Meiosis in allopolyploid Crepis capillaris. II. Autotetraploids.

Meiotic chromosome pairing of autotetraploid Crepis capillaris was analysed by electron microscopy of surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I chromosome configurations. Prophase I quadrivalent frequencies are high in all three tetrasomes. (A, D, and C) and partially dependent on chromosome size. At metaphase I quadrivalent frequencies are much lower and strongly dependent on chromosome size. There is no evidence for multivalent elimination during prophase I in this system, and the reduction in multivalent frequency at metaphase I can be explained by an insufficiency of appropriately placed chiasmata. The high frequencies of prophase I quadrivalents far exceed the two-thirds expected on a simple model with two terminal independent pairing initiation sites per tetrasome, suggesting that multiple pairing initiation occurs. Direct observations reveal relatively high frequencies of pairing partner switches (PPSs) at prophase I, which confirms this suggestion. The numbers of PPSs per tetrasome show a good fit to the Poisson distribution, and their positional distribution along chromosomes is random and nonlocalized. These observations favour a model of pairing initiation based on a large number of evenly distributed autonomous pairing sites each with a uniform and low probability of generating a PPS.     

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     

Further insights on chromosomal pairing of autopolyploids: a triploid and tetraploids of rye

Chromosomal pairing of one triploid and three tetraploid plants of rye, Secale cereale, was analyzed by electron microscopy in surface-spread prophase I nuclei and compared with light microscopic observations of metaphase I cells. Prophase I is characterized by: (i) the weak alignment showed by the three or four unsynapsed or partially homologous synapsed axes; (ii) the low number ber of pairing partner switches (PPSs) displayed by both trivalents and quadrivalents; and (iii) the existence of complex multivalents in which up to 13 chromosomes in the triploid and 22 chromosomes in the tetraploids were involved. However, only few heterologous chromosomal associations were maintained at metaphase I. The results obtained are discussed under the assumptions of the random end pairing model with some modifications.     

Trisomics of ryegrass and their transmission

Summary Primary trisomics of perennial ryegrass, Lolium perenne L., were studied for meiotic behaviour, fertility, morphology and trisome transmission. Trisomics differed from each other in mean meiotic association, pollen fertility, seed set and morphology. The combined cytomorphological data suggested that the investigated trisomic plants included trisomes 2 to 7. No pollen transmission of trisomes was detected. Female transmission of trisomes ranged from 12% for tri 3 to 37% for tri 4 with a mean of 24% for the six trisomes. Trisome transmission was not related to either chromosome size or trivalent/univalent frequency, although the larger trisomes formed trivalents more frequently than the smaller trisomes.Part of M.Sc.Agr. Thesis, submitted by senior author to the National University of Ireland, Dublin     

Chromosome pairing and potential for intergeneric recombination in some hypotetraploid somatic hybrids of Lycopersicon esculentum (+) Solanum tuberosum.

Three somatic hybrids resulting from protoplast fusions of a diploid kanamycin-resistant line of tomato (Lycopersicon esculentum) and a dihaploid hygromycin-resistant transformant of a monohaploid potato (Solanum tuberosum) line were used for a cytogenetic study on chromosome pairing and meiotic recombination. Chromosome counts in root-tip meristem cells revealed two hypotetraploids with chromosome complements of 2n = 46 and one with 2n = 47. Electron microscope analyses of synaptonemal complex spreads of hypotonically burst protoplasts at mid prophase I showed abundant exchanges of pairing partners in multivalents involving as many as eight chromosomes. In the cells at late pachytene recombination nodules were found in multivalents on both sides of pairing partner exchanges, indicating recombination at both homologous and homoeologous sites. Light microscope observations of pollen mother cells at late diakinesis and metaphase I also revealed multivalents, though their occurrence in low frequencies betrays the reduction of multivalent number and complexity. Precocious separation of half bivalents at metaphase I and lagging of univalents at anaphase I were observed frequently. Bridges, which may result from an apparent inversion loop found in the synaptonemal complexes of a mid prophase I nucleus, were also quite common at anaphase I, though the expected accompanying fragments could be detected in only a few cells. Most striking were the high frequencies of first division restitution in preparations at metaphase II/anaphase II, giving rise to unreduced gametes. In spite of the expected high numbers of balanced haploid and diploid gametes, male fertility, as revealed by pollen staining, was found to be negligible.     

Meiosis in four trisomic and one double trisomic males of the newt Pleurodeles waltlii

In the newt Pleurodeles waltlii, meiosis was studied in four trisomic and one double trisomic males. Study of first prophase shows that trivalent frequencies and trivalent configurations are correlated with chromosome length; moreover, trivalent configurations indicate that long chromosomes have multiple points of initiation of synapsis whereas two points might be adequate to secure synapsis of short chromosomes. From the study of metaphase II it appears that the extra chromosomes segregate in half of the spermatocytes II. Some abnormal spermatocytes, resulting from nondisjunction of chromosomes at mitosis or at first division of meiosis, or from precocious division of chromosomes at first division of meiosis were observed. In the male double trisomic meiosis fails at anaphase of second division; this accounts for the sterility of the individual.     

An indirect test for a role of the synaptonemal complex in the establishment of sister chromatid cohesiveness

The meiotic behavior of a special maize trisome was quantitatively observed at pachytene, metaphase I, anaphase I, prophase II, metaphase II and anaphase II. The data obtained are consistent with (but do not prove) the model that sister chromatid cohesiveness at anaphase I may be established during pachytene synapsis of the chromosome regions involved. The data suggest, however, that the normal prophase II maintenance of dyad integrity by cohesiveness of sister chromatid centromere regions does not depend upon prior synapsis of these regions, although monads separated from each other on the anaphase I spindle may be delivered to the same prophase II daughter nucleus. — The strands which some of the time connect sister chromatids which are separating equationally at anaphase I show a positive Feulgen staining reaction.     

The distribution of male meiotic pairing sites on chromosome 2 of Drosophila melanogaster: meiotic pairing and segregation of 2-Y transpositions

The distribution of meiotic pairing sites on a Drosophila melanogaster autosome was studied by characterizing patterns of prophase pairing and anaphase segregation in males heterozygous for a number of 2-Y transpositions, collectively coveringall of chromosome arm 2R and one-fourth of chromosome arm 2L. It was found that all transpositions involving euchromatin from chromosome 2, even short stretches, increased the frequency of prophase I quadrivalents involving the sex and second chromosome bivalents above background levels. Quadrivalent frequencies were the same whether the males carried both elements of the transposition or just the Dp (2;Y) element along with two normal chromosome 2s, indicating that pairing is non-competitive. The frequency of quadrivalents was proportional to the size of the transposed region, suggesting that pairing sites are widely distributed on chromosome 2. Moreover, all but the smallest transpositions caused a detectable bias in the segregation ratio, in favor of alternate segregations, indicating that the prophase associations were effective in orienting centromeres to opposite poles. One transposition involving only heterochromatin of chromosome 2 had no effect on quadrivalent frequency, consistent with previous evidence that autosomal heterochromatin lacks meiotic pairing ability in males. One region at the base of chromosome arm 2L proved to be especially effective in stimulating quadrivalent formation and anaphase segregation, indicating the presence of a strong pairing site in this region. It is concluded that autosomal pairing in D. melanogaster males is based on general homology, despite the lack of homologous recombination.by A.C. Spradling     

Genetic control of synapsis and recombination in Lolium amphidiploids

Homologous bivalent formation in amphidiploids of Lolium is promoted during meiosis by diploidising genes carried by A-chromosomes and by supernumerary B-chromosomes. The site and mode of action of these diploidising factors were investigated by comparing the relative frequencies of pairing configurations at meiotic prophase and metaphase I in several different hybrid genotypes. The results indicate that diploidising genes act predominatly by increasing the stringency of synapsis at early stages of meiotic prophase. By contrast, B-chromosomes appear to promote bivalent formation by ensuring that homoeologously paired chromosome segments within multivalents do not crossover. The results show that the additive effects of diploidising genes and B-chromosomes are to a certain extent separable in terms of their mode of action and timing during meiosis.     

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.     

Synaptic behaviour of hexaploid wheat haploids with different effectiveness of the diploidizing mechanism

Haploids of three cultivars of Triticum aestivum (Thatcher, Chris, and Chinese Spring) were obtained from crosses with Zea mays. The level of chromosome pairing at metaphase I and the synaptic behaviour at prophase I was studied. There were differences in the meiotic behaviour of the haploids from different cultivars. Thatcher and Chris haploids had significantly higher levels of pairing at metaphase I than Chinese Spring haploids. This metaphase I pairing was correlated with higher levels of synapsis achieved in the Thatcher and Chris prophase I nuclei than in the Chinese Spring nuclei. Variation in the effectiveness of the diploidizing mechanism among cultivars of wheat is proposed to have a genetic origin and the role of the Ph1 locus in the different haploids is discussed.     

Further examination of the production-line hypothesis in mouse foetal oocytes

Pachytene oocytes from foetal mice heterozygous for the translocation T(14; 15)6Ca were screened for evidence of a production-line effect on chromosome pairing. Metaphase I oocytes from adult heterozygotes were also examined to determine whether any such effect on pahytene chromosome pairing is subsequently repeated during adult reproductive life as anticipated by the production-line hypothesis. It was found that as gestation proceeded the proportion of pachytene oocytes with a translocation quadrivalent declined and that with a trivalent and univalent correspondingly increased. That is, there was evidence of variation in pairing behaviour of the translocation at different times of gestation. In contrast, the proportions of metaphase I cells with either a quadrivalent or a trivalent plus univalent did not vary between adult females of different ages. Thus if the variation observed at pachytene was the result of a production-line effect, clearly this was not reflected in the behaviour of the translocation at metaphase I. Our observations therefore do not support the production line hypothesis for the maternal age effect on nondisjunction.     

Chromosome structure and pairing preferences in autotetraploid rye (Secale cereale).

The influence of chromosome structure upon pairing behaviour during meiosis was investigated by comparing four autotetraploid genotypes of rye (Secale cereale) containing homologous chromosome sets with different degrees of structural similarity. The series provided a range of genotypes that, at one extreme, contained structurally identical chromosome sets and, at the other extreme, sets that are certainly more heterozygous in the genic sense and probably also more diverse from a purely structural viewpoint. Relative frequencies of pairing configurations at meiotic prophase and metaphase I were compared by electron microscopy of whole-mount surface-spread synaptonemal complex complements and light microscopy of squash preparations. Despite unexpectedly low quadrivalent frequencies over all four genotypes, higher mean bivalent frequencies appeared to be associated with greater homologue diversity. In other words, greater structural divergence between chromosome sets appears to facilitate more efficient discrimination between homologous and identical chromosomes that drives the formation of bivalents. Statistical comparisons were not able to confirm in some cases the significance of the observed pattern of pairing behaviour.     

Correlation between Pairing Initiation Sites, Recombination Nodules and Meiotic Recombination in Sordaria Macrospora

The decrease of meiotic exchanges (crossing over and conversion) in two mutants of Sordaria macrospora correlated strongly with a reduction of chiasmata and of both types of "recombination nodules." Serial section reconstruction electron microscopy was used to compare the synapsis pattern of meiotic prophase I in wild type and mutants. First, synapsis occurred but the number of synaptonemal complex initiation sites was reduced in both mutants. Second, this reduction was accompanied by, or resulted in, modifications of the pattern of synapsis. Genetic and synaptonemal complex maps were compared in three regions along one chromosome arm divided into well marked intervals. Reciprocal exchange frequencies and number of recombination nodules correlated in wild type in the three analyzed intervals, but disparity was found between the location of recombination nodules and exchanges in the mutants. Despite the twofold exchange decrease, sections of the genome such as the short arm of chromosome 2 and telomere regions were sheltered from nodule decrease and from pairing modifications. This indicated a certain amount of diversity in the control of these features and suggested that exchange frequency was dependent not only on the amount of effective pairing but also on the localization of the pairing sites, as revealed by the synaptonemal complex progression in the mutants.     

Cytological evidence that the Sxr fragment of XY,Sxr mice pairs homologously at meiotic prophase with the proximal testis-determining region

Self-pairing of the Y chromosome at prophase of meiosis in XY,Sxr male mice appears to take place in many cells to the exclusion of pairing between the Y and the X. This phenomenon offers an explanation for the high level of X-Y separation seen in these males at prophase of meiosis, additional separations being evident, however, when metaphase I (MI) cells are examined. A minority of prophase cells show the Y paired both autologously and with a sub-terminal region of the X which could be the normal pairing region. The balloon-like configurations observed when self-pairing occurs suggest that the distal Sxr fragment is inverted on the Y chromosome of Sxr carrier males in relation to the normal proximal testis-determining (Td)-containing region.     

Homolog pairing and sister chromatid cohesion in heterochromatin in <Emphasis Type="Italic">Drosophila</Emphasis> male meiosis I

Drosophila males undergo meiosis without recombination or chiasmata but homologous chromosomes pair and disjoin regularly. The X–Y pair utilizes a specific repeated sequence within the heterochromatic ribosomal DNA blocks as a pairing site. No pairing sites have yet been identified for the autosomes. To search for such sites, we utilized probes targeting specific heterochromatic regions to assay heterochromatin pairing sequences and behavior in meiosis by fluorescence in situ hybridization (FISH). We found that the small fourth chromosome pairs at heterochromatic region 61 and associates with the X chromosome throughout prophase I. Homolog pairing of the fourth chromosome is disrupted when the homolog conjunction complex is perturbed by mutations in SNM or MNM. On the other hand, six tested heterochromatic regions of the major autosomes proved to be largely unpaired after early prophase I, suggesting that stable homolog pairing sites do not exist in heterochromatin of the major autosomes. Furthermore, FISH analysis revealed two distinct patterns of sister chromatid cohesion in heterochromatin: regions with stable cohesion and regions lacking cohesion. This suggests that meiotic sister chromatid cohesion is incomplete within heterochromatin and may occur at specific preferential sites.     

Chromosomally-Induced Meiotic Drive in Drosophila Males: Checkpoint or Fallout?

In male Drosophila melanogaster, anomalies in sex chromosome pairing at meiosis often lead to complete or partial sperm dysfunction. This observation has led to the suggestion that defects in either the efficiency or configuration of chromosome pairing at metaphase trigger a checkpoint mechanism that leads to the elimination of meiotic products. Here, we discuss this model in consideration of recent observations on the conservation of metaphase checkpoint components in male meiosis, and on the phenotype of new alleles of the male-specific meiotic mutant teflon. Based on these observations, we propose an alternative hypothesis for the cause of sperm dysfunction in cases of chromosomal sterility and drive. We suggest that disruption of the prophase compartmentalization of sex chromatin, rather than abnormal pairing at metaphase, may be the causative defect. Such disruption may occur as a result of perturbations in sex chromosome pairing, or by translocations involving autosomal and sex chromatin. We discuss how this hypothesis may account for previously described examples chromosomal causes of meiotic drive and sterility in Drosophila. This revised version was published online in July 2006 with corrections to the Cover Date.     

Meiotic mutants of rye Secale cereale L.

Summary A mutant form of weedy rye characterized by male and female sterility and having a hereditary block in the chromosome synapsis has been found and described. Genetic analysis has shown the synapsis block to be determined by the recessive allele of a gene designated as sy-1. Electron microscopy of surface-spread microsporocyte nuclei revealed the complete absence of the synaptonemal complex over the whole meiotic prophase I, although the axial cores were perfectly formed by each chromosome. Only univalents were observed at metaphase I, their average number ranging from 13.1 to 14.0 per cell. A precocious distribution of univalents at the poles is observed at metaphase I. All of the later stages of meiosis were irregular and resulted in the formation of abnormal microspores. Thus, the mutant proves to be asynaptic because of the blocked initiation of synapses at prophase I.     

Chromosome systems in the eriococcidae (Coccoidea Homoptera)

Spermatogenesis is described in two eriococcid species and the observations are compared to those previously reported. In Gossyparia spuria the diploid chromosome number is 28 in both males and females. In the female all the chromosomes are euchromatic. In most male tissues 14 of the chromosomes are euchromatic (E) and 14 are heterochromatic (H). Prior to the first meiotic division in males the number of H chromosomes was reduced. During prophase I all the cells showed 14 E chromosomes and from 1 to over 9 H chromosomes. The range of chromosome numbers in metaphase I was similar to that in prophase I. All the chromosomes divided in anaphase I, and, following differential uncoiling at interkinesis, the E and H groups of chromosomes segregated from each other at anaphase II. Only the E groups formed sperm. The presence of a variable number of H chromosomes and a haploid number of E chromosomes in spermatogenesis suggested the presence of the multiple-D variant of the Comstockiella chromosome system. In this system some of the H chromosomes become euchromatic prior to prophase I of spermatogenesis and pair with their E homologues. All the remaining H chromosomes are thus univalents, while among the E elements, some are univalents and the rest are bivalents. The observed reduction in the number of H chromosomes in the first meiotic division which was previously attributed to pairing among the H chromosomes, is now interpreted to be the result of the return of some of the H chromosomes to a euchromatic state and to their subsequent pairing with their E homologues. Spermatogenesis in Eriococcus araucariae was similar to that of G. spuria except that the reduction in the number of H chromosomes was not as extensive. The chromosome systems of the two species are compared to those of other eriococcids and the differences are briefly discussed.Supported by grant GB1585 from the National Science Foundation, Washington, D. C.     

Chromosome pairing in the allotetraploid Aegilops biuncialis and a triploid intergeneric hybrid.

Chromosome pairing behaviour of the natural allotetraploid Aegilops biuncialis (genome UUMM) and a triploid hybrid Ae. biuncialis x Secale cereale (genome UMR) was analyzed by electron microscopy in surface-spread prophase I nuclei. Synaptonemal-complex analysis at zygotene and pachytene revealed that synapsis in the allotetraploid was mostly between homologous chromosomes, although a few quadrivalents were also formed. Only homologous bivalents were observed at metaphase I. In contrast, homoeologous and heterologous chromosome associations were common at prophase I and metaphase I of the triploid hybrid. It is concluded that the mechanism controlling bivalent formation in Ae. biuncialis acts mainly at zygotene by restricting pairing to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in the homoeologous associations. In the hybrid the mechanism fails at both stages. Key words : Aegilops biuncialis, allotetraploid, intergeneric hybrid, pairing control, synaptonemal complex.