Centromere Orientation of Quadrivalents of Heterozygous Translocations and an Autoploid of GOSSYPIUM HIRSUTUM L

Cytological observations of quadrivalents of heterozygous translocations in Gossypium hirsutum L. demonstrate that, in addition to alternate-1 and alternate-2 orientations, a third alternate orientation (alternate-3), which occurs as a three-dimensional, V-type configuration, can be identified.—Two additional types of disjunctions, the centromere orientations of which are rotational modifications of either adjacent or alternate configurations, were also observed in quadrivalents of a translocation heterozygote. These two types are rare, and both appear in the form of the Roman numeral X . The X and the alternate-3 types also occur in quadrivalents of an autoploid of G. hirsutum.—The two X types, along with adjacent-1, adjacent-2, alternate-1 and alternate-2 orientations, represent the six possible types of planar 2 x 2 random orientation of the four centromeres of a quadrivalent. Including the three-dimensional alternate-3 type, there are seven types of orientation.     

Meiotic behaviour of chromosomes 1R, 2R and 5R in autotetraploid rye

The meiotic behaviour of chromosomes 1R, 2R and 5R was studied in C-banded preparations of autotetraploid rye. Analysis of pairing and chiasma formation was based on metaphase I configurations, using the model designed by Sybenga, with slight modifications. Frequencies of two modes of pairing (one quadrivalent or two bivalents) differed from those expected for random pairing. Although preferential pairing for some arm pairs of chromosome 2R was detected, this did not seem to be the cause of the increased bivalent pairing. This increase was attributed to either the spatial separation of the four homologous chromosomes in some premeiotic cells into two groups of two, or a correction of the synaptonemal complex, or both. The number of chiasmate associations showed variation between chromosomes and between arms within the same chromosome. It was closely related to arm length, but different after quadrivalent and bivalent pairing. This is suggested to be a consequence of partner exchange interfering with pairing and, consequently, with chiasma formation, and a different chiasma distribution after quadrivalent pairing. Variation between chromosomes in the frequencies of alternate and adjacent co-orientation in metaphase I quadrivalents without interstitial chiasmata suggests that the relative positions of the centromeres in the quadrivalent influence their co-orientation.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations

In autotetraploids, chromosome pairing may be in the form of quadrivalents or bivalent pairs. Whether or not the quadrivalents are maintained until first meiotic metaphase depends on the formation of chiasmata. The relative frequencies of M I configurations thus contain information both on pairing and on chiasma formation. With distal chiasma localisation six configurations can be recognised and their relative frequencies determined: ring quadrivalents, chain quadrivalents, trivalents (with univalent), ring bivalents, open (rod) bivalents, univalent pairs. These represent five degrees of freedom permitting five parameters to be estimated: the frequency (f) of quadrivalent pairing; the frequencies of chiasmate association of the two ends (arms in metacentrics), a′, b′, after quadrivalent pairing, and a, b after bivalent pairing. — The appropriate formulae have been derived and applied to observations on Tradescantia virginiana (4n=24) which has pronounced distal chiasma localisation. Slight modifications make the model applicable to autotetraploids with interstitial in addition to distal chiasmata. In T. virginiana, chromosome pairing appeared to be random between homologues (65.8% quadrivalent pairing; 55.4% observed at M I). After quadrivalent pairing chiasmate association is frequent in the “average long” arm (95.0%) and much less so in the other arm (60.5%). This is attributed to partner exchange. After bivalent pairing chiasma frequencies are still different for the two arms (93.8% and 83.5% association respectively) but much less pronounced. Various complications are discussed.     

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.     

Meiotic analysis of two human reciprocal X-autosome translocations

Two cases of human reciprocal X-autosome translocation, t(X;12) and t(X;2), are described in sterile males, along with meiotic findings. Each carrier had inherited the translocation from his mother. Both showed azoospermia and germ-cell maturation arrest at the primary spermatocyte level, with most cells being arrested at the pachytene stage. A few metaphase I (MI) divisions were found, with occasional metaphase II cells being seen in the t(X;2) carrier. MI air-dried preparations gave clear evidence of chain quadrivalent formation. In the t(X;2) heterozygote, the pairing characteristics of the quadrivalent at pachytene were also analyzed in electron microscopic spreads. Disturbance of pairing around the breakpoints characterized most quadrivalents, and there was evidence in about 20% of the cells that nonhomologous pairing had taken place between the translocated chromosomes and the normal chromosome 2. Comparisons are made with similar nonhomologous pairing configurations seen at pachytene in quadrivalents of male reciprocal X-autosome translocations of the mouse.     

Interchange quadrivalents and chromosome order at meiotic metaphase I in Briza L. (Gramineae)

In interchange heterozygotes of Briza humilis and B. media the interchange quadrivalent is shown to be preferentially positioned in flattened, lateral spreads at metaphase I. The positioning of the interchange quadrivalents is different in the two species but in both the frequency of alternate or adjacent orientation is different for different positions on the metaphase plate. B chromosomes in B. humilis are found to alter the positioning of the quadrivalent and the B chromosomes themselves are also found to show a nonrandom distribution on the metaphase plate.     

Chiasma interference and centromere co-orientation in a spontaneous translocation heterozygote of Euchorthippus pulvinatus gallicus (Acrididae; Orthoptera)

The meiosis of an individual of the species Euchorthippus pulvinatus gallicus heterozygous for a reciprocal translocation involving chromosomes 3 and 6 has been analysed using the Giemsa C-banding technique. It is concluded that: (i) Chiasma interference in the quadrivalent seems to act only at the arm level. There is no interference across the translocation break point. No interchromosomal chiasma interference could be demonstrated, (ii) The results concerning the co-orientation of the quadrivalent suggest that the length of the chromosomal segments between two adjacent centromeres at metaphase I is related with their orientation behaviour.     

A human 9;20 reciprocal translocation associated with male infertility analyzed at prophase and metaphase I of meiosis

Details are given of a meiotic prophase analysis, carried out by spreading, of a human 9;20 reciprocal translocation ascertained in a subfertile, oligospermic male. Air-dried meiotic preparations revealed the presence of translocation quadrivalents at metaphase I. Germ-cell degeneration was evident from the early prophase of meiosis onward. Associations between the translocation quadrivalent and XY bivalent at pachytene were seen in only 20% of the cells and seemed not to be the prime cause of germ-cell failure. Pairing disruption around the breakpoints of the translocation at pachytene and/or pairing failure in one arm of the pachytene cross was observed in a total of 87% of all cells analyzed. This could have contributed significantly to germ-cell atresia.     

Meiosis in translocation heterozygotes in the mosquito Culex pipiens

Adult Culex pipiens males irradiated with both X-rays and neutrons were crossed to untreated females and F₁-egg rafts were checked for dominant lethality. F₁-progenies were outcrossed with normal individuals in order to obtain lines with inherited semisterility. From a total of 120 lines that showed a certain amount of sterility 12 lines were studied cytologically. 10 lines showed reciprocal chromosome exchanges.—At late pachytene and diplotene cross configurations with large asynaptic regions at the center of the cross are obligatory. Bivalents, chains of three, chains of four, and ring configurations are present at metaphase and anaphase I. The different frequencies of the occurrence of such multiples are dependent on the chromosomes involved in the exchange, the length of the pairing segments and the chiasma frequencies in these segments. Chiasma frequency in the interstitial segments is reduced by means of chiasma interference over the centromere and by asynapsis near the breakage points. — Alternate, adjacent-1- and adjacent-2-distributions are present to a different extent. Alternate distribution is most, adjacent-2-distribution least frequent. — The role of translocations and the probability of their becoming effective in pest eradication programs is discussed.     

Meiotic pairing and segregation of translocation quadrivalents in yeast

Meiotic pairing and segregation were studied in three different heterozygous reciprocal translocation strains of the baker’s yeast, Saccharomyces cerevisiae. Pachytene translocation quadrivalents were identified by a combination of immunofluorescence and fluorescence in situ hybridization and the karyotypes of meiotic products were determined by pulsed-field gel electrophoresis. The translocations differed with respect to the relative sizes of the chromosomes involved and the positions of translocation breakpoints, and produced translocation quadrivalents of widely different shapes. This allowed us to study the influence of the morphology of quadrivalents on their segregation behaviour. In all cases alternate predominated over adjacent segregation. 3:1 disjunction of chromosomes was more frequent when translocation breakpoints were close to the centromeres. If a translocation breakpoint was distant from the centromere, the occurrence of an intervening chiasma influenced the pattern of segregation. In general, quadrivalent formation and segregation resembled the behaviour of translocation heterozygotes in most higher eukaryotes. We therefore conclude that, although chromosome condensation does not occur in yeast metaphase, centromere orientation and chromosome disjunction are governed in a way similar to that of higher eukaryotes. Received: 6 February 1998; in revised form: 19 May 1998 / Accepted: 23 May 1998     

C-band morphology of T(8;9)/8;9 in Blattella germanica

Summary Centromere position and each arm of the T(8;9)/8;9 quadrivalent at late pachytene can be recognized by C-banding. The chromosome 9 breakpoint lies immediately adjacent to the centromeric C-band; that of 8, in the general region of the centromere but the relationship to centromeric bands was not determined since the latter stain very faintly. Chromosome 9 differs from no. 8 in the presence of a complex series of intercalary bands, heavy centromeric bands and, overall, a larger amount of C-band material. Possible implications of these differences with respect to chromosome breakability and the nature and distribution of mutant loci are noted. Earlier identification of adjacent-1 and adjacent-2 metaphase I ring orientations, made on the basis of orientation pattern, was confirmed.     

Prometaphase I and anaphase I in an interchange heterozygote of Allium triquetrum (Liliaceae)

Four unstable malorientations in a chain quadrivalent, as well as maloriented bivalents, were studied in fixed prometaphase I pollen-mothercells from an interchange heterozygote of Allium triquetrum. The relative frequencies of these malorientations, in cells from pollen sacs of different developmental ages, suggest that maloriented bivalents, on the average, reorient before maloriented quadrivalents; and that, similarly, there are differences in the timing of reorientation amongst the four types of maloriented quadrivalents. — Further, the proportion of anaphase I cells derived from alternate orientation in the interchange quadrivalent is lower than expected in pollen sacs with a high percentage of cells in mid-anaphase; but higher than expected in pollen sacs in which relatively few cells have as yet proceeded into anaphase, and also in those in which most cells have already passed through anaphase. Arguably, these data are a direct outcome of the differential behavior of unstable maloriented quadrivalents in the preceeding prometaphase.     

Centromere orientation,reorientation, and segregation in an interchange quadrivalent during anther development in pearl millet

Prometaphase I orientation, reorientation and anaphase I segregational behaviour of a chain-forming interchange quadrivalent involving one of the long chromosomes and the long arm of the seventh (nucleolar) chromosome was studied during anther development in pearl millet. The data obtained from 34 anthers showed that by early prometaphase I about 90% of the bivalents have attained stable bipolar orientation but about 48% of the quadrivalents are mal-oriented. There seems to be an interaction between bivalents and quadrivalents during mal-orientation and reorientation. The mal-oriented bivalents reoriented before the quadrivalents. For quadrivalent mal-orientation four types, 4/0, 3/1, 2/1/1/1 and 2/2 (adjacent 1), were distinguished in addition to the regular types, adjacent 2 and alternate. Based on their potential to reorient, the order of the mal-oriented quadrivalent types was 4/0 > 3/1 > 2/1/1; 2/2 led to anaphase I disjunction as for an adjacent 1 segregation. The data from 36 anthers at anaphase I showed alternate segregation of chromosomes in nearly 50% of pollen mother cells (PMCs) up to a developmental index of about 65. In late anthers about 35% PMCs showed alternate segregation. This suggests that the PMCs that reached metaphase I later had more adjacent 2 orientations since mal-oriented configurations delay meiotic development, and implies preferential reorientation behaviour of the maloriented quadrivalent types.     

Chromosomal identification and meiotic behavior of reciprocal translocations in a rye polymorphic population. Evolutionary implications

Summary The spontaneous interchange polymorphism of rye cultivar Ailés is composed, as can be deduced from the chromosomal identification of the interchanges analyzed, of several different reciprocal translocations in which the chromosomes of its haploid complement are involved with a similar frequency, except for chromosomes 4R and 6R. Several features of chromosome behavior at metaphase I, such as configuration and orientation of quadrivalents and frequency of chiasmata, were analyzed in structural heterozygotes for different interchanges. The two main factors affecting the orientation of quadrivalents at metaphase I proved to be the morphology of these chromosome associations at metaphase I and, in particular, the frequency of bound chromosome arms that they showed. A genotypic control of alternate orientation of quadrivalents independent of chiasmata frequency was not detected. In addition, the frequency of alternate orientation shows no relation to the fitness. Possible evolutionary implications of the results obtained are discussed.     

Quantitative analysis of diploid translocation heterozygotes: test of models and equations

Summary Equations have been derived for two different models of chromosome pairing and chiasmata distribution. The first model represents the normal condition and assumes complete synapsis of homologous bivalents and the arms of interchange quadrivalents. This is followed by a nonrandom distribution of chiasmata among bivalents and multivalents such that each bivalent or bivalent-equivalent always has at least one chiasma. Univalents occur only as part of a III, I configuration at diakinesis or metaphase I. The second model assumes that a hologenomic mutation is present in which all chromosomes of a genome are equally affected. Two different assumptions can be made for such a mutation, and both give the same results: (1) homologous or homoeologous chromosome arms may be randomly paired or unpaired, but synapsis always leads to a crossover; (2) homologous or homoeologous arms always pair, but chiasmata are randomly distributed among the arms. The meiotic configurations at diakinesis or metaphase I are the same for both assumptions. Meiotic configurations of normal diploid interchange heterozygotes show good agreement with numbers predicted by the equations for nonrandom chiasmata distribution among configurations. Inter-specific hybrids with supernumerary chromosomes produced meiotic configurations frequencies in agreement with predictions of equations for random chiasmata distribution, but a hybrid without supernumeraries fitted the nonrandom expectations.     

Disjunction types and their frequencies in two heterozygous reciprocal translocations of Blattella germanica (L.)

Adjacent-1, alternate-1, adjacent-2, and alternate-2 disjunction configurations were observed cytologically at metaphase I in heterozygotes from two reciprocal chromosome translocations in the German cockroach, Blattella germanica (L.). T(7; 12) shows random disjunction and the frequencies of the above four types were in a ratio of 2112 (p>0.90). T(3; 12) has directed disjunction (about 70% alternate) which is attributable to a heavy preponderance of alternate-2. No interpretable ratio occurs here except for the equality of alternate-1 and adjacent-2 types. These observations confirm the existence of two types of alternate disjunction, and provide insight into the basis for random vs directed disjunction.     

Modification of the properties of a translocation in the German cockroach by selection

Summary A stock of Blattella germanica bearing the interchange T(3; 12)/3;12 was subjected to close inbreeding with selection for random disjunction at metaphase I. After 3–4 generations of selection, interchange quadrivalent chiasma frequency decreased, variability in free bivalent chiasma frequency increased sharply, and individuals with either random or directed disjunction were present in the stock. Random disjunction was modified from a ratio of 2112 (adj.-1; alt.-1; adj.-2; alt.-2) to a ratio of 1111. After 7–8 generations of selection, chiasma frequency appeared to stabilize at lower than normal levels and variability decreased for both quadrivalents and free bivalents. Directed disjunction was modified from a ratio of 2114 to 1112, and no individuals with the original high level of directed disjunction were detected. Chains-of-four tended to orient randomly, especially in individuals where the ring quadrivalents showed directed disjunction. Relaxation of inbreeding, but not selection, produced an increase in chiasma frequency and variability in both free bivalents and quadrivalents, but the modified ratios for both random and directed disjunction were retained. These results are discussed with respect to inbreeding effects and genetic control of chiasma frequency and metaphase I disjunction in interchange quadrivalents.     

Anther development and the orientation of the interchange quadrivalent in pearl millet

The effects of developmental stage of the anther, ageing of the plant, inbreeding and season on meiotic segregation of an interchange chain quadrivalent were studied in pearl millet, Pennisetum americanum (L.) Leeke. The frequency of adjacent orientation (and segregation) increased with developmental stage of the anther, independent of other factors. Plant age and degree of inbreeding had no demonstrable effects, but there was an indication that high temperature favoured adjacent orientation. Chromosome contraction as measured by change in chromosome length appeared to be negligible during metaphase-anaphase. Therefore, increased adjacent orientation cannot readily be explained by metaphase reorientation resulting from the straightening of chromosomes caused by an increase in their rigidity. It is probable that the unoriented or mal-oriented quadrivalents observed regularly at early metaphase I continue to straighten out prior to their delayed orientation. When they finally orientate late in metaphase, their orientation will more likely be adjacent than alternate.     

Position and orientation in the metaphase equator of an interchange quadrivalent of hybrid rye

The position and orientation of an interchange quadrivalent in flattened lateral views of metaphase I were studied in pollen mother cells of hybrid rye. Five quadrivalent types showed three positional distributions in the equator, these distributions having elements both similar to and very different from those found previously for an interchange quadrivalent of Allium triquetrum.     

Centromere co-orientation in a spontaneous translocation heterozygote of Euchorthippus pulvinatus gallicus (Acrididae,Orthoptera)

The co-orientation of a quadrivalent in an individual of E. pulvinatus gallicus heterozygous for a reciprocal translocation is studied. The data agree with the hypothesis that the co-orientation behaviour of two adjacent centromeres in a quadrivalent depends on the relative distance between them at metaphase I, this distance being determined by chiasmata.