long homologous segments may remain unrecognized in meiosis and will not be involved in chiasmate pairing.     

Preferential elimination of chromosome 5R of rye in the progeny of 5R5D dimonosomics

Transmission of chromosome 5R of rye (Secale cereale L.) and chromosome 5D of common wheat (Triticum aestivum L.) through gametes of 5R5D dimonosomics (2n = 42, 20W″ + 5R′ + 5D′) was studied. Chromosome 5R was found to have lower competitiveness as compared to 5D. Gametes with the rye chromosome were two times less often involved in the formation of a progeny. The combined frequency of the karyotypes of wheat (5D5D) and wheat monosomics (5D) was 11.6-fold higher than the frequency of the karyotypes of substitution lines (5R5R) and monosomics for the rye chromosome (5R). The karyotypes of 10.38% of hybrid plants had aberrant 5R chromosomes with different translocations formed as a result of breakages in the centromere and in the proximal region of the long arm. Telocentrics for the short arm t5RS, i5RS isochromosomes, and chromosomes with a terminal deletion T5RS.5RL-del were identified. The absence of amplification of SSR markers mapped on 5RS and the detection of PCR products for a number of 5RL markers (including the genome-specific rye marker Xrms115) permitted nine plants carrying only the long arm of chromosome 5R to be revealed. Since t5RL telocentrics were not detected by the cytological analysis, the results obtained allow us to suggest the presence of small intercalary translocations of the long arm of chromosome 5R in chromosome 5D or in other wheat chromosomes.     

Variation in chiasma frequency among eight accessions of Arabidopsis thaliana

Natural variation in meiotic recombination frequency in Arabidopsis thaliana has been assessed by analyzing chiasma frequency variation among a range of geographically and ecologically diverse accessions. Fifty pollen mother cells at metaphase I of meiosis were analyzed from each of eight accessions and fluorescence in situ hybridization was applied to enable identification of all 10 chromosome arms. There was no significant variation in mean chiasma frequency between plants within accessions, but there was significant variation between accessions. Further analysis confirmed this finding and identified two particular accessions, Cvi and Ler, as having chiasma frequencies significantly lower than those of the other accessions. The analysis also revealed that the pattern of chiasma distribution between arms and among chromosomes is not consistent over accessions. Further detailed analyses were conducted on each individual chromosome (1-5) in turn, revealing that chromosome 4, one of the acrocentric chromosomes, is the least variable while the other acrocentric chromosome (2) is the most variable. These findings indicate the existence of recombination regulatory elements in Arabidopsis and we conclude that it may be possible in the future to identify these elements and determine their mode of action. The practical implications of such developments are considerable.     

Cytogenetic analysis of some Brazilian marsupials (Didelphidae: Marsupialia)

Three species of marsupials from the Amazon region (Marmosa cinerea, Caluromys lanatus, and Didelphis marsupialis) and two from the region of S?o Paulo (Didelphis marsupialis and Didelphis albiventris) were studied. The G-banding pattern of the species with 2n = 14 (M. cinerea and C. lanatus) was very similar, as well as the pattern of G-bands in the species with 22 chromosomes (Didelphis). All of the autosomes of M. cinerea and D. albiventris have centromeric C-bands and the Y chromosome is totally C-band positive. The long arm of the M. cinerea X chromosome is completely C-band positive except for a negative band close to the centromeric region. In D. albiventris the long arm of the X chromosome is C-band positive except for a negative band close to the telomeric region. In M. cinerea the silver-stained nucleolar organizer regions (Ag-NORs) are found in the acrocentric chromosomes, being located in the telomeric region of one pair and in the centromeric region of the other pair. Caluromys lanatus has centromeric Ag-NORs in one acrocentric and in one submetacentric chromosome pairs. Didelphis marsupialis has three chromosome pairs with telomeric Ag-NORs. In D. albiventris the Ag-NORs are terminal and located in both arms of one pair and in the long arm of two pairs of chromosomes.