Lampbrush chromosomes and chiasmata of sex-reversed crested newts

Triturus cristatus carnifex provides a particularly clear example of sexual dimorphism for chiasma frequency and localisation. Oocytes from normal XX females routinely carry one proximal chiasma on each arm of their lampbrush bivalents. Spermatocytes from normal XY males have more numerous and relatively distal chiasmata. Lampbrush chromosomes from the oocytes of sex-reversed XY neofemales are found to resemble those from normal oocytes in having one proximal chiasma on each bivalent arm. A comparison of particular markers on the heteromorphic long arm of chromosome 1 provides evidence to equate the lampbrush 1A to somatic 1A, and confirms previous reports that lampbrush chromosome 1A is slightly longer than 1B. The XY sex bivalent of neofemales does not show any obvious heteromorphy of recognised marker loops. Received: 9 September 1997 / Accepted: 16 October 1997     

Mapping of pachytene bivalents of female codling moth Cydia pomonella (Linnaeus) (Lep., Tortricidae)

Spread pachytene nuclei of codling moth Cydia pomonella (Linnaeus) (Lep., Tortricidae) females of a Syrian strain (SY) were used to investigate chromomere patterns of chromosome bivalents and determine their length. The karyotype of female codling moths consists of 28 chromosome bivalents, of which seven are clearly distinguishable using chromosome length and the number and size of the chromomeres in the pachytene stage. One autosome bivalent has two nucleolar organizing regions (NORs) that are located at the opposite ends of the chromosome and appear as distinct structural landmarks. In female codling moths, the WZ sex chromosome bivalent was easily identified in pachytene oocytes according to the heterochromatic thread of the W chromosome. This study contributed to the knowledge and identification of pachytene chromosomes of female codling moths.     

Complete pachytene chromomere karyotypes of human spermatocyte bivalents

Summary Well-spread human pachytene spermatocyte bivalents were obtained allowing specific identification of each bivalent within its total complement according to its chromomere sequence combined with further staining of its centromeric heterochromatin. The total number of chromomeres was found to be related to the degree of bivalent contraction: 396 in condensed bivalents and 511 in decondensed bivalents. A striking correspondance between chromomeres and mitotic G-bands was observed; on account of the variability of bivalent contraction, condensed bivalents corresponded to prometaphase somatic chromosomes and decondensed bivalents to mid/late prophase chromosomes.     

Chromosome-specific desynapsis in the n = 2 race of Haplopappus gracilis (Compositae)

During cytological screening for pollen sterility in a wild population of Haplopappus gracilis (n = 2), several partially sterile plants were found that had good pachytene pairing but varying numbers of univalents. Some plants had chromosome A bivalents or A univalents, while in the same cells chromosome B had only bivalents. In other plants the reverse condition occurred; the B chromosome had B bivalents or B univalents and only A bivalents. This demonstrates a chromosome-specific effect for the desynapsis genes. Hybridization between the two homozygous mutant genotypes produced only normal bivalents; this indicates the two mutants are not alleles and each is recessive. An F2 generation showed independent assortment of the desynaptic mutations. The chromosome A bivalent is the larger of the two and normally has one or two chiasmata; the B bivalent normally has a single chiasma. Chiasmata distribution was tested in the desynaptic mutant A bivalents and showed an acceptable fit to a binomial distribution. This occurs also in heterozygous, asynaptic pairing control gene mutations. Analysis of the NOR bivalent in two hologenomic desynaptic mutations in tomato also showed a good fit to a binomial distribution of chiasmata. This indicates the same methods are applicable to diverse species.     

Meiosis in a male with Down's syndrome

Summary The chromosomes in mitotic and meiotic phases were investigated in a male Down's syndrome case, aged 45. Information was obtained that based on blood and tunica vaginalis cultures, the somatic chromosome complement was found to possess 47 chromosomes with the standard 21-trisomy, and further that the majority of cells from biopsied testicular specimens examined showed the chromosome number 47 in spermatogonia, and 22 autosomal elements consisting of 21 bivalents and a trivalent, together with an X-Y bivalent in the first spermatocytes. The seminiferous tubules contained no mature spermatozoa.Contribution No. 688 from the Zoological Institute, Faculty of Science, Hokkaido University, Sapporo. This paper is dedicated to Professor Sajiro Makino, Zoological Institute, Hokkaido University, Sapporo, in honor of his sixtieth birthday, June 21, 1966.     

Meiosis in Drosophila melanogaster

Chromosome pairing during meiosis I in D. melanogaster males was investigated ultrastructurally by examining complete bivalents in electron micrographs of serial thin sections. The XY bivalent is characterized by the presence of unique material located between the two half-bivalents at the site of synapsis. The material has a fibrillar appearance and is less electron dense than the surrounding chromatin. YY bivalents in XYY males and XY bivalents containing the X chromosome, In(1)sc ^4Lsc^8R, where the pairing sites of the X chromosome are inverted and partially deleted also possess this material. The material is not associated with autosomal bivalents and may represent a morphological manifestation of the hypothetical cohesive elements (collochores) which are thought to function in conjunction of the X and Y chromosomes (Cooper, 1964).     

Identification of the Z-W bivalent in the silkworm,Bombyx mori

None of the 56 chromosomes including sex chromosomes have been identified in the silkworm so far, though the 28 linkage groups have been determined (Doira, 1986). The present study aims to demonstrate the sex chromosome bivalent in the oocyte by using a particular strain, the sex-limited yellow cocoon (Sy), in which a large fragment of the second chromosome was translocated onto the W chromosome. Among 28 bivalents in the oocyte of the Sy strain, an asymmetrical synaptonemal complex was observed, while in the oocyte of the control strains no such complex was found. We consider this complex as the Z-W bivalent in the silkworm.     

Haploid meiosis and its bearing on the phylogeny of pearl millet,Pennisetum typhoides stapf et hubb.

Meiotic chromosome associations in a spontaneously originated haploid plant of pearl millet have been studied and their phyletic significance discussed. Chromosome pairing could be observed at pachytene and diplotene. Out of a total of 285 PMC's studied at diakinesis-metaphase 1, 43 showed one bivalent and 7 had two bivalents per cell. Both rod- and ring-bivalents were observed. Apart from synapsis accompanied by chiasma formation, close associations of univalent chromosomes were observed. Out of 150 cells without true bivalents, 41 showed 1 s-s association and five, 2 s-s pairs per cell. On the basis of the realization of a maximum of two bivalents per cell, as also of a maximum of 2 s-s pairs, it has been inferred that the chromosome complement ofP. typhoides (n=7) has evolved from a basic set ofn=5 chromosomes. Other available evidence supporting this inference is also discussed.     

The induction of orientational instability and bivalent interlocking at meiosis

The administration of 40° C heat-treatments was found to induce bivalent orientational instability and interlocking at male meiosis in the locust Locusta migratoria. Only the longest members of the complement showed orientational instability and these usually possessed single distally sited chiasmata, with near-maximal intercentromeric distances. An effect on the stability of spindle fibre microtubule association, or attachment to the chromosome, is considered to be a possible explanation of the behaviour found. Bipolar orientation was generally achieved prior to anaphase I so that chromosome segregation was usually normal. Diamphitelic bivalents provided the most common exception to this rule. They sometimes lagged at anaphase, with the separation of half-bivalents and the production of structures indistinguishable from lagging univalents. The bivalent interlocking also involved the longest members of the complement. Most combinations of rod/rod, rod/ring and ring/ring types of interlocking were found. Usually only two bivalents were interlocked in any one cell, although occasionally three were found interlocked. All types appeared to involve an effect on the regulation of chromosome pairing, although at least one of the cells found showed interlocking caused by the metaphase orientational instability. In most cells, interlocked bivalents showed stable orientation and this usually involved the unipolar orientation of each bivalent's two centromeres. Such configurations provide concrete support for the importance of physical tension in the maintenance of metaphase orientational stability. They lead to double non-disjunction at anaphase I. Interlocked bivalents showed normal congression to a mid-equatorial position with no tendency for the re-adjustment of arm ratios to equalise centromere distances from the poles. This behaviour is discussed in relation to spindle fibre dynamics and it is concluded that no hypothesis of congression currently available can satisfactorily explain all that we know of the behaviour of univalents, bivalents, multivalents and interlocked bivalents.     

Analysis of meiosis and non-random bivalent formation in desynaptic mutants of Pennisetum americanum (L.) Leeke

Meiotic behaviour of induced desynaptic mutants of Pennisetum americanum (L.) Leeke in general was described.The frequency distribution of bivalents in induced desynaptic mutants was compared with that expected on the basis of the binomial series. The deviation from the binomial distribution was tested as to its conformity with models based on intra-and intercellular differences in bivalent formation. It is suggested that in these desynaptic mutants of Pennisetum americanum bivalent formation is non-random, which is largely due to the result of intracellular differences in chromosome behaviour regarding their requirements for chiasma formation.     

Nature and role of accessory chromosomes in silver-gray foxes. IV. The behavior of accessory chromosomes in meiosis

Comparison of chromosome number at somatic and spermatogonial mitoses has demonstrated the increase in the number of additional chromosomes in cells of germinal tissue. This may evidence a mechanism of B-chromosomes accumulation in foxes. B-chromosomes may lag as univalents, may form bivalent associations, or occasionally form trivalents at the stage of diakinesis-metaphase I, and they may associate with macrobivalents (A-chromosome bivalents). The analysis of metaphase II has shown that the distribution of B-chromosomes in the second metaphase is random resulting in gametes with various numbers of B-chromosomes.     

The effect of B chromosomes on homoeologous pairing in species hybrids

The effect of B chromosomes on chromosome pairing at meiosis was investigated in the species hybrid Lolium temulentum x L. perenne at both the diploid and tetraploid level. The presence of B chromosomes drastically reduced association of homoeologous chromosomes in both the diploids and tetraploids. This was evident from the high frequency of univalents recorded in PMC's of diploid hybrids with B's and from the predominantly bivalent association of homologous chromosomes in tetraploids of this type. In the absence of B's homoeologous pairing was extensive giving a high frequency of bivalents in the diploids and multivalents as well as bivalents and univalents in the tetraploids.     

Chiasma frequency,distribution and interference maps of mouse autosomes

Chiasma frequencies were analysed and chiasma positions measured in diakinesis/metaphase I autosomal bivalents from oocytes and spermatocytes of F₁ hybrid C3H/HeH×101/H mice. Twenty chromosome size ranks, including the presumptive X bivalent, could be distinguished in oocytes, and nineteen autosomal ranks plus the XY pair spermatocytes. Overall, mean cell chiasma frequencies of the two sexes did not differ significantly once the contribution of the presumptive X bivalent and the XY pair were taken into account. Sex related differences in chiasma distribution patterns were evident, however. In monochiasmate bivalents, the chiasma was most commonly located interstitially in oocytes while in spermatocytes it could be either interstitial or distal. In dichiasmate bivalents, the chiasmata tended to be more centrally located in oocytes than in spermatocytes. Minimum inter-chiasma distances did not appear to show any great variation in chromosome pairs of different sizes, however, mean inter-chiasma distances did increase with the bivalent length. The minimum-inter chiasma distance data suggest that chiasma interference is complete over a chromosomal segment equating to approximately 60 Mb. Measurement of the positions of chiasmata along chromosome arms open up the possibility of producing chiasma-based genetic maps for all the autosomes of the mouse.     

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.     

Chiasma distribution in asynaptic Locusta migratoria

The formation of chiasmata in six full sib male partially asynaptic individuals of Locusta migratoria has been studied. The mean chiasma frequency per cell was 2.3 both at diplotene and metaphase I. Chiasmata tended to be distributed evenly among the bivalents. The frequency and distribution of the chiasmata in each type of bivalent (L, M, or S) depended on the level of asynapsis and on interference between the bivalents. Short bivalents were the most affected by interference, while M bivalents associated independently of L and S bivalent behaviour.     

The meiotic structure and behavior of the strongly heteromorphic X/Y sex chromosomes of neotropical plethodontid salamanders of the genus Oedipina

Plethodontid salamanders in the genus Oedipina are characterized by a strongly heteromorphic sex-determining pair of X/Y chromosomes. The telocentric X chromosome and the subtelocentric Y chromosome are clearly distinguished from the autosomes and their behavior during meiosis can be sequentially followed in squash preparations of spermatocytes. In Oedipina the sex chromosomes are not obscured by an opaque sex vesicle during early meiotic stages, making it possible to observe details of sex bivalent structure and behavior not directly visible in other vertebrate groups. The sex chromosomes can first be distinguished from autosomal bivalents at the conclusion of zygotene, with X and Y synapsed only along a short segment at their non-centromeric ends, forming a bivalent that contrasts sharply with the completely synapsed autosomes. During pachytene, the XY bivalent becomes progressively shortened and more compact, disappearing as a visible structure when pachytene progresses into the diffuse stage of male meiosis. Diplotene bivalents gradually emerge from the diffuse nuclei, presumably by the return of the loops of chromatin into their respective chromomeres. During early diplotene, the X/Y bivalent is clearly visible with a single chiasma within the synapsed segment. This chiasma is terminalized by first meiotic metaphase with the X and Y appearing either in end-to-end synaptic contact or as univalents separated at opposite poles relative to the equatorially distributed autosomal bivalents. In C-banded preparations, the Y is entirely heterochromatic while the X contains a large centromeric C-band and another block of heterochromatin located at the telomeric end, in the region of synapsis with the Y. We find no cytological evidence of dosage compensation, such as differential staining of the X chromosomes or Barr bodies, in mitotic or interphase cells from female animals.     

Induced chromosomal exchange directs the segregation of recombinant chromatids in mitosis of Drosophila.

In meiosis, the segregation of chromosomes at the reductional division is accomplished by first linking homologs together. Genetic exchange generates the bivalents that direct regular chromosome segregation. We show that genetic exchange in mitosis also generates bivalents and that these bivalents direct mitotic chromosome segregation. After FLP-mediated homologous recombination in G2 of the cell cycle, recombinant chromatids consistently segregate away from each other (x segregation). This pattern of segregation also applies to exchange between heterologs. Most, or all, cases of non-x segregation are the result of exchange in G1. Cytological evidence is presented that confirms the existence of the bivalents that direct this pattern of segregation. Our results implicate sister chromatid cohesion in maintenance of the bivalent. The pattern of chromatid segregation can be altered by providing an additional FRT at a more proximal site on one chromosome. We propose that sister chromatid exchange occurs at the more proximal site, allowing the recombinant chromatids to segregate together. This also allowed the recovery of reciprocal translocations following FLP-mediated heterologous recombination. The observation that exchange can generate a bivalent in mitotic divisions provides support for a simple evolutionary relationship between mitosis and meiosis.     

CYTOLOGICAL STUDIES OF NONTRUE–BREEDING MUTANTS IN SORGHUM OBTAINED AFTER COLCHICINE TREATMENT

Sanders , Mary E., and Clifford J. Franzke . (South Dakota State Coll.. Brookings.) Cytological studies of nontrue-breeding mutants in sorghum obtained after colchicine treatment. Amer. Jour. Bot. 49(9): 990–996. Illus. 1962.—Although pollen mother cells of nontrue-breeding mutant plants obtained after colchicine treatment of sorghum seedlings, line ‘Experimental 3,’ showed normal chromosome behavior (10 bivalents) at phases of meiosis I, some abnormalities were found at corresponding stages in first- and second-generation self-progeny plants of some of them. The most frequent chromosome irregularities were an increase over ‘Experimental 3’ in number of cells containing univalents, and mixoploid tissues with tetraploid and diploid cells. The higher polyploid groups (6n, 8n, 10n) also present in 2 plants might be related to their male-sterile condition rather than being an indication of the chromosome complement. Abnormalities in progenies suggest that some of the mutants might have been chimeras in which abnormalities were missed and raise the question whether chromosome changes are involved in the formation of the mutants in spite of the preponderance of normal diploid cells with 10 bivalents during prophase and metaphase of meiosis I. This could occur if sorghum contains a genetic mechanism which promotes bivalent rather than multivalent pairing. That such might be a possibility is indicated by the large numbers of bivalents and small numbers of multivalents found in polyploid cells or groups.     

Absence of chiasmata from the heteromorphic region of chromosome I during spermatogenesis in Triturus cristatus carnifex

Analysis of squash preparations of spermatocytes from crested newts, Triturus cristatus carnifex, has shown that in most cells at least one large bivalent regularly fails to form chiasmata in one arm-pair. Feulgen microphotometry of diplotene and metaphase bivalents has shown that it is the largest bivalent in each cell which shows chiasma failure in one arm-pair. A C-banding technique which identifies chromosome I by virtue of a long, darkly stained region in its long arm, was used to confirm the absence of chiasmata from one arm-pair of the longest bivalent, and specifically from the darkly stained region. The achiasmate region which chromosome I exhibits during spermatogenesis, corresponds to the heteromorphic region of oocyte lampbrush bivalent I in which chiasmata never form. A possible correlation between the complete absence of crossing-over from the heteromorphic region and unusual cytological and molecular features which it exhibits, are discussed.     

Chiasmata distribution in the normal karyotype of mice

The number of chiasmata per cell and variance of chiasmata numbers were studied, as well as the recombinational interaction between different bivalents in CBA/Lac mice male line. No competition of bivalents for chiasmata was discovered in mice; at the same time, the chiasmata within one arm of the chromosome interfere with each other. The number of chiasmata per bivalent is estimated for each chromosome independently. The number of chiasmata per chromosome is limited both from below (minimum one chiasma independently of its size) and from above (positive interference of chiasmata).