SUPERNUMERARY CHROMOSOMES IN TRADESCANTIA II. EFFECT OF COLCHICINE ON MEIOTIC ISOCHROMOSOME CONFIGURATIONS

The effect of colchicine on meiotic pairing and configuration frequencies of three homologous isosupernumerary chromosomes was investigated. In the absence of colchicine, the three isochromosomes displayed a high degree of interchromosomal pairing and chiasma formation. As a consequence, a high frequency of bivalents and trivalents were observed at diakinesis-metaphase I. The unique structure of isochromosomes enables them to pair intrachromosomally (i.e., foldback pairing) yet the preferential occurrence of interchromosomal pairing suggests that all six arms of the three isosupernumerary chromosomes were in close association prior to or upon initiation of synapsis. Supernumerary chromosomes in microsporocytes treated during presynapsis or early synapsis with colchicine exhibited a significant reduction (P < 0.001) in the number of bivalents and trivalents at diakinesis. However, there was no reduction in overall chiasma frequency among supernumeraries due to the induction of increased intrachromosomal pairing and chiasma formation. A colchicine-sensitive association or alignment of homologues preceding effective pairing has been demonstrated in standard chromosomes of a number of plant species. This study provides the first evidence to indicate that at least certain supernumerary chromosomes may display presynaptic association as well. The results also support the strongly held contention that colchicine is not directly preventing or inhibiting the actual formation of chiasmata, since no reduction in chiasma frequency was observed in the isochromosomes.     

Relationships between frequency,localization and errors in chiasma formation in desynaptic rye

Four inbred lines of rye (Secale cereale) and the F₁ and F₂ from the cross between two of them have been studied. The results indicate that the genotypes used show variation in chiasma frequency, chiasma error frequency and chiasma localization. Significant correlations between these characters have been found: as chiasma frequency decreases both chiasma error frequency and distal localization increase. These correlations lead us to the assumption that these anomalies are in fact secondary effects of the failure of some preconditions for exchange. It is suggested that one such exchange precondition may be effective pairing.     

The effect of colchicine on meiosis in Lilium speciosum cv. “Rosemede”

Chromosome pairing and chiasma frequency were studied in meiocytes at diakinesis of Lilium speciosum cv. Rosemede fixed up to 21 days after the start of either continuous or 3 day pulse colchicine treatment. The two treatments gave similar results. In pulse treated pollen mother cells (PMCs) the mean chiasma frequency per cell fell from 26.4 in controls to 8.5 after fourteen days while the mean number of univalents per cell increased from 0.05 to 17.58. There was a negative correlation between mean chiasma frequency per bivalent and per PMC in colchicine treated buds; univalents were preferentially induced in bivalents with one chiasma, and preferentially excluded in bivalents with 4 chiasmata. Some chiasmata were redistributed to surviving bivalents despite the concurrent reduction in chiasma frequency per meiocyte. — Colchicine sensitivity began in premeiotic interphase and extended to mid or late zygotene in PMCs; ongoing synapsis was unaffected. However, susceptibility to univalency was asynchronous between bivalents occurring at zygotene in short chromosomes but at late premeiotic interphase in the longest chromosomes. The number of chiasmata per bivalent could be altered by colchicine without inducing univalents, but the ultimate effect was to reduce the number of chiasmata per bivalent (or per chromosome arm) directly to zero. The major factors determining the order and extent of reduced pairing and chiasma number were total chromosome length and arm length. Pairing and chiasma formation in embryo sac mother cells were less sensitive to colchicine than in PMCs, but their behavior was otherwise similar.     

Unusual chromosome pairing and B chromosomes inBriza spicata (Poaceae)

One plant from a population ofBriza spicata (Poaceae) was found to have highly irregular meiotic behaviour. It is characterized by having a reduced chiasma frequency, a large between cell variance in chiasma frequency and the formation of multivalents involving pairs of A chromosomes. The B chromosome present in this plant also forms multivalents with a pair of A chromosomes. It is suggested that the normal control of strict bivalent pairing has broken down and homoeologous chromosomes are associating as multivalents. Furthermore, the partial homology of the B chromosome with a pair of A chromosomes is revealed.     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on meiosis is described in the diploid and tetraploid interspecific hybrid Lolium multiflorum x Lolium perenne. Although the parental species are very closely related, the presence of B chromosomes in the diploid hybrid reduced both chiasma frequency and the number of bivalents at meiosis by a small but significant amount. However at the tetraploid level the presence of B chromosomes did not seem to alter the pairing pattern and chiasma frequency in any way. The use of B chromosomes to stabilize meiosis in amphiploids of this type between closely related outbreeding species is therefore ruled out.     

Suppression of multivalent formation by B chromosomes in natural and artificial autopolyploids of scurvy-grass (Cochlearia L.)

Summary In diploid Cochlearia pyrenaica, its established natural autotetraploid C. officinalis, and their newly induced autotetraploid and auto-octoploid derivatives, B chromosomes change the normal pattern of chromosome association by suppressing homologous pairing. Frequency of bivalents increases at the expense of multivalents from lower to higher numbers of B chromosomes. The reduction of multivalents due to the direct influence of the B chromosomes, independent of pollen mother cell chiasma frequency, is suggested as being related to the mechanism that prevents A/B chromosome pairing.     

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.     

Recombination and chiasmata: few but intriguing discrepancies.

The paradigm that meiotic recombination and chiasmata have the same basis has been challenged, primarily for plants. High resolution genetic mapping frequently results in maps with lengths far exceeding those based on chiasma counts. In addition, recombination between specific homoeologous chromosomes derived from interspecific hybrids is sometimes much higher than can be explained by meiotic chiasma frequencies. However, almost the entire discrepancy disappears when proper care is taken of map inflation resulting from the shortcomings of the mapping algorithm and classification errors, the use of dissimilar material, and the difficulty of accurately counting chiasmata. Still, some exchanges, especially of short interstitial segments, cannot readily be explained by normal meiotic behaviour. Aberrant meiotic processes involving segment replacement or insertion can probably be excluded. Some cases of unusual recombination are somatic, possibly premeiotic exchange. For other cases, local relaxation of chiasma interference caused by small interruptions of homology disturbing synaptonemal complex formation is proposed as the cause. It would be accompanied by a preference for compensating exchanges (negative chromatid interference) resulting from asymmetry of the pairing chromatid pairs, so that one side of each pair preferentially participates in pairing. Over longer distances, the pairing face may switch, causing the normal random chromatid participation in double exchanges and the relatively low frequency of short interstitial exchanges. Key words : recombination frequency, map length, chiasmata, discrepancy, chromatid interference.     

Genetic interactions between wheat and rye genomes in triticale

Summary Six primary triticale lines were produced from two advanced breeding lines of Triticum durum and three inbred genotypes of Secale cereale. The wheat and rye parents and the triticale derivatives were crossed in all possible combinations within each species group. Chiasma and univalent frequency of parents and hybrids were determined. The primary triticale lines had more univalents and less chiasmata per pollen mother cell than the corresponding wheat and rye parents together. The parental wheat F₁ exhibited negative heterosis for chiasma frequency whereas all rye hybrids had much higher chiasma frequencies than their inbred parents. Triticale F₁s generally showed lower chiasma frequencies and more univalents than their parents, but the degree of pairing failure was dependent upon which of the parental species within the triticale, wheat or rye, was in the heterozygous state. F₁s with heterozygous wheat genome only showed the least reduction in chiasma number (presumably caused by gene actions within the wheat genome), while F₁s with heterozygous rye genome showed high reduction in chiasma frequency and an increase in pairing failure (induced by negative interactions between the heterozygous rye and the wheat genome in triticale). A high correlation was found between the frequency of undisturbed pollen mother cells and the frequency of aneuploids in the subsequent generation. A higher number of aneuploids occurred in those populations which were heterozygous for the rye genome.     

The relation between pairing preference and chiasma frequency in tetrasomics of rye.

The association pattern of marked tetrasomes of Secale chromosome 1R at meiotic first metaphase was analyzed. Two of the four chromosomes were identical with terminal C-bands at both arms; the other two were also identical but lacked C-bands and were homologous or homeologous with the first two. Four different types of heterozygotes for 1R were studied: (i). autotetraploid hybrids between genetic variants within Secale cereale subsp. cereale, (ii). tetraploid hybrids between subspecies of Secale cereale, (iii). tetraploid hybrids between species of Secale, and (iv). autotetrasomes of S. cereale in a wheat background. Earlier observations that heterozygous associations (banded with unbanded) had consistently higher chiasma frequencies than homozygous associations were extended and confirmed. To analyze this phenomenon more closely, the possible relations between this correlation and several other meiotic phenomena were studied. For this analysis, three genetically different autotetraploid hybrids within S. cereale were selected that differed with respect to the relation between pairing type and chiasma frequency. Special attention was given to different patterns of interference and other meiotic phenomena in the two chromosome arms of chromosome 1R. No relations between such phenomena and the relation between pairing type and chiasma frequency could be established. A hypothesis is formulated assuming that long-distance homologue attraction is concentrated in a limited number of sites and that in different genotypes, different patterns of active sites are present. Moderately weak attraction sites can pair with strong homologous sites under favorable genetic conditions, but two weak sites cannot. Then, heterozygotes have more effective pairing initiation and consequently chiasma formation than homozygotes. Under less favorable conditions, only strong sites are effective, and then, homozygotes pair better, but the chiasma frequency is lower. A model of the forces involved in homologue attraction is presented.     

Non-homologous chromosome pairing and crossover formation in haploid rice meiosis

While many studies have provided significant insight into homolog pairing during meiosis, information on non-homologous pairing is much less abundant. In the present study, fluorescence in situ hybridization (FISH) was used to investigate non-homologous pairing in haploid rice during meiosis. At pachytene, non-homologous chromosomes paired and formed synaptonemal complexes. FISH analysis data indicated that chromosome pairing could be grouped into three major types: (1) single chromosome paired fold-back as the univalent structure, (2) two non-homologous chromosomes paired as the bivalent structure, and (3) three or more non-homologous chromosomes paired as the multivalent structure. In the survey of 70 cells, 65 contained univalents, 45 contained bivalents, and 49 contained multivalent. Moreover, chromosomes 9 and 10 as well as chromosomes 11 and 12 formed non-homologous bivalents at a higher frequency than the other chromosomes. However, chiasma was always detected in the bivalent only between chromosomes 11 and 12 at diakinesis or metaphase I, indicating the pairing between these two chromosomes leads non-homologous recombination during meiosis. The synaptonemal complex formation between non-homologs was further proved by immunodetection of RCE8, PAIR2, and ZEP1. Especially, ZEP1 only loaded onto the paired chromosomes other than the un-paired chromosomes at pachytene in haploid.     

The genetics of meiotic chromosome pairing in Lolium temulentum x Lolium perenne tetraploids

Summary The degree of preferential pairing of homologous chromosomes was estimated in a series of tetraploid hybrids of Lolium temulentum x Lolium perenne by means of cytological and genetic analyses. The correlations between the frequency of bivalents at first metaphase of meiosis in the hybrid tetraploids and the degree of preferential pairing calculated from the segregation pattern of isozyme alleles in a test cross was extremely high. The results showed clearly that suppression of heterogenetic pairing in these Lolium tetraploids is achieved by a genetic system involving the A chromosomes as well as the B chromosome system which has been known for some time. Certain similarities with the genetic system controlling pairing in polyploid wheats are discussed.     

Chromosomal DNA variation,genomic constraints and recombination in Lathyrus

There is approximately a doubling of the total nuclear DNA between the 8 Lathyrus species and there are significant differences in the amounts of DNA in euchromatin and heterochromatin. Between the 8 species chiasma frequency and total nuclear DNA are not correlated but within complements it is positively correlated with the amount of DNA in the chromosomes. There is no significant correlation between chiasma frequency and euchromatin DNA nor between chiasma frequency and heterochromatin DNA among species, but among chromosomes, as with total DNA, it is positively correlated with euchromatin DNA and heterochromatin DNA. Results show that despite the large differences in DNA amounts between species there are genomic constraints underlying the frequency and distribution of chiasmata in the chromosome complements.     

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 genotypic control of homoeologous chromosome association in Lolium temulentum × Lolium perenne interspecific hybrids

Two contrasting genotypes of Lolium perenne and two inbred lines of L. temulentum were examined with regard to their effect on homoeologous chromosome pairing in interspecific hybrids derived from them. Substantial differences in chiasma frequency were observed between the hybrid progeny of the different parental types. The background genes involved were found to operate in the presence and in the absence of B chromosomes. The combination of A chromosome genes present in some of the 0B hybrids was found to result in a considerable suppression of chiasma formation at the diploid level, and the restriction of pairing to strict homologues at the tetraploid level. It appears, therefore, that genes are present within the diploid species of the genus Lolium which are capable of performing a function similar to that of the Ph locus in wheat.     

Chromosome pairing relationships among the A,B, and D genomes of bread wheat

Summary Chromosome pairing and chiasma frequency were studied in bread wheat euhaploids (2n = 3x = 21; ABD genomes) with and without the major pairing regulatorPh1. This constitutes the first report of chromosome pairing relationships among the A, B, and D genomes of wheat without the influence of an alien genome. AllPh1 euhaploids had very little pairing, with 0.62–1.05 rod bivalents per cell; ring bivalents were virtually absent and mean arm-binding frequency (c) values ranged from 0.050 to 0.086. In contrast, theph1b euhaploids had extensive homoeologous pairing, with chiasma frequency 7.5–11.6 times higher than that in thePh1 euhaploids. They had 0.53–1.16 trivalents, 1.53–1.74 ring bivalents, and 2.90–3.57 rod bivalents, withc from 0.580 to 0.629. N-banding of meiotic chromosomes showed strongly preferential pairing between chromosomes of the A and D genomes; 80% of the pairing was between these genomes, especially in the presence of theph1b allele. The application of mathematical models to unmarked chromosomes also supported a 21 genomic structure of theph1b euhaploids. Numerical modeling suggested that about 80% of the metaphase I association was between the two most related genomes in the presence ofph1b, but that pairing under Ph1 was considerably more random. The data demonstrate that the A and D genomes are much more closely related to each other than either is to B. These results may have phylogenetic significance and hence breeding implications.This paper is dedicated to the memory of the late Ernest R. SearsCooperative investigations of the USDA-Agricultural Research Service and the Utah Agricultural Experiment Station, Logan, UT 84322, USA. Approved as Journal Paper No. 3986     

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.     

The structure,meiotic behaviour and effects of B chromosomes in Briza humilis Bieb. (Gramineae)

A single population of Briza humilis contained two types of B chromosome, one a large (B^L) and the other a small (B^S) acrocentric. DNA measurements show that the B^L chromosome contains approximately twice as much DNA per unit length as the members of the regular complement. The meiotic pairing behaviour of the Bs is variable and B^L and B^S are seen to pair in some cells. The presence of B^L depresses the chiasma frequency of the regular complement but the chiasma frequency of A and B chromosomes does not appear to be related. The transmission rate of the B chromosomes is variable and the B^L shows a non-disjunction mechanism during microsporogenesis that is absent during megasporogenesis. For the B^S chromosome the transmission rate is very low and there is no evidence of a non-disjunction mechanism. In general seeds containing B^L chromosomes germinate more slowly than those without B chromosomes.     

Chromosome pairing in tetraploid hybrids between Lolium perenne and L. multiflorum

Summary Chromosome association at first meiotic metaphase in tetraploid hybrids between Lolium perenne and L. multiflorum was compared with that in autotetraploid L. perenne. The hybrids were found to have significantly higher levels of bivalent frequency, and lower levels of multivalent and chiasma frequency. A significant increase in multivalent frequency with increasing chiasma formation was found in both groups, but the increase was much less in the hybrids. These differences in chromosome associations between the two groups must therefore reflect differences in chiasma distribution and it is suggested that the results indicate a significant degree of preferential bivalent pairing in the hybrids.     

Synaptonemal complex spreading in Allium cepa and A. fistulosum

The general features and fine structure of homologous chromosome alignment and pairing have been investigated in two species of Allium (A. fistulosum and A. cepa), which have similar karyotypes but very different patterns of chiasma distribution. Although there is no support for the occurrence of a general pre-meiotic alignment of homologous chromosomes, both species show some alignment of homologues as an immediate prelude to synaptonemal complex (SC) formation. In both species pairing usually commences at sub-terminal sites and is succeeded by numerous separate intercalary initiations of pairing in interstitial and distal regions and then in proximal regions. The last parts to pair, in both species, are pericentromeric and telomeric regions. There is, therefore, no evident relationship between the sequence of pairing and chiasma distribution in these species. Regularly alternating convergences and divergences of aligned axial cores (ACs), termed multiple association sites, are frequently observed. It is proposed that these represent potential pairing initiation sites and from observations on their spatial distribution it is argued that they may be evenly distributed through most of the genome. Small spherical or ellipsoid nodules are found at association sites and between closely aligned ACs which persist in the SC segments present during zygotene, but most of them disappear abruptly at the end of zygotene. These are termed zygotene nodules (ZN) and it is proposed that they are involved in matching corresponding sites on homologous chromosomes as well as possibly having a recombinational role. Their composition, structure, mode of action and relationship to pachytene recombination nodules are at present unknown.