Unpaired Chromosomes Research Articles

Synaptonemal complex defects in a man with nonobstructive azoospermia

Objective: We have previously demonstrated an association between decreased recombination and nondisjunction in sperm. The aim of this study was to analyze early stages of meiosis in infertile men to determine if any abnormalities in chromosome pairing or recombination could be discerned. Design:The stages of meiotic prophase, extent of chromosome pairing and the number of recombination foci were compared in a man with nonobstructive azoospermia and controls. Materials and Methods: Immunofluorescence methods were used to identify the synaptonemal complex in various stages of prophase. Antibodies were used to identify the synaptonemal complex (SCP1/SCP3), the centromere (CREST) and sites of recombination (MLH1). Results: Only 36 meiotic spreads could be recovered from the infertile man with nonobstructive azoospermia compared to hundreds of spreads available from controls. One-third of his cells were delayed in the early stages of meiosis (zygotene). The infertile man had a greatly reduced frequency of recombination with a mean of only 33.4% foci (range 1–67) compared to a mean of 50 foci (range 36–67) in controls. Also 34.8% of pachytene cells had at least one bivalent with no recombinant foci and 52.2% of cells had unpaired chromosome regions. Lack of chromosome pairing is known to cause meiotic arrest in humans. Conclusion:This is the first infertile man identified to have an arrest in meiosis because of an error in the processing of recombination foci or a defect in SCP 1 leading to incomplete chromosome pairing.

New methods for comparison of chromosomes within and between species

Abstract A new method (called New Relative Length) was developed for comparison of chromosome size when both paired and unpaired chromosomes are present. All chromosomes were used in the calculation. A method of graphical display of karyotype data (called a Karyotype Graph), plotting New Relative Length on the vertical axis and arm ratio on the horizontal axis was used to indicate sampling error, to pair chromosomes and to match similar chromosomes in related species. Terms for Chromosome Size were also defined namely Small, Medium or Large depending on New Relative Length as a proportion of total chromosome length. These terms could also be used to compare chromosomes within species and between related species.

091 Cytogenetics of Inland Saltgrass

Inland saltgrass, Distichlis spicata var. stricta (Torr.) Beetle, is a native western U.S. grass that has potential in turf situations. Elite clones with outstanding potential have been selected. Poor seed production appears to be the main limiting factor for its use as turf in saline conditions. In order to better understand seed set, 40 genotypes were examined for chromosome number and morphology. Chromosome numbers of root tip metaphase spreads varied with the most common being 2n = 38. However, chromosome numbers of 39, 40, 42, and 74 were also observed. Meiotic examination of anthers revealed 19 bivalents for those with 38 chromosomes while those with 42 had 20 bivalents with 2 unpaired chromosomes. The unpaired chromosomes lagged at anaphase. Crosses among genotypes of different chromosome number have resulted in good seed set. Harvested seed are germinable and plants from these crosses are being grown for further studies.

Are unpaired chromosomes spermicidal?: A maximum-likelihood analysis of segregation and meiotic drive in Drosophila melanogaster males deficient for the ribosomal-dna.

Meiosis in Drosophila melanogaster males is achiasmate and requires special systems to ensure normal segregation. Several situations that yield frequent nondisjunction also produce high levels of chromatin-dependent sperm lethality, suggesting the possibility of a simple and direct connection between defective disjunction and defective sperm development. One hypothesis that has been offered is that pairing not only ensures disjunction, but also changes the physical state of chromosomes so that they can be packaged in sperm. Here, I present an analysis of extensive data on disjunction and sperm survival in rDNA-deficient males collected by B. McKee and D. Lindsley. This analysis demonstrates that, although nondisjunction and sperm lethality are indeed correlated, the basis of this is not the presence of unpaired chromosomes in the sperm. Chromosomes that have failed to disjoin are not themselves spermicidal.

Chromosomal inheritance patterns of intergeneric hybrids of ictalurid catfishes: odd diploid numbers with equal parental contributions

Hybrid chromosomal compositions of channel catfish Ictalurus punctatus× black bullhead Ameiurus melas and channel catfish × flathead catfish Pylodictis olivaris were analysed by a computer-based method. The karyotype of each hybrid was highly asymmetric, and the diploid numbers and arm numbers were intermediate to the parental types. The hybrid offspring of channel catfish × black bullhead possessed a diploid number of 59 chromosomes, with an arm number estimate of 87. The hybrid offspring of the channel catfish × flathead catfish cross possessed a diploid number of 57 chromosomes, also with an arm number estimate of 87. Nucleolus organizer regions (NORs) were located on a single pair of chromosomes with symmetric staining intensity in channel catfish and in black bullhead, and on a single pair of chromosomes with asymmetric staining intensity in flathead catfish. The channel catfish × black bullhead hybrid had two unpaired chromosomes that stained positively for NORs. The channel catfish × flathead catfish had three unpaired chromosomes that stained positively for NORs. Specific marker chromosomes were identified in each hybrid. There was no evidence of androgenesis, gynogenesis, polyploidy or aneuploidy in the hybrids. Results of this study, plus information reported previously, indicate that chromosomes of ictalurid catfishes are inherited stably in a haploid pattern with an equal contribution to the genomes of F1 hybrids, even in intergeneric crosses involving divergent numbers of parental chromosomes.

Diplosporous and aposporous apomixis in a pentaploid race of Paspalum minus

Paspalum minus is a small grass widely distributed from North America and the Greater Antilles to Paraguay and Bolivia in South America. Previous studies indicated that diploid, tetraploid, and pentaploid races exist. Cytoembryological studies in pentaploid plants indicate irregular meiotic behavior with mostly unpaired chromosomes. Melosis is interrupted after the stage of metaphase I and the nucleus is restored enabling mitosis to occur. Each male meiocyte results in two pollen grains with unreduced nuclei, 2 n=5 x=50. In the ovule, two unreduced megaspores arise from the archesporial cell. The micropilar one degenerated and the other forms a diplosporous embryo sac. The cytologicaly unreduced egg cell forms the embryo by parthenogenesis. In addition, 1–3 aposporous embryo sacs develope from nucelar cells beside the diplosporous sac in more than one half of the ovules. Thus, diplospory ( Taraxacum type) and apospory occur in the same species and in the same ovule. This is the first report of diplospory and apospory occurring simultaneously in the grass family. The genetic specificity for embryo sac development is restricted to the archespore in diplosporous apomicts. In P. minus this specificity may express in both the archespore and in the surrounding nucelar cells.

Cytogenetic behaviour of the interspecific hybrid of Manihot neusana Nassar and cassava, M. esculenta Crantz, and its backcross progeny

An interspecific hybrid between cassava, Manihot esculenta Crantz, and M. neusana Nassar and the backcrossed progenies was examined meiotically and mitotically. The F1 hybrid showed irregular meiosis due to unpaired chromosomes. The backcrossed progeny ranged in fertility from 17.7 to 35.8%. Meiotic restitution was also observed in all of the hybrids. Diploid pollen formation also occurred and ranged from 0.6 to 1.6%. Irregular meiosis among the F1 hybrid produced aneuploidy in backcrossed progeny. When the F1 hybrid was used as the maternal plant in backcrosses with cassava, this improved seed setting in the BC1 progenies, whereas the reciprocal backcross produced sterile progenies. Key words: Wild species, meiotic restitution, aneuploidy, diploid gamete

Synaptonemal complex analysis of domestic sheep (Ovis aries) with Robertsonian translocations. II. Trivalent and pairing abnormalities in Massey I and Massey II heterozygotes

Zygotene and pachytene spermatocytes from Massey I (t1 5;26) and Massey II (t2 8; 11) translocation heterozygotes each contained one trivalent, often delayed in pairing, while cells from double Massey translocation heterozygotes had two such trivalents. As meiosis progressed, trivalents became fully paired, with acrocentric axes in a cis configuration. Abnormal pairing configurations often resulted from interactions between unpaired chromosome axes or segments. However, when two Massey trivalents were present in the same nucleus, there was no pairing interaction between them. In different Massey translocation heterozygotes, trivalent-involved pairing abnormalities occurred in 14-28% of cells, with XY-trivalent and XY-bivalent-trivalent associations being as high as 7.1-23.1%. In spermatocytes from single and double Massey translocation heterozygotes with normal-sized testes, the total SC abnormality frequency was 34.4% for the t1 heterozygotes, 27.1% for the t2 heterozygotes, and 21.4% for the double heterozygote. One Massey II heterozygote with one normal and one small testis had significantly higher SC abnormality frequency (54%) than normal rams. A trisomic cell was recorded in one ram and two hyperdiploid cells in another ram, but these were unrelated to the translocations. It is suggested that resolution of pairing abnormalities by synaptic adjustment is important in reducing the effects on fertility of the translocations.

Reconstruction in wheat of complete chromosomes 1B and 1R from the 1RS.1BL translocation of 'Kavkaz' origin

Translocation 1RS.1BL originally from the 'Aurora' and 'Kavkaz' wheats has spread to bread wheats all over the world. It offers several resistance genes and appears to significantly increase yields. For future study of the arm location of the yield-increasing factors, complete chromosomes 1B and 1R were reconstructed from the 1RS.1BL translocation. This was accomplished by centric misdivision and fusion in double monosomies 20 II + 1RS.1BL I + 1BS.1RL I. The frequency of misdivision and misdivision-fusion of the two unpaired chromosomes was 23.1%, which was much higher than expected but comparable with that observed in other stocks grown under similar conditions. Transmission of the reconstructed chromosomes to progeny was normal. In addition to all possible misdivision and misdivision-fusion products, two chromosomes of ambiguous origin were recovered: chromosome 1B with a proximal insert of about 40% of 1RS and chromosome 1R with a proximal insert of about 30% of 1BS. The study demonstrates that centric translocations can misdivide and fuse again to produce stable chromosomes with new arm combinations.

Evolution and the meaning of metaphase*

We used an evolutionary test to ask whether the congression of chromosomes to the spindle equator is important in itself or just a mitotic happenstance. If congression matters, then it might evolve if absent initially. Previous workers established that newly made trivalents, meiotic units of three chromosomes, generally do not congress to the spindle equator. Instead, these young trivalents lie close to the pole to which two of the three chromosomes are oriented. We studied ancient sex-chromosome trivalents that arose hundreds of thousands to several million years ago in several species of praying mantids and one grasshopper. All these old trivalents lie near the spindle equator at metaphase; some of them congress as precisely to the equator as the ordinary chromosomes in the same cells. We conclude that congression evolved independently two or three times in the materials studied. Therefore, the metaphase position of chromosomes midway between the poles appears to matter, but why? In the praying mantids, the evident answer is that metaphase is a quality-control checkpoint. Sometimes the three chromosomes are not associated in a trivalent but rather are present as a bivalent plus an unpaired chromosome, which lies near one pole. Earlier workers showed that such cells are blocked in metaphase and eventually degenerate; this prevents the formation of sperm with abnormal combinations of sex chromosomes. We suggest that the quality-control system would have trouble distinguishing an unpaired chromosome from an uncongressed, newly arisen trivalent, both of which would lie near a spindle pole. If so, the confused quality-control system would block anaphase imprudently, causing a loss of cells that would have produced normal sperm. Hence, we conclude that the congression of the trivalent to the equator probably evolved along with the metaphase quality-control checkpoint. The mechanism of congression in old trivalents is uncertain, but probably involves an interesting force-sensitive regulation of the motors associated with particular chromosomes. We also examined the congression of two newly made quadrivalents when they orient with three kinetochores to one pole and one to the other. As others have described, one of these quadrivalents does not congress, while the other quadrivalent comes closer than expected to the spindle equator. Such variation in the extent of congression may provide materials on which natural selection can act, leading to the evolution of congression. The trivalents of praying mantids are attractive materials for further studies of the mechanism of congression and of the idea that metaphase is a checkpoint for progression through the cell cycle.

Meiosis in Saccharomyces cerevisiae mutants lacking the centromere-binding protein CP1.

CP1 (encoded by the CEP1 gene) is a centromere binding protein of Saccharomyces cerevisiae that binds to the conserved DNA element I (CDEI) of yeast centromeres. To investigate the function of CP1 in yeast meiosis, we analyzed the meiotic segregation of CEN plasmids, nonessential chromosome fragments (CFs) and chromosomes in cep1 null mutants. Plasmids and CFs missegregated in 10-20% of meioses with the most frequent type of aberrant event being precocious sister segregation at the first meiotic division; paired and unpaired CFs behaved similarly. An unpaired chromosome I homolog (2N + 1) also missegregated at high frequency in the cep1 mutant (7.6%); however, missegregation of other chromosomes was not detected by tetrad analysis. Spore viability of cep1 tetrads was significantly reduced, and the pattern of spore death was nonrandom. The inviability could not be explained solely by chromosome missegregation and is probably a pleiotropic effect of cep1. Mitotic chromosome loss in cep1 strains was also analyzed. Both simple loss (1:0 segregation) and nondisjunction (2:0 segregation) were increased, but the majority of loss events resulted from nondisjunction. We interpret the results to suggest that CP1 generally promotes chromatid-kinetochore adhesion.

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

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

Synaptonemal complex analysis of a three-breakpoint translocation in a subfertile bull.

Somatic chromosome analysis of a subfertile Brown Swiss bull demonstrated a three-breakpoint translocation involving chromosomes 1, 8, and 9 in G- and R-banded karyotypes. Based on standard bovine chromosome nomenclature, the translocation was defined as t(1;8;9)(q43;q13;q26). Synaptonemal complex analysis of the chromosome aberration by electron microscopy revealed a hexavalent configuration in 52 of 53 pachytene cells. Twenty-seven cells (51%) had a completely paired hexavalent configuration showing distinctly nonhomologous pairings between normal and/or translocated chromosomes involved in the exchanges. Thirteen cells showed a hexavalent configuration with centrally unpaired chromosome segments but with completely paired terminal arms. In 13 cells (including one at zygotene) the translocation chromosomes formed an open hexavalent, and in one cell there were two completely paired trivalents. Thirty-two cells at diakinesis-MI demonstrated 28 configurations, including one large hexavalent. Testicular histology, testis size, and seminal characteristics were normal.

Metaphase I pairing of deficient chromosomes and genetic mapping of deficiency breakpoints in common wheat

Metaphase I pairing of deficient chromosomes was analyzed in a set of 'Chinese Spring' (CS) wheat (Triticum aestivum L. em. Thell.) plants with varying lengths of deficiencies in the long arm of chromosome 4A (6, 8, 11, 17, 23, 34, 36, 39, and 50% missing), the long arm of chromosome 5B (49% missing), and the long arm of chromosome 2B (33% missing). Pairing in homologous chromosomes between deficient and complete arms was greatly reduced even by small differences in arm length. In deficiency homozygotes and in an isochromosome derived from a deficient 4AL arm, pairing of the two deficient arms was high and approached that of two complete arms. In plants where deficient and complete arms competed for pairing partners, pairing was exclusively between arms of the same length. These results suggest that in wheat, pairing initiation sites are distributed throughout at least the distal halves of the arms and that the alignment of telomeres may be critical for pairing success. Genetic mapping of the deficiency breakpoints was confounded by misdivision of unpaired chromosomes and abnormal transmission rates. Genetic distances between centromeres and breakpoints appeared to be proportional to metaphase I pairing frequencies.Key words: bread wheat, deficiency, chromosome pairing competition, mapping, telomere, pairing initiation.

The Paternal-Sex-Ratio Chromosome of Nasonia

Paternal sex ratio (PSR) is a supernumerary chromosome that is transmitted through sperm to fertilized eggs and then gains a transmission advantage by causing supercondensation of the paternal chromosomes (except itself). Because of haplodiploidy, this converts diploid females into haploid (PSR) males. PSR gains a transmission advantage because the unpaired chromosome has a higher frequency of transmission through male (mitotic) gametngenesis than through female (meiotic) gametogenesis. PSR increases in panmictic populations that produce more than 50% fertilized eggs. PSR frequency is typically reduced by population subdivision because of increased local competition among PSR males and reduced availability of females resulting from PSR action. Genetic and molecular studies of PSR have recently begun. PSR is highly heterochromatic and contains large tandem arrays of repetitive DNA unique to it. PSR apparently acts by "imprinting" the set of paternal chromosomes, resulting in their supercondensation during mitosis. Deletion studies are under way to characterize the genetic basis of PSR action and "protection" from its own action.

Ultrastructure, meiotic behavior, and evolution of sex chromosomes of the genus Ellobius

The whole-mount SC preparations from males of three species of the genus Ellobius (Ellobius fuscocapillus, Ellobius lutescens), and Ellobius tancrei1 were studied by electron microscopy. In the males of Ellobius fuscocapillus, behavioral peculiarities of the sex bivalent (viz. the normal male heterozygosity) are characterized by early complete desynapsis of sex chromosomes (X, Y), occurring at late pachytene-early diplotene. The karyotype of species Ellobius lutescens is unique for mammals. In both sexes it is characterized by an odd number of chromosomes (2n=17). At prophase I the unpaired chromosome 9 is not involved in synapsis with other chromosomes and forms a sex body at the end of pachytene.

Chromosome pairing in the Triticum monococcum complex: evidence for pairing control genes

The occurrence and behavior of pairing control genes at the diploid level were analyzed by using the models and equations developed by Jackson and co-workers. It appears that all pairing control genes are codominant and they are detectable only as heterozygotes in diploids. The phenotypic expressions of such genes are the production of univalents, and this is positively correlated with the occurrence of unpaired chromosome segments at pachytene. Analyses of a large number of accessions of the Triticum monococcum complex and various hybrid combinations have also shown that pairing control gene alleles occur within accessions of the same diploid species and in intraspecific and interspecific hybrids. Strength differences among the pairing control alleles are indicated by different frequencies of univalents at the diploid level, and the occurrence of pairing control genes is not correlated with taxonomic units.Key words: mathematical models, meiotic analyses, pairing control gene, heterozygotes, pachytene, Triticum diploids.

Crossability and genomic relationships between Solanum lucani F. Muell.*S. melongena L.

The chromosome pairing in F1 hybrid synthesized from morphologically dissimilar species of Solanum lucani F. Muell. and S. melongena L., at metaphase I, was very irregular. The hybrid exhibited significantly low level of bivalents (5.72) and this coupled with high frequency of unpaired chromosomes (7.29) was indicative of less genetic similarity between the parents. Majority of the chromosome associations in hybrid were asymmetrical chain, spoon or V-shaped, tri-, quadri- and multivalents. About 66.67% cells have 2-3 trivalents which indicates that 5 pairs of chromosomes are involved in bringing about three interchanges and that both the parental species have heterozygosity between them for reciprocal translocation in hybrid. During first anaphase 21.56 % cells suffered from 1-3 chromatid bridges with 10.0% acentric fragments (0-2) which gave further indications of heterozygosity between the parents. Lagging, elimination or unequal separation of chromosomes generally formed micronuclei to affect pollen and plant fertility significantly. Present observations are suggestive of the species evolving from different genetic stocks.

Synaptonemal Complex Analysis of Spermatocytes in Hybrids of Silver Fox and Blue Fox.

Investigations of male meiosis in silver fox x blue fox hybrids have revealed meiotic arrest at the first prophase stage. Synaptonemal complex analysis using light and electron microscopy demonstrated the occurrence of multivalents, bivalentlike structures, and unpaired axes. We conclude that the sterility of male hybrid foxes probably is due to pairing problems of chromosomes caused by extensive karyotypical differences of the two species, resulting in unpaired chromosomes, chromosome segments, and broken chromosomes.

Cytogenetics of Mammalian Embryonic Development

x+362pp. Oxford: Clarendon Press, 1987This monograph has an interesting history. It was first published in Russian by Nauka (Moscow) in 1978. The authors kindly sent copies to some fellow cytogeneticists in Britain who, after perusing it as well as they could, agreed that the Englishspeaking world should be given the chance of reading it too. Largely through the good offices of Dr Anne McLaren, Oxford University Press undertook to publish an English translation of an updated version and here it is. The translation has been accomplished very stylishly by one of the authors, V. S. Baranov, and OUP should be congratulated on their bold decision.The book deals in a systematic and thorough manner with an expanding field of enquiry. It is mainly devoted to experimental work on laboratory mammals, especially the mouse, but human examples of related phenomena (e.g. hydatidiform moles versus androgenetic embryos) are often described. The first seventy pages deal with effects of haploidy, triploidy etc. and how they arise. The authors then describe how numerical and structural changes come about, with emphasis on nondisjunctional mechanisms. Strangely, they do not include the fact that practically all existing reciprocal translocations in the mouse have been induced by radiation or radiomimetic chemicals whereas Robertsonian (whole arm) ones all seem to have arisen spontaneously in laboratory and wild populations. Next comes a detailed coverage of the various aneuploidy conditions in mice, on which they have worked extensively themselves. Inevitably, there is some overlap here and elsewhere with Epstein’s very useful monograph on the consequences of chromosome imbalance but they have different points of view, so the effect is additive rather than subtractive. The authors then describe in detail their studies on the developmental effects of chromosome imbalance generated by the well-known T6Ca translocation and some others, but comment that little work has been done in this field. This gap in our knowledge has been narrowed recently (see Progress and Topics in Cytogenetics, vol. 8) but much more needs to be done, especially on regions conserved between mouse and man.In a stimulating chapter on the functional activity of chromosomes and control mechanisms in early embryonic development, the authors discuss, inter alia, their own hypotheses on the initial regulation of development by ‘early genes’ (e.g. for actin and tubulin) clustered together on a few autosomes (Dyban) and on the importance of interchromosomal relationships and the mutual activation of homologous chromosomes in early embryogenesis (Baranov). This latter hypothesis would imply that, for the chromosomes concerned, monosomy and nulhsomy should have equally severe effects, since loci on a single unpaired chromosome would be more or less completely inactivated. We have recently disproved this for one mouse chromosome, since nullo-15’s die earlier than mono-15’s (Beechey & Searle, Cytogenet. Cell Genet.) but possibly the phenomenon, if it exists, is connected with chromosomes involved in genomic imprinting, which exonerates no. 15.The influence of specific genes on early mammalian development is also dealt with at some length and there is an interesting chapter on higher nervous activity in animals with chromosome anomalies. The latter is largely based on work by Soviet scientists. This brings out one very valuable aspect of this book, namely its inclusion of much relevant work by East European scientists which might be missed otherwise. In the Preface it is stated that Russian scientists “are always adequately informed on all scientific work published in English or in any other Western language.” Perhaps this is a slight exaggeration but it does point to a general informational imbalance, which this book helps to redress in one branch of biology at least The book is well illustrated and has about a thousand references, as well as a subject index. The authors’ goal for this new edition was to produce a book “of use to the English readers”; I feel sure that this will be.