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.     

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.     

Metaphase I orientation of chain-forming interchange quadrivalents: a theoretical consideration

The orientation behavior of chain forming interchange quadrivalents at metaphase I was studied in three interchange heterozygotes of pearl millet [Pennisetum americanum (L.) Leeke] which involve chromosomes 1, 3, 6 and 7 in various combinations. Of these, two combinations predominantly produced rings and the third was a chain-forming type. The chain quadrivalents derived from the two ring-forming interchanges, as well as the chain quadrivalent generated by the third interchange, all showed one adjacent orientation at metaphase I (adjacent-1 or -2, depending upon the formation or failure of chiasmata and their positions in the different segments of the pachytene cross). Homologous centromere co-orientation leading to adjacent-1 and alternate-1 occurs following chiasma failure in the noncentric arms of the pachytene cross, and nonhomologous centromere co-orientation leading to adjacent-2 and alternate-2 occurs following chiasma failure in the centric arms of the pachytene cross. Thus, it has been proposed that, unlike in ring quadrivalents, a specific chain quadrivalent will have only homologous or nonhomologous centromere co-orientations at metaphase I.     

From the phenomenon of nondisjunction to the problem of chromosome co-orientation (75th anniversary of Bridges' article]

The concepts of the mechanism of chromosome nondisjunction in Drosophila are described in a historical retrospective. Evidences are given for the appropriateness of the term co-orientation in the traditional sense used by geneticists treating nondisjunction. There are 6 variants of co-operation in Drosophila meiosis depending upon the number and particular chromosomes involved in co-orientation. The classical chromosome nondisjunction is a variant of co-orientation in the bivalent composed of two homologous chromosomes. By comparing the different variants of pairing (pairing in bi- and multivalents) resulting in co-orientation, the elementary events preceding co-orientation may be identified. The author reviews his recent data concerning the similarities of the co-orientation of two homologs and the co-orientation of two nonhomologs in Drosophila meiosis. The concept of the role of pairing in the precentromeric heterochromatic region during chromosome co-orientation is considered, and the hypothesis of delayed pairing in this region during meiotic prophase is put forward. Based on the suggested hypothesis clarified are (i) the relationship of pairing, crossing over, and disjunction of homologous chromosomes (ii) the relationship of crossing over and co-orientation of nonhomologous chromosomes, and (iii) the time when the contact resulting in nonhomolog co-orientation takes place.     

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 at metaphase I in interchange heterozygotes of rye,Secale cereale L.

Three plants of cultivated rye heterozygous for reciprocal translocations have been used to analyze the centromere co-orientation in ring quadrivalent associations at metaphase I. New cytological evidence of the existence of the alternate-II configuration is presented. From the statistical analysis of the data one can deduce that: (i) the four configurations (adjacent-I, adjacent-II, alternate-I and alternate-II) appear with different frequencies, (ii) alternate co-orientations are much more frequent than adjacent ones, (iii) co-orientations of type I are more frequent than those of type II, (iv) among the alternate configurations those of type I are much more frequent than those of type II, (v) among the type I co-orientations the alternate configuration is much more frequent than the adjacent one, (vi) there is no correlation between the percentages of type I and type II classes within the adjacent and alternate co-orientations.     

Fertility and selection in tetraploid Lolium

Selection over five generations increased the fertility (seed set) in a population of tetraploid Lolium perenne from 67.57 per cent in the second (C ₂) to 73.95 per cent in the seventh (C ₇) generation. The increased fertility is associated with a high quadrivalent frequency and a low frequency of bivalents, trivalents and univalents. The high quadrivalent frequency in the selected population was achieved partly by a slight overall increase in chiasma frequency but mainly by a redistribution of chiasmata.     

An unusual chromosome system in a leafhopper (Homoptera auchenorrhyncha)

An unidentified species of the genus Austroagalloides is shown to have 7_II+1_IV instead of the normal 11_II+XO configuration at metaphase I of meiosis in males. The quadrivalent manifests two types of pairing which ensures regular disjunction; normal chiasma-type pairing and distance pairing. It is suggested that the 7_II+1_IV form is derived from the 11_II+XO form by a series of fusions.     

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.     

The spatial arrangement ofAllium triquetrum chain quadrivalents at metaphase I as viewed by confocal microscopy

We examined the three-dimensional arrangement of bivalents and, in particular, a chain of four chromosomes (chain quadrivalent) in the metaphase I spindle of pollen mother cells ofAllium triquetrum by confocal microscopy. Firstly, we show by optical sectioning and three-dimensional image reconstruction that the cooriented pairs of centromeres of all seven bivalents lie virtually parallel to each other in the metaphase I spindle, parallel to the long axis of the spindle. Secondly, we like-wise show that the four centromeres of the chain quadrivalent are aligned in the metaphase I spindle in, essentially, atwo-dimensional array, not in a three-dimensional array, as proposed by some other authors. This two-dimensionality has its basis, we argue, in the principle that poleward directed spindle forces minimise centromere-to-pole distances and therefore align pairs of centromeres connected to opposite poles most axially (vertically) in the spindle. These distances are minimised for the quadrivalent as a whole only when it lies in two dimensions, i.e. in aplane parallel to the spindle axis.     

Cytogenetics of pearl millet

Summary The somatic karyotype of pearl millet Pennisetum americanum (L.) Leeke. (2n = 14) has been studied in several cultivars, but few cytological markers have been discovered which could help in the easy identification of the chromosomes. Analysis of pachytene bivalents permits such identification but is feasible only in a few cultivars. Recently, several lines having telocentric chromosomes have been produced and classified but their potentialities as cytogenetic tools have yet to be explored. Some African populations of pearl millet carry B-chromosomes in their karyotype. Cytogenetics of B-chromosomes has been reported in great detail. Bs undergo spontaneous changes to produce deficient- and iso-chromosomes. The main effect of B-chromosomes is on chiasma frequency which is exerted by the relative amounts of chiasma promoting euchromatin and the chiasma depressing heterochromatin in the Bs. Haploid plants occur occasionally and sometimes show a low degree of seed set, offering a possibility of establishing homozygous inbred lines. Cytogenetics of several spontaneous and induced autotetraploids have been reported. In general quadrivalent formation between the seven sets of four homologues was random. Seed set of the autotetraploids could be improved by selection; improved seed fertility was found to be associated with increased chiasma frequency, increased quadrivalent frequency and regular distribution of chromosomes at anaphase I. Genes controlling morphological characters of plant phenotype segregate independent of those controlling fertility and in pearl millet polyploidy per se is not limiting to plant vigour. Primary trisomics represent the best studied among the aneuploids of pearl millet. All the seven primary trisomics have been identified and described. Some were used in assigning genes to specific chromosomes but in general trisomies have poor vigour and fertility, and show low frequency of transmission. Apart from B-chromosomes, cytogenetics of interchanges has been the best studied aspect of pearl millet. The frequency of co-orientation of an interchange complex at metaphase I, which determines the fertility or sterility of the interchange heterozygote, is influenced by the genetic background and thus is theoretically amenable for selection leading to improved fertility of the heterozygote. Interchange tester-stocks have been assembled which can be used to identify the chromosomes involved in any newly obtained interchange. A complex interchange line involving all the chromosomes of the complement has also been produced, but the ring-of-fourteen produces total male and female sterility.Genotypic control of mitosis and meiosis has been reported, with reference to chromosome numerical mosaicism, multiploid sporocytes, desynapsis and chromosome fragmentation, and male sterility. Pearl millet being a largely outbreeding species, forced inbreeding was mainly found to result in loss of morphological vigour and reduction in mean chiasma frequency per PMC. Interspecific hybrids between pearl millet and several related species have been cytologically investigated and homology of the seven chromosomes of pearl millet with seven of the fourteen chromosomes of P. purpureum has been demonstrated. Cytogenetic evidence from haploids, autopolyploids and interspecific hybrids has indications to suggest that the haploid number of x = 7 is derived from x = 5, but the evidence is inconclusive and needs critical evaluation.     

Chromosome association and fertility in tetraploid ryegrass

In colchicine treated diploid ryegrass,Lolium perenne L. (2n=14), in addition to normal diploids, tetraploids and mixoploids, cytologically aberrant plants were observed. The latter included one aneuploid (2n=25), four translocation heterozygotes (diploid and mixoploid), and one mixoploid desynaptic plant which was a chimera of normal and desynaptic tissues as shown by successive clonal propagations. The normal diploids had chromosome association of 7 II, with an average chiasmata frequency of 12 per pollen mother cell and had 85.6 per cent pollen fertility. The tetraploids showed a mean chromosome association of 0.88 I, 4.83 II, 0.29 III and 3.91 IV, with an average chiasmata frequency of 25 per microsporocyte, and had 83.8 per cent fertile pollen. In the tetraploids, pollen fertility was negatively correlated with quadrivalent frequency and positively so with bivalent and trivalent frequencies, there being no significant correlation with univalent frequency. Among the most frequent types of quadrivalents, only the alternate chain quadrivalent frequency was positively correlated (r=+.9297) with the plllen fertility. On this basis, an increased pollen fertility in the succeeding generations of tetraploids could result from a decrease in the frequency of quadrivalent types other than the alternate chains.     

Induced allopolyploidization of autotetraploidBrassica campestris

Seeds of a variety of allogamous Brown Sarson (Brassica campestris) were irradiated with gamma rays either before or after duplication of their chromosome number by colchicine. Tetraploids from seeds irradiated before duplication had mean quadrivalent and chiasma frequencies of 1.85 and 28.35 respectively. Those from seeds irradiated after duplication had frequencies of 2.82 and 29.94, while unirradiated tetraploids had 2.77 and 30.53 respectively. Small structural aberrations inducing preferential pairing are assumed to have caused the lower quadrivalent frequency after the first treatment. The results suggest that it may be more expedient to induced, allopolyploidlike chromosome behaviour by irradiation at the diploid than at the tetraploid level.     

High-resolution physical mapping reveals that the apospory-specific genomic region (ASGR) in Cenchrus ciliaris is located on a heterochromatic and hemizygous region of a single chromosome

An apomictic mode of reproduction known as apospory is displayed by most buffelgrass (Cenchrus ciliaris) genotypes, but rare sexual individuals have been identified. Previously, intraspecific crosses between sexual and aposporous genotypes allowed linkage to be discovered between the aposporous mode of reproduction and nine molecular markers that had been isolated from an aposporous relative, Pennisetum squamulatum. This region was described as the apospory-specific genomic region (ASGR). We now show an ideogram of the chromosome complement for aposporous tetraploid buffelgrass accession B-12-9 including the ASGR-carrier chromosome. The ASGR-carrier chromosome has a region of hemizygosity, as determined by in situ hybridization of BAC clones and unique morphological characteristics when compared with other chromosomes in the genome. In spite of its unique morphology, the ASGR-carrier chromosome could be identified as one of the chromosomes of a meiosis I quadrivalent. A similar partially hemizygous segment was also detected in the ASGR-carrier chromosome of the aposporous buffelgrass genotype, Higgins, but not in the sexual accession B-2S. Two non-recombining BACs linked to apospory were physically mapped on a highly condensed chromatin region of the short arm of B-12-9, and the distance between the BACs was estimated to be ∼11 Mbp, a distance similar to what previously has been shown in P. squamulatum. The short arm of the ASGR-carrier chromosome was highly condensed at pachytene and extended only 1.7–2.7 fold that of mitotic chromosomes. Low recombination in the ASGR may partially be due to its localization in heterochromatin.     

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.     

Interchromosome effects on chiasma distribution due to translocations in Locusta migratoria

Two different strains carrying translocations were obtained by irradiation of Locusta migratoria males. The presence of translocations produced an increase in chiasma number in the chromosomes not involved in the translocation, compared to the same chromosomes in wild-type individuals. This increase was higher in the individuals carrying the octovalent than in those carrying the quadrivalent.     

Cytological studies on triploid Trigonella corniculata L.

Two spontaneous triploid plants of Trigonella corniculata (3n=24) were observed in a population raised from irradiated seeds. Up to seven trivalents per cell were observed. One of the plants appeared to be a double translocation heterozygote involving two non-homologous chromosomes, and therefore could form a quadrivalent, a ring trivalent and bivalents without corresponding univalents.     

A quadrivalent studied in living and fixed grasshopper spermatocytes

A single specimen of the grasshopper Melanoplus differentialis was discovered which was heterozygous for an interchange involving two of the largest chromosomes of the complement. The resulting quadrivalent occurred as a ring-of-four, chain-of-four, and, rarely, as two heteromorphic bivalents. Metaphase orientation and anaphase disjunction of some of these quadrivalents were recorded in living cells. The chain-of-four was stable in the configuration in which two pairs of kinetochores were oriented to each pole. The configuration in which three pairs of kinetochores were oriented to one pole and one pair to the other pole was also stable, but was shifted off the equator toward the pole to which three pairs of kinetochores were oriented. Given the nature of this quadrivalent the unusual stability of the 31 configuration is expected from current hypotheses of chromosome orientation and reorientation.     

CYTOLOGICAL CONSEQUENCES OF DNA AMPLIFICATION IN AN ANTHER CULTURE-DERIVED DOUBLED HAPLOID LINE OF NICOTIANA TABACUM

An increase in nuclear DNA, without an increase in chromosome number, has been found to occur as a result of anther culture in Nicotiana tabacum L. The objective of this study was to determine the cytological consequences of this DNA amplification in F₁ hybrids between a doubled haploid that had undergone a substantial increase in DNA and the cultivar from which that doubled haploid was derived. Mitotic and meiotic analyses were performed on plants obtained from reciprocal crosses of N. tabacum cv. NC95 and NC95 SCDHL 12, a doubled haploid line that has 41% more DNA than the parental cultivar. While no cytological abnormalities were observed in either parental line, numerous abnormalities were seen in both somatic and meiotic tissues of the F₁ hybrid. Chromosome losses, which appeared to result from spindle errors, were observed in these tissues. It is speculated that the spindle errors may be the result of a genetic unbalance caused by combining genomes with widely differing amounts of DNA. In addition to the spindle errors, a quadrivalent with an atypical morphology was observed in meiotic diplotene and metaphase I cells of the hybrid. The quadrivalent configuration was interpreted to represent pairing between amplified homologous regions in homeologous chromosomes. Further investigations of additional doubled haploid × cultivar lines is required before the significance of these findings to the anther culture process in N. tabacum can be fully assessed.