Non-homologous Associations Research Articles

Centric-fusion translocation and whole-arm heterochromatin in the karyotype of the blue fox (<i>Alopex lagopus</i> L.): synaptonemal complex analysis

Conventional observations of mitotic chromosomes from two male blue foxes, revealing a centric-fusion translocation and whole-arm heterochromatin, were verified by synaptonemal complex analysis. This analysis revealed that the centric fusion had been preceded by a conspicuous loss of chromosome material in the two one-armed chromosomes involved, but the chromosomal origin of the centric-fusion kinetochore could not be established. The nontranslocated chromosomes of the trivalent, which in all cells but one were in cis configuration, had reached by early pachytene a stage in which almost complete homologous pairing and nonhomologous association or pairing of the free ends of the chromosomes could be observed. In later stages, complete pairing of the nontranslocated chromosomes with the corresponding arms of the centric-fusion translocation was seen occasionally. One to six autosomal bivalents demonstrated unpaired heterochromatic arms in early pachytene, and the heterochromatic chromosome arms were sometimes unpaired even in late pachytene. Some of them showed a distinct size heteromorphism in late zygotene and early pachytene. In most late-pachytene cells, however, the heteromorphic chromosomes were completely length-adjusted. Only a small fraction of the cells showed pairing interference between nonhomologous chromosomes.

Analysis of synaptonemal complexes in the amphidiploid of Lolium multiflorum × Festuca drymeja

Synaptonemal complexes in the amphidiploid Lolium multiflorum (4χ) × Festuca drymeja (4χ) have been examined by the whole mount spreading technique in nuclei with between 49 and 100% pairing. At mid to late zygotene nonhomologous associations are formed, with multivalents involving more than half the total axial element length in some cases. However, they are corrected by pachytene. There is evidence of differences in the timing or rate of pairing between the two sets of chromosomes; the L. multiflorum chromosomes seem more advanced than the F. drymeja chromosomes in their pairing at mid and late zygotene, and it is possible that this asynchrony places a constraint on intergenomic chromosome pairing.Key words: Lolium-Festuca, amphidiploid, synaptonemal complexes, nonhomologous pairing, correction mechanism.

Prophase of meiosis in human spermatocytes analysed by EM microspreading in infertile men and their controls and comparisons with human oocytes

Observations at the electron microscope (EM) level have been made on 1883 primary spermatocytes from 40 chromosomally normal subfertile men and 566 spermatocytes from 10 fertile controls, using the technique of microspreading. Spermatocytes of infertile men in general showed greater indications of degeneration including higher levels of background silver deposition, nucleolar organising region - XY associations, fragmentation of synaptonemal complexes and overproduction of XY excrescences. A few oligospermic men also showed an immature morphology of the XY pair and/or a reduced extent of XY synapsis. Dissociation of the sex chromosome axes at prophase was found to occur with a much lower frequency than that recorded for separated X and Y chromosomes at metaphase I. In a single spermatocyte, synaptonemal complex formation was observed between Xqter and Yqter, a situation that could enable rare XqYq interchange. A proteinaceous stalked body exists on the Y axis towards its non-pairing end; this structure might have a functional relationship with the gene for spermatogenesis, (AZF), located at the euchromatin/heterochromatin interface. Compared with human oocytes, spermatocytes show fewer anomalies of synapsis, i.e. asynapsed segments or whole axes, non-homologous associations, interchanges, interlocks. These latter data agree well with findings from the mouse.

Assessment of chromosome associations in haploids and their parental accessions in Hordeum

Meiotic pairing was studied in the following species and their haploid derivatives: Hordeum cordobense 2x, H. marinum 2x and 4x, H. secalinum 4x, H. capense 4x, H. jubatum 4x, H. brachyantherum 4x and 6x, H. lechleri 6x, and H. procerum 6x. The study revealed (i) homologous pairing in diploid species and very little nonhomologous associations in their mono-haploids; (ii) the alloploid nature of the polyploid taxa; (iii) a certain degree of homoeologous pairing in polyhaploids despite the diploid-like meiotic behaviour of the polyploids; (iv) genetic variation in the suppression of homoeologous chromosome pairing in different Hordeum species.Key words: Hordeum, meiotic pairing, haploids.

Immunodeficiency, centromeric heterochromatin instability of chromosomes 1, 9, and 16, and facial anomalies: the ICF syndrome.

Instability of the heterochromatic centromeric regions of chromosomes 1, 9, and 16 associated with immunodeficiency was found in a four year old girl. Similar phenotypic and chromosomal abnormalities were described in a previous patient studied by us and in four other published cases. All these patients have facial anomalies in addition to combined immunodeficiency and chromosomal instability. Stretching of the heterochromatic centromeric regions of chromosomes 1, 16, and to a lesser extent, 9 and homologous and non-homologous associations of these regions were the most common cytogenetic findings in all the patients. Multi-branched configurations and whole arm deletions of chromosomes 1 or 16 or both were also found. Comparing clinical and chromosomal data we conclude that immunodeficiency, centromeric heterochromatin instability, and facial anomalies form a new syndrome, for which we propose the acronym ICF. A mutation interfering with the normal process of condensation of part of the centromeric heterochromatin is postulated as the basic chromosome defect in this syndrome.

Colchicine action at meiotic prophase revealed by SC-spreading

In the present paper earlier findings on the interference of colchicine with presynaptic alignment and synaptonemal complex formation in Allium ursinum are corroborated and several other direct or indirect consequences of colchicine treatment, like deformations and decomposition of lateral elements and a high incidence of non-homologous associations are reported. Mechanisms of meiotic alignment and pairing are discussed with respect to their susceptibility to the known cytological and biochemical activities of colchicine.

Karyological Studies ofMetrioptera ArnoldiRme (Decticidae) andAncistrura Nigrovittata(Br.) (Phaneropteridae: Tettigonioidea-Orthoptera)

SUMMARYThe karyotype of Metrioptera amoldi Rme and Ancistrura nigrovittata (Br.) is described. Both species possess the primitive character of karyotype (2n = 31♂, all chromosomes are acrocentric, XO and XX sex determining mechanism). A small number of chiasmata were found, Metrioptera amoldi having two or three in L and one in M and S, while Ancistrura nigrovittata has only one chiasma in all bivalents. Both species have non-homologous association between X and small autosomes and association between different large middle M and S chromosomes. The karyotype of Metrioptera arnoldi is much more flexible than that of Ancistrura nigrovittata. In Metrioptera amoldi the chromosomes of pairs L1, L2, and M5 show diferences in staining in the length and thickness of terminal elements of bivalents. Metrioptera amoldi is compared in its karyological aspects with 3 other species belonging to the genus Metrioptera.

Oocyte development in XO foetuses of man and mouse: the possible role of heterologous X-chromosome pairing in germ cell survival.

The pairing characteristics of the X axis in XO human and mouse oocytes were studied by the spreading technique throughout meiotic prophase. In three human XO foetuses, germ cell development was seen to be largely blocked at the preleptotene stage. In XO mice on the other hand, oocytes surviving through pachytene increasingly show the X axis making a non-homologous association with itself or with an autosome. Such associations take the form hairpins or rings when self pairing occurs or triradial structures when involvement is with an autosome. Pairing initiation in the autosomes involved is disturbed by the X axis suggesting that the heterologous pairing seen is taking place at the earliest stage of synaptonemal complex formation, namely zygotene. It is suggested, that in the XO mouse, and perhaps also in rare fertile XO humans, survival, of a population of oocytes into the adult is ensured by the ability of the X axis to pair non-homologously at meiotic prophase, thus satisfying pairing requirements.

MOST OF THE HOMOEOLOGOUS PAIRING AT METAPHASE I IN WHEAT-RYE HYBRIDS IS NOT CHIASMATIC

The use of telomeric C-bands in wheat-rye hybrids has made it possible to distinguish three types of wheat-wheat (1B(L)) and wheat-rye associations (a, end-to-end extremely distal; b, end-to-ed distal; and c, interstitial) between homoeologous chromosomes at different metaphase I stages (early, middle and late) and also to estimate the actual recombination frequencies for such associations at anaphase I. There was a decrease of the a and b association frequencies during the different metaphase I stages, whereas the c type remained without variation in all stages. A good fit between the frequencies of c associations at metaphase I and the number of recombinant chromosomes at anaphase I, assuming a maximum of one chiasma per bond, was found; however, there was no correspondence between metaphase I and anaphase I data when all associations (a + b + c) were considered. In addition, rye-rye homologous pairing was observed at metaphase I, but no evidence for rye-rye recombination was found at anaphase I. The results indicate that most of end-to-end (a and b) homoeologous and nonhomologous associations are actually nonchiasmatic and are a remnant of prophase pairing.

The association of centromeres of nonhomologous chromosomes at meiotic prophase in Beta vulgaris L.

Centromere association involving two or three nonhomologous bivalents at meiotic prophase of Beta vulgaris and monosomic additions of B. vulgaris × B. procumbens are described. The associations, most conspicuous at late pachytene and persisting until diakinesis, occur abundantly in a small group of genetically related plants. The presence of normal bivalents at metaphase I and the absence of irregularities at later stages show that centromere associations do not disturb meiotic recombination and transmission. A mathematical model proposed to test for randomness with respect to the number of bivalents involved in centromere associations, showed significant difference between observed and expected frequencies. On the basis of our own observations and data from the literature, it is suggested that the associations at pachytene and diplotene are delayed separations of centromere aggregates that arise at premeiotic interphase or during the formation of the synizetic knot. A possible molecular mechanism is discussed.Key words: centromere association, meiotic prophase, pachytene chromosomes, nonhomologous association, sugar beet, Beta vulgaris.

The meiotic process and aneuploidy.

Normal females developing at 25 degrees C produce their first population of oocytes at 132 +/- 2 hr post oviposition. Entrance of the oocytes into premeiotic interphase signals initiation of DNA replication which continues for 30 hr. Coincidentally, extensive SCs appear, averaging 50 microns (132 hr), peaking at 75 microns (144 hr), and continuing into early vitellarial stages. Recombinational response to heat, evidenced by enhancement or induction of exchange, is limited to the S-phase with a peak at 144 hr coinciding with maximal extension of the SC. Coincidence of synapsis and recombination response with S at premeiotic interphase is contrary to their conventional localization at meiotic prophase. The interrelationship between exchange and nondisjunction has been clarified by the Distributive Pairing Model of meiosis. Originally revealed through high frequencies of nonrandom assortment of nonhomologous chromosomes, distributive pairing has been shown to follow and to be noncompetitive with exchange, to be based on size-recognition, not homology, and as a raison d'etre, to provide a segregational mechanism for noncrossover homologs. Rearrangements, recombination mutants, and aneuploids may contribute noncrossover chromosomes to the distributive pool and so promote the nonhomologous associations responsible for nondisjunction of homologs and regular segregation of nonhomologs. Further information concerning early meiotic events and their relation to segregation has been revealed by studies of the ts rec-1(26) mutant. Application of the restrictive temperature (31 degrees C) at sequential times during development shows wild-type activity to be drastically reduced beginning at 126/132 hr, terminating at 162 hr, and so coinciding with S and the heat-sensitive period of the normal genome. EMs of serially sectioned oocyte nuclei maintained at the restrictive or control temperature reveal SCs to be indistinguishable, and implicate recombination rather than synapsis as the target of the mutant. Activity of rec-1(26) in the range 17 degrees to 31 degrees C reveals a sharp decline between 28+ degrees and 31 degrees C, typical of a denaturation curve. If denaturation of the rec protein by the restrictive temperature marks its active phase, it follows that recombination terminates at the end of S when the restrictive temperature becomes ineffective. The notion that synapsis ensures regular segregation (35) is invalidated by rec-1(26) whose normal synapsis at the restrictive temperature is followed by a 200-fold increase in nondisjunction at segregation, as compared to the control.(ABSTRACT TRUNCATED AT 400 WORDS)

On the interchromosomal connections in plants

Using Giemsa C-banding technique, the existence of interchromosomal connections in well spread root tip metaphase plates in a variety of plant species has been demonstrated. Various types of interchromosomal connections are observed, involving satellite, telomeres, interstitial regions and in a few cases centromeres, too. The possible role of these interchromosomal connections in establishing the homologous and non-homologous association in the somatic chromosomes and maintaining the spatial relationship of the genetic apparatus is indicated. In the majority of the cases the connections are made up of constitutive heterochromatin.

C-banding and non-homologous associations

A chromosome complement formed by 16 autosomes and an Xyp sex chromosome system was found in Epilachna paenulata Germar (Coleoptera: Coccinellidae). All autosomes were metacentric except pair 1 which was submetacentric. The X and the Y chromosomes were also submetacentric but the Y was minute. The whole chromosome set carried large paracentric heterochromatic C-segments representing about 15% of the haploid complement length. Heterochromatic segments associated progressively during early meiotic stages forming a large single chromocenter. After C-banding, chromocenters revealed an inner networklike filamentous structure. Starlike chromosome configurations resulted from the attachment of bivalents to the chromocenters. These associations were followed until early diakinesis. Thin remnant filaments were also observed connecting metaphase I chromosomes. Evidence is presented that, in this species, the Xyp bivalent resulted from an end-to-end association of the long arms of the sex chromosomes. The “parachute” Xyp bivalent appeared to be composed of three distinct segments: two intensely heterochromatic C-banded corpuscles formed the “canopy” and a V-shaped euchromatic filament connecting them represented the “parachutist” component. The triple constitution of the sex bivalent was interpreted as follows: each heterochromatic corpuscle corresponded to the paracentric C-segment of the X and Y chromosomes; the euchromatic filament represented mainly the long arm of the X chromosome terminally associated with the long arm of the Y chromosome. The complete sequence of the formation of the Xyp bivalent starting from nonassociated sex chromosomes in early meiotic stages, and progressing through pairing of heterochromatic segments, coiling of the euchromatic filament, and movement of the heterochromatic corpuscles to opposite poles is described. These findings suggest that in E. paenulata the Xyp sex bivalent formation is different than in other coleopteran species and that constitutive heterochromatic segments play an important role not only in chromosome associations but also in the Xyp formation.

HYBRID DYSGENESIS IN DROSOPHILA MELANOGASTER: FACTORS AFFECTING CHROMOSOMAL CONTAMINATION IN THE P-M SYSTEM

The two interacting components of the P-M system of hybrid dysgenesis are chromosomally associated elements called P factors and a susceptible cytoplasmic state referred to as M cytotype. Previous experiments have indicated that P factors are a family of multiple-copy transposable genetic elements dispersed throughout the genome of P strains but absent in long-established M strains.-Evidence is presented that the sterility and male recombination-inducing potential of P elements may be acquired by X chromosomes, derived from M strains, through nonhomologous association with P strain autosomes, a process referred to as "chromosomal contamination." The frequencies of chromosomal contamination of X chromosomes by P strain autosomes were highly variable and depended on a number of factors. M cytotype (as opposed to P cytotype) was essential for high frequencies of P factor contamination. There were large differences in contamination potential among individual female families, and a weak negative correlation existed between family size and contamination frequency. Chromosomal contamination in the P-M system was shown to be independent of that in the I-R system.-Frequency distributions suggested that the relationship between sterility production and P factor insertion is complex. The majority of P element transpositions, identified by in situ hybridization in one X chromosome, were not associated with gonadal sterility. However, high sterility potential was found to be associated with the presence of at least one P element inserted into the X chromosome. This potential was lost at a rate of about one-sixth per generation in M cytotype but was stabilized in P cytotype. Various hypotheses concerning the relationship between transposition and chromosomal contamination are discussed.

Supernumerary segments in five species of grasshoppers (Orthoptera: Acridoidea)

Supernumerary segments have been observed in five species of grasshoppers: Calliptamus barbarus, Oedipoda fuscocincta, Acrotylus insubricus, Omocestus raymondi and Chorthippus ariasi. In four cases they are located in the S-chromosomes, but in A. insubricus they are carried by the megameric pair (M9). In C. barbarus and O. fuscocincta we have observed non-homologous association during diplotene between the extra segments and the X-chromosome. The supernumerary segments of these two species are distally located. However, in anaphase-I the unequal bivalents divide reductionally in 30% and 20% of the cells, respectively. Finally, the supernumerary segments of C. barbarus do not produce any effect on mean chiasma frequency, but they decrease significantly the between-cell variance of chiasmata in males carrying them.

Cytogenetics of the trigeneric hybrid, (Hordeum vulgare ×Triticum aestivum) ×Secale cereale

Chromosome pairing was studied in hybrids of (Hordeum vulgare ×Triticum aestivum) ×Secale cereale. Chiasma frequency per cell varied from 1.94 to 3.16 between the different hybrids. This variation was attributed to genetic variability in rye parents which affected homoeologous pairing. The pairing of rye chromosomes as revealed by Giemsa C-banding was a combination of nonhomologous association between rye chromosomes and associations with chromosomes of wheat and barley.

Pachytene analyses in Atylosia scarabaeoides and Cajanus cajan*A. scarabaeoides hybrid.

All the eleven pachytene chromosomes of A. scarabaeoides were identified on the basis of their relative length, arm ratio, chromomere pattern and nucleolar association. The pachytene complement of A. scarabaeoides consists of one median, 9 submedian and one subterminal chromosomes. On the basis of direct comparison of parental pachytene chromosomes, 7 chromosomes are common to A. scarabaeoides and C. cajan. In 8 bivalents of the hybrid the homoeologues participating in pairing were identified. Out of these 8, one bivalent showed non-homologous association and 4 were found to be heteromorphic. Two translocations and an interstitial duplication were noticed in a few microsporocytes. On the basis of the common chromosomes involved in translocations and those exhibiting heteromorphism in the 2 hybrids, C. cajan×A. sericea and C. cajan×A. scarabaeoides but not in the C. cajan×A. lineata hybrid it is concluded that A. sericea and A. scarabaeoides are closer to each other than either of them to A. lineata.

Nonhomologous associations of C-heterochromatin at human male meitoic prophase.

In human male meiotic prophase, nonhomologous pairings and connections between C-heterochromatic regions were demonstrated by C-banding and G-11 staining. Approximately 40% of the pachytene stages exhibited bivalent associations at regions of C-heterochromatin. Nonhomologous associations were seen between all possible morphological types of bivalents. Evidence is submitted suggesting that nonhomologous associations between regions of C-heterochromatin are a fairly common event in many organisms, including man, and the possible roles of this phenomenon in meiosis are discussed.

Unusual Chromosome Associations inKrigia Cespitosa(Compositae)

SUMMARYAn investigation of Krigia cespitosa Raf. (2n=8) revealed an unusually high frequency of non-homologous chromosome associations due to stickiness. Despite this, meiotic behaviour and plant fertility were unaffected.

Meiosis in polyhaploid Hordeum: hemizygous ineffective control of diploid-like behaviour in a hexaploid?

Meiotic chromosome behaviour was studied in the hexaploid Hordeum parodii (2n=6x=42) and in six haploids (2n=3x=21) obtained from a cross between H. parodii and H. bulbosum (2n=2x=14) whereby all bulbosum chromosomes were selectively eliminated. The alloploid nature of H. parodii was evident from the exclusive bivalent formation at the hexaploid level and the low and variable number of bivalents in its haploid derivatives. In haploids, both nonhomologous (intragenomic) and homoeologous (intergenomic) chromosomes paired at prophase. Foldbacks in single chromosomes, bivalents and trivalents were observed at prophase and metaphase I. At diakinesis, the associations involved a maximum of 20 chromosomes which decreased to 12 by metaphase I. This decrease was attributed to the failure of the non-homologous associations to persist until metaphase I. A “hemizygous-ineffective” control for the diploid-like behaviour of the hexaploid parodii is proposed to explain the homeologous chromosome pairing in its haploid derivatives.