Meiotic delay of mouse spermatocytes carrying x-ray-induced translocations.

The kinetics of spermatocyte progression through meiotic prophase in cells with or without induced translocations were studied in mice that had been exposed to x-rays. Pulse-labeling experiments using 3H-thymidine, followed by autoradiographic analysis, indicated that at higher x-ray doses (6 and 7 Gy), translocation-carrying cells tend to spend more time in meiotic prophase than do normal cells. At 2 Gy, no such delay seemed to be present. The observed delay may explain the reduction in transmission of translocations to the next generation reported by others.     

Univalent sex chromosomes in spermatocytes of Sxr-carrying mice

Pachytene configurations of the sex chromosomes were studied in whole-mount, silver-stained preparations of spermatocytes in mice with XY,Sxr, XX,Sxr, XO,Sxr, XO,Sxr+5¹² and T(X;4)37H,YSxr chromosomes, and non-Sxr-carrying controls. XY,Sxr males showed an increased number of X and Y univalents and of self-synapsed Y chromosomes. In T(X;4)37H,YSxr males an increased proportion of trivalent+Y configurations was also accompanied by higher numbers of self-paired Y univalents; the proportion of trivalent+X⁴ was not increased, but that of self-synapsed X⁴ univalents was. There was more selfsynapsis in cells containing one univalent than in cells containing two univalents. Spermatocytes of XX,Sxr mice contained single univalent X, which was never seen to be self-synapsed, but self-synapsis of the X occurred in a proportion of cells in XO,Sxr males. There were no self-paired X chromosomes in the XO,Sxr+5¹² mouse although lowlevel pairing of the 5¹² chromosome occurred. All four XX,Sxr and XO,Sxr males contained testicular sperm, and testicular sperm were also present in one T(X;4)37H male, while another such male had sperm in the caput. It is concluded that (1) self-synapsis of univalents is affected by variable conditions in the cell as well as by the DNA sequences of the chromosome, and (2) that the level of achievable spermatogenesis is not always rigidly predetermined by a chromosome anomaly but can be modulated by the genetic background.     

Meiotic chromosome pairing in Stethophyma grossum spermatocytes studied by a surface-spreading and silver-staining technique

Spermatocytes of Stethophyma grossum were prepared for light microscope examination by a surface-spreading and silver-staining technique adapted with very little modification from a mammalian procedure. Very clear preparations of a variety of prophase I stages were obtained which revealed many details of chromosome and nuclear organisation. These observations and preliminary observations on two other grasshopper species demonstrate the ready applicability of these techniques to Orthopteran spermatocytes. A detailed study of six pachytene stage spermatocytes gave conclusive confirmation that three bivalents achieve full pairing in male meiosis of Stethophyma grossum, the other eight bivalents being partially paired at their procentric ends only.     

Meiotic segregation of chromosomes in Drosophila melanogaster oocytes

A new technique was developed for a light microscopic analysis of meiosis in Drosophila oocytes. — When the nuclear envelope breaks down the bivalents, till then compressed into a karyosome, separate in early prometaphase. The homologues remain associated by chiasmata except for the fourth chromosomes which are no longer associated. Non-homologous chromosomes regularly segregating from each other in genetic experiments are also unconnected after karyosome disintegration but during metaphase I the fourth chromosomes and the heterologous pairs coorient on the same arc of the spindle and move precociously towards opposite poles. Nondisjunction and other irregularities are not infrequent in oocytes having an uneven number of achiasmatic elements. The fourth chromosomes and the Xs or the large autosomes, when lacking chiasmata, may be involved in non-homologous segregation. In c3G homozygotes all chromosomes appear as univalents in prometaphase. Segregation is variable but the observations suggest the polar distribution of equal numbers of chromosomes in variable combinations irrespective of the size. — Coorientation of univalents may be accounted for if the centromeres, whether homologous or non-homologous, are associated in pairs during early meiotic prophase, and that in the karyosome these pairing relationships are preserved until spindle organization at the onset of prometaphase.     

Meiotic segregation of compound-3 chromosomes in Drosophila

Analysis of crossos between genetically marked stocks of Drosophila melanogaster showed, that the compound-3 chromosomes C(3L)RM and C(3R)RM segregate preterentially in female meiosis, and the following two types of eggs are formed predominantly: C(3L)RM; 0 and 0; C(3R)RM. In male meiosis segregation is almost random and four types of sperm are formed: 1. C(3L)RM; C(3R)RM, 2. 0; 0, 3. C(3L)RM; 0, 4. 0; C(3R)RM. The frequencies of these sperm types vary with the genotypes tested. In the stock C(3L)RM, st; C(3R)RM, p^p, males produce 76.8% type 1 and 2, and 23.2% type 3 and 4; males of the stock C(3L)RM, ri; C(3R)RM, sr form 63.2% type 1 and 2, and 36.8% type 3 and 4.The segregational behaviour of compound-3 chromosomes found in female meiosis is expected according to the distributive pairing hypothesis. In the male however, where there is no distributive pairing, the stock-specific segregation of compound-3 chromosomes may be due to the presence of small homologous chromosome segments near the centromere which influence chromosome distribution.     

Robertsonian chromosomes and the nuclear architecture of mouse meiotic prophase spermatocytes

Background

The nuclear architecture of meiotic prophase spermatocytes is based on higher-order patterns of spatial associations among chromosomal domains from different bivalents. The meiotic nuclear architecture depends on the chromosome characteristics and consequently is prone to modification by chromosomal rearrangements. In this work, we consider Mus domesticus spermatocytes with diploid chromosome number 2n = 40, all telocentric, and investigate a possible modification of the ancestral nuclear architecture due to the emergence of derived Rb chromosomes, which may be present in the homozygous or heterozygous condition.

Results

In the 2n = 40 spermatocyte nuclei random associations mediated by pericentromeric heterochromatin among the 19 telocentric bivalents ocurr at the nuclear periphery. The observed frequency of associations among them, made distinguishable by specific probes and FISH, seems to be the same for pairs that may or may not form Rb chromosomes. In the homozygote Rb 2n = 24 spermatocytes, associations also mediated by pericentromeric heterochromatin occur mainly between the three telocentric or the eight metacentric bivalents themselves. In heterozygote Rb 2n = 32 spermatocytes all heterochromatin is localized at the nuclear periphery, yet associations are mainly observed among the three telocentric bivalents and between the asynaptic axes of the trivalents.

Conclusions

The Rb chromosomes pose sharp restrictions for interactions in the 2n = 24 and 2n = 32 spermatocytes, as compared to the ample possibilities for interactions between bivalents in the 2n = 40 spermatocytes. Undoubtedly the emergence of Rb chromosomes changes the ancestral nuclear architecture of 2n = 40 spermatocytes since they establish new types of interactions among chromosomal domains, particularly through centromeric and heterochromatic regions at the nuclear periphery among telocentric and at the nuclear center among Rb metacentric ones.     

Meiotic recombination at the ends of chromosomes in Saccharomyces cerevisiae

Meiotic reciprocal recombination (crossing over) was examined in the outermost 60-80 kb of almost all Saccharomyces cerevisiae chromosomes. These sequences included both repetitive gene-poor subtelomeric heterochromatin-like regions and their adjacent unique gene-rich euchromatin-like regions. Subtelomeric sequences underwent very little crossing over, exhibiting approximately two- to threefold fewer crossovers per kilobase of DNA than the genomic average. Surprisingly, the adjacent euchromatic regions underwent crossing over at twice the average genomic rate and contained at least nine new recombination "hot spots." These results prompted an analysis of existing genetic mapping data, which showed that meiotic reciprocal recombination rates were on average greater near chromosome ends exclusive of the subtelomeres. Thus, the distribution of crossovers in S. cerevisiae appears to resemble that found in several higher eukaryotes where the outermost chromosomal regions show increased crossing over.     

The attachment of pachytene chromosomes to the nuclear membrane in mammalian spermatocytes

• 1.1. Electron microscopic studies and statistical analyses have been made of the points of attachment of the synaptinemal complex to the nuclear membrane of primary spermatocytes in the mouse, field vole and golden hamster.
• 2.2. The findings of the investigation support the view that the synaptinemal complex, and hence the bivalent, are attached at both ends and not intermediately, even when the chromosome possesses a sub-terminal centromere.

     

Meiotic behaviour of individual chromosomes in allotriploid Alstroemeria hybrids

Chromosome association and chiasma formation were studied in pollen mother cells at metaphase I of four allotriplod BC1 plants (2n=3x=24) obtained from the backcross of the hybrid Alstroemeria aurea x A. inodora with its parent A. inodora. We distinguished the chromosomes of both parental species by genomic in situ hybridization (GISH), whereas the individual chromosomes were identified on the basis of their multicolour FISH banding patterns obtained after a second hybridization with two species-specific satellite repeats as probes. All the four BC1 plants possessed two genomes of A. inodora and one of A. aurea. Variable numbers of recombinant chromosomes, resulting from meiotic recombination in the interspecific hybrid, were present in these plants. The homologous A. inodora chromosomes generally formed bivalents, leaving the homoeologous A. aurea chromosomes unassociated. High frequencies of trivalents were observed for the chromosome sets that contained recombinant chromosomes, even when the recombinant segments were small. Chromosome associations in the trivalents were restricted to homologous segments. The implications of the absence of homoeologous chromosome pairing on gamete constitution and prospects for introgression in Alstroemeria are discussed.     

The behaviour and structure of the sex chromosomes in spermatocytes of Microtus oeconomus

The meiotic behaviour and structure of the sex chromosomes of Microtus oeconomus (2n=30) in Giemsa stained preparations are described. The X-Y pair appears as a sex vesicle at late zygotene. At late pachytene an unfolded sex vesicle is visible. A condensed sex vesicle appears during pre-diffuse diplotene and starts to unfold again during post-diffuse diplotene. At diakinesis and metaphase I the X and Y chromosomes can be recognized in an end-to-end association. During anaphase I, interkinesis and metaphase II the sex chromosomes are heteropycnotic and can therefore easily be recognized during the final stages of meiosis. During spermiogenesis the X and Y chromosomes can be identified in Giemsa stained preparations until the stage of spermatid elongation.     

Meiotic cohesion requires accumulation of ORD on chromosomes before condensation

Cohesion between sister chromatids is a prerequisite for accurate chromosome segregation during mitosis and meiosis. To allow chromosome condensation during prophase, the connections that hold sister chromatids together must be maintained but still permit extensive chromatin compaction. In Drosophila, null mutations in the orientation disruptor (ord) gene lead to meiotic nondisjunction in males and females because cohesion is absent by the time that sister kinetochores make stable microtubule attachments. We provide evidence that ORD is concentrated within the extrachromosomal domains of the nuclei of Drosophila primary spermatocytes during early G2, but accumulates on the meiotic chromosomes by mid to late G2. Moreover, using fluorescence in situ hybridization to monitor cohesion directly, we show that cohesion defects first become detectable in ord(null) spermatocytes shortly after the time when wild-type ORD associates with the chromosomes. After condensation, ORD remains bound at the centromeres of wild-type spermatocytes and persists there until centromeric cohesion is released during anaphase II. Our results suggest that association of ORD with meiotic chromosomes during mid to late G2 is required to maintain sister-chromatid cohesion during prophase condensation and that retention of ORD at the centromeres after condensation ensures the maintenance of centromeric cohesion until anaphase II.     

Meiotic loss of the B chromosomes of maize is influenced by the B univalent co-orientation and the TR-1 knob constitution of the A chromosomes

The suppression of meiotic loss when the maize B chromosomes are unpaired is genetically determined. Two genotypes were selected in 1B x 0B crosses: the H line where the B transmission rate is Mendelian (50%) and the L line where the B is present in only about 40% of the progeny. Using the ZmBs probe located at the centromere and at the distal portion of the B chromosome in FISH, we found that the centromeric and telomeric ends of the B univalent co-orient at metaphase I. This feature seems to promote proper centromere orientation causing the lack of meiotic loss of the unpaired B. The co-orientation was observed in both lines, however in the L line the B univalents were not always properly oriented, showing amphitelic orientation in about 25% of the metaphase I cells. We also studied plants of the H and L lines with FISH to test the possible relation between the knob constitution and B loss. It has been found that the plants of both lines are similarly variable for the 180-bp knob repeat, but they differ in the TR-1 350-bp repeat, the L line having more TR-1 knobs. The use of a 45S rDNA probe which labels chromosome 6, allowed us to determine that this chromosome shows the main variability between the two lines: the L line has TR-1 in both arms, showing a large TR-1 knob on the long arm. The H line has only one, generally located on the short arm besides the NOR.     

A quantitative study on anaphase movement of chromosomes in living grasshopper spermatocytes

Summary The present investigation has been undertaken to obtain data for the analysis of the chromosome movement at anaphase and the formation of a cleavage furrow. The study is based on simultaneous measurements of the spindle and cell diameters as well as of the chromosome separation in living spermatocyte divisions of the grasshoppers, Podisma sapporense and Acrydium japonicum.Evidence from the present investigation shows that the movement of chromosomes to the poles and the elongation of the spindle are separated in time; the spindle length remains unchanged through out anaphase. Spindle elongation is not associated with the separation of daughter chromosomes. The cell, and the spindle as well, elongate after the chromosomes have reached the poles. Cell elongation may follow the stretching of the spindle, and cause sufficient tension to distort the cell wall, resulting in the subsequent formation of a cleavage furrow.Contribution No. 327 from the Zoological Institute, Faculty of Science, Hokkaido University, Sapporo, Japan. Aided by a grant from the Scientific Research Fund of the Ministry of Education.     

Meiotic chromosomes in a female with primary trisomic Down's syndrome

Summary We report the first analysis of chromosomes at the first meiotic metaphase in a non-mosaic female with primary 21-trisomic Down's syndrome. A 21-univalent and a high chiasma number were found. These findings are discussed in comparison with the observations on normal subjects and on meiosis in Down's syndrome males.