Differential meiotic behaviour of diploid and tetraploid cells in a partially asynaptic mutant

A partially asynaptic individual of the grasshopperEuchorthippus pulvinatus (2n = 16 + X) was analysed at meiosis, using a Giemsa C-banding technique. Long chromosomes formed univalents less frequently than did medium and short ones. Homologues which succeeded in forming bivalents showed reduced chiasma frequency, the long chromosomes being affected by most. Changes in chiasma distribution were also observed. The presence of univalents at metaphase I seems to affect the function of the spindle, since most cells at the second division were unreduced. Cytokinesis was also subsequently suppressed in a great number of these products of restitution, resulting in the formation of diploid and tetraploid spermatids. Fifteen tetraploid metaphase I cells were also found in which pairing level and chiasma frequency were almost twice the average value in normal diploid individuals. The nature of this mutant is discussed.     

Chromosome-specific control of chiasma formation in Crepis capillaris

An experimental population of Crepis capillaris (2n=6) displays frequent chromosome-specific univalence affecting all three chromosome pairs of the complement independently in different plants. The frequency of univalence in the population varies from 0% in some plants to about 40% of pollen mother cells in other plants. Most commonly, affected pollen mother cells contain just one pair of univalents and wherever the frequency of cells containing univalents exceeds 10%, a chromosome-specific effect almost invariably appears. Univalence affecting the A, C and D chromosome pairs is about equally frequent in the population. The mean cell chiasma frequencies of affected plants are generally lower than those of normal plants from the same population, despite suggestions of compensating increases in the chiasma frequencies of unaffected chromosomes pairs in the presence of specific univalence of a third pair. Breeding tests have been carried out, and observations made on pachytene stages which demonstrate that the specific univalence is due to recessive genes causing desynapsis following apparently normal pairing of homologues.     

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.     

Asynapsis in Lolium perenne

Eighteen plants displaying varying degrees of asynapsis ranging from weak to very strong were found among four out of six populations of Lolium perenne L. (2n=14) which had been subjected to three cycles of directional phenotypic selection for productivity of green material. No plants were found displaying univalents in the original generation but the incidence increased with cycles of selection, indicating the genetic control and differential distribution of asynaptic genes among these populations. — The analysis of univalents and chiasma frequency of pollen mother cells (PMC) of six partially asynaptic plants chosen for detailed study revealed that univalents occurred throughout all PMC chiasma classes irrespective of chiasma frequency, but the higher the chiasma frequency of any PMC the less the likelihood of univalents occurring. The relationship between chiasma frequency and univalent frequency per PMC per plant was negative. — Mean chiasma frequency per bivalent increased for the asynaptic cells in comparison with the normal in both the weak and medium asynaptic groups which was explained by the availability of additional chiasmata for redistribution.     

The effects of ageing on the meiotic chromosomes of male and female mice

The effects of age on the chiasma frequencies, chiasma position and numbers of univalents at MI in males and females of three strains of mouse were examined. Males showed a slight but non significant rise in chiasma frequency in age due to an increase in bivalents with two chiasmata at the expense of single chiasmata bivalents. In contrast, females exhibited a significant decrease in chiasma frequency with age due to the loss of two chiasma bivalents with a corresponding increase in single terminal chiasmata bivalents. In both males and females there was no significant increase in univalents with age in the strains studied. Of interest was the finding of a greater degree of contraction of the MI chromosomes in the oocytes of old relative to young females, a differential contraction that was independent of culture time. This finding is discussed with regard to the production line theory and non disjunction at Anaphase in other strains of mice.     

Chiasma distribution in rye chromosomes of diploid rye and in wheat/rye addition lines in relation to C-heterochromatin

Summary In five genetically different inbred lines of rye and in the seven Chinese Spring/Imperial wheatrye addition lines, chiasma distribution in rye chromosomes was studied with respect to the amount and position of constitutive heterochromatin (Giemsa C-bands). In all inbred lines, rye chromosomes with one primary terminal band were more frequently found as univalents than those with primary bands on both telomeres. These chromosomes were most probably 5R and/or 6R. In the addition lines a highly significant reduction in the number of arms bound by chiasmata was found for rye chromosomes 5R and 6R. Because of the similar chiasma distribution in the inbred lines and in the rye chromosomes of the addition lines, no effect of the wheat genome on the number of chiasmata in the rye chromosomes can be ascertained. However, a relationship between chiasma frequency and chromosome arm length seems to exist, since under reduced chiasma conditions the two shortest arms of the rye complement, those of chromosomes 5R and 6R, frequently fail to form a chiasma. No effect of the large blocks of constitutive heterochromatin in the telomeres of the rye chromosomes on the position of chiasmata within a bivalent could be established.This study was financially supported by the Deutsche Forschungsgemeinschaft     

Desynaptische Mutanten mit Unterschieden im Univalentenverhalten

After application of neutrons on dry seeds ofPisum sativum three recessive mutants were isolated showing irregularities in the course of meiosis. A cytogenetical analysis showed that at metaphase I, a varying number of univalents are formed, most likely as a result of reduced chiasma frequencies. At anaphase I, some univalents divide precociously in mutant 2982 but none do so in mutants 2989 and 2552. As a consequence, most cells of 2989 and 2552 built up more than two spindles at anaphase II, whereas the majority of cells in 2982 form two spindles. This situation is reflected in the frequency distribution of gones per PMC at tetrad stage. The discussion deals with the possible causes of univalent formation at meiotic prophase and the variability of univalent behaviour at anaphase I.     

Chiasma frequency variation inCepaea hortensis (Müll.) and a comparison withC. nemoralis (L.)

Chiasma frequency variation has been studied in 12 populations ofCepaea hortensis and the results compared with those previously obtained forC. nemoralis from the same populations. Variation in chiasma frequency is effectively restricted to a single large bivalent. In both species there are significant differences between populations in chiasma frequency. Although mean population mean chiasma frequencies are similar inC. hortensis andC. nemoralis, C. hortensis is less variable thanC. nemoralis. This can be seen, to a greater or lesser extent, at several levels-(i) within cells, (ii) between cells, (iii) between snails, and (iv) between populations-and is most noticeable when interstitial chiasmata are considered separately from terminal chiasmata. Data from 6 populations suggest that chiasma frequency remains stable between successive years. It is suggested that, since sympatric populations were studied, the most likely explanation for differences in chiasma frequency statistics between the two species is thatC. hortensis andC. nemoralis are responding differently to similar selective pressures.     

Experimental hybridization within the genus Triturus (Urodela: Salamandridae)

The spermatogenesis of 9 F₁ hybrids of Triturus cristatus carnifex × T. vulgaris meridionalis was studied in squash preparations of testicular fragments, treated by the C-staining method. The chromosome number of these hybrids was examined in spermatogonial metaphases and found to be diploid. The two parental sets were always recognized, which means that a regular, although heterospecific, amphimixis occurred (2n=n+n). Meiotic prophase I is greatly altered owing to a failure of typical chromosome pairing and chiasma formation. At metaphase I and/or meta-anaphase I, the effects of the hybrid combination of the 2 specific parental sets are clearly visible. Most primary spermatocytes contain only univalents. A few show chromosome associations (bivalents, trivalents and, more rarely, quadrivalent chains) besides univalents. Such associations are of 2 types: (a) intragenomal associations = associations of 2 chromosomes by a terminal (a¹) or subterminal chiasma (a²); (b) intergenomal associations = associations of 2 chromosomes by a terminal (b¹) or subterminal chiasma (b²). Univalents segregate at random while the associations often lag on the equatorial plane or migrate entire to a spindle pole. Primary spermatocytes with chromosome multivalents can encounter greater difficulties in accomplishing the first cytokinesis. Secondary spermatocytes are numerically and qualitatively unbalanced; however, some of them undergo spermiogenesis and can give rise to a small number of sperms, generally abnormal and never united in bundles. — Problems related to the occurrence of anomalous chiasmata and of intra- and inter-genomal homologies are discussed.     

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.     

Synthesis and cytological characterization of trigeneric hybrids involving durum wheat,Thinopyrum bessarabicum,and Lophopyrum elongatum

Summary In an attempt to transfer genes for salt tolerance and other desirable traits from the diploid wheatgrasses, Thinopyrum bessarabicum (2n=2x=14; JJ genome) and Lophopyrum elongatum (2n=2x=14; EE genome), into durum wheat cv Langdon (2n=4x=28; AABB genomes), trigeneric hybrids with the genomic constitution ABJE were synthesized and cytologically characterized. C-banding analysis of somatic chromosomes of the A, B, J, and E genomes in the same cellular environment revealed distinct banding patterns; each of the 28 chromosomes could be identified. They differed in the total amount of constitutive heterochromatin. Total surface area and C-banded area of each chromosome were calculated. The B genome was the largest in size, followed by the J, A, and E genomes, and its chromosomes were also the most heavily banded. Only 25.8% of the total chromosome complement in 10 ABJE hybrids showed association, with mean arm-pairing frequency (c) values from 0.123 to 0.180 and chiasma frequencies from 3.36 to 5.02 per cell. The overall mean pairing was 0.004 ring IV + 0.046 chain IV + 0.236 III + 0.21 ring II + 2.95 rod II + 20.771. This is total pairing between chromosomes of different genomes, possibly between A and B, A and J, A and E, B and J, B and E, and J and E, in the presence of apparently functional pairing regulator Ph1. Because chromosome pairing in the presence of Ph1 seldom occurs between A and B, or between J and E, it was inferred that pairing between the wheat chromosomes and alien chromosomes occurred. The trigeneric hybrids with two genomes of wheat and one each of Thinopyrum and Lophopyrum should be useful in the production of cytogenetic stocks to facilitate the transfer of alien genes into wheat.     

Evidence for separate genetic control of crossing over and chiasma maintenance in maize

The meiotic cytological behavior of chromosomes in maize microsporocytes homozygous for the recessive mutant desynaptic was studied at various stages. It was found that following apparently normal pachytene synapsis there appears to be sporadic precocious desynapsis. By diakinesis bivalents heterozygous for a distal knob have often separated to pairs of univalents, each with a knob-carrying and a knobless chromatid. From the frequency of such events it is inferred that the crossover process is probably not affected by the mutant and that the genetic defect affects instead a distinct function concerned with chiasma maintenance following crossing over. Since precocious separation of dyads to monads at prophase II was also found in the desynaptic material, it is suggested that normal chiasma maintenance until anaphase I and normal dyad integrity maintenance between anaphase I and anaphase II may depend upon the same mechanism; it is also suggested that this may involve a special tendency for cohesiveness of sister chromatids during meiosis, beyond that which is ordinarily found at mitosis.     

Cytogenetics of Crotalaria VI. Chiasma frequency and position,and univalent behaviour in a (partially) asynaptic mutants of C. juncea

In Crotalaria juncea (n=8) a plant exhibiting partial asynapsis was isolated in the M₁ of a combined treatment of 50 kR gamma rays +0.2% EMS. The majority (48.14%) of PMCs at diplotene, diakinesis and metaphase I had 16 univalents. The bivalents in the asynaptic mutant were always rod-shaped with one terminal chiasma. In comparison, controls had on the average 7.08 ring bivalents. The asynapsis is genetically controlled, monofactorially recessive, and it is concluded that chromosome pairing is interrupted at a very early stage. There is a possible correlation between the number of bivalents and the arrangement of the univalents at metaphase I. When there were less than four bivalents, the univalents tended to be polar, and when there were more than four, the univalents were more equatorial in arrangement. The arrangement of univalents was random and apparently not influenced by the bivalents, when their number (4) was exactly half the zygotic number.     

Meiotic nondisjunction in oocytes from aged Djungarian hamsters correlates with an alteration in meiosis rate but not in univalent formation

Summary The effect of maternal ageing on the meiotic rate, on chiasma and univalent frequency as well as on heteroploidy in secondary oocytes from Djungarian hamsters was exammed. The frequency of hyperhaploid oocytes increased from 0.6% in young (8–14 weeks) to 2.8% in middle-aged (26–46 weeks) and reached 3.6% in the oldest females (49–75 weeks). On the basis of malsegregated bivalents per oocyte, nondisjunction occurred most often in the middle-aged group (5.42x10^-2 bivalents per oocyte). Hereby, the large meta- and submetacentric A-D chromosomes were preferentially involved. Furthermore, the pattern of nondisjunction was not different from that expected on the basis of chromosome length or induced by colchicine. The large A-D chromosomes did not show any alteration in chiasma or univalent frequency. Terminalized chiasmata were only detected in the E group and univalents increased slightly, but not significantly in the small chromosomes (G group). At higher ages, both chromosome group were not preferentially involved in nondisjunction. Presegregation slightly increased with age and affected more or less all bivalents, whereas the incidence of diploidy significantly decreased. With respect to the rate of meiosis in oocytes from aged females, the resumption was delayed at metaphase I. Our data suggest that failures in the control of oocyte proliferation are involved in nondisjunction rather than the production-line. Furthermore, a model is proposed to explain nondisjunction of specific bivalents at certain maternal ages.     

Genetic Collection of Meiotic Mutants of Rye <Emphasis Type="Italic">Secale cereale</Emphasis> L.

Genetic collection of meiotic mutants of winter rye Secale cereale L. (2n = 14) was created. Mutations were detected in inbred F₂ generations after self-fertilization of the F₁ hybrids, obtained by individual crossing of rye plants (cultivar Vyatka) or weedy rye with plants from autofertile lines. The mutations cause partial or complete plant sterility and are maintained in collection in a heterozygous state. Genetic analysis accompanied by cytogenetic study of meiosis has revealed six mutation types. (1) Nonallelic asynaptic mutations sy1 and sy9 caused the formation of only axial chromosome elements in prophase and anaphase. The synaptonemal complexes (SCs) were absent, the formation of the chromosome “bouquet” was impaired, and all chromosomes were univalent in meiotic metaphase I in 96.8% (sy1) and 67% (sy2) of cells. (2) Weak asynaptic mutation sy3, which hindered complete termination of synapsis in prophase I. Subterminal asynaptic segments were always observed in the SC, and at least one pair of univalents was present in metaphase I, but the number of cells with 14 univalents did not exceed 2%. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19, which caused partially nonhomologous synapsis: change in pairing partners and fold-back chromosome synapsis in prophase I. In metaphase I, the number of univalents varied and multivalents were observed. (4) Mutation mei6, which causes the formation of ultrastructural protrusions on the lateral SC elements, gaps and branching of these elements. (5) Allelic mutations mei8 and mei8-10, which caused irregular chromatin condensation along chromosomes in prophase I, sticking and fragmentation of chromosomes in metaphase I. (6) Allelic mutations mei5 and mei10, which caused chromosome hypercondensation, defects of the division spindle formation, and random arrest of cells at different meiotic stages. However, these mutations did not affect the formation of microspore envelopes even around the cells, whose development was blocked at prophase I. Analysis of cytological pictures of meiosis in double rye mutants reveled epistatic interaction in the mutation series sy9 > sy1 > sy3 > sy19, which reflects the order of switching these genes in the course of meiosis. The expression of genes sy2 and sy19 was shown to be controlled by modifier genes. Most meiotic mutations found in rye have analogs in other plant species.     

Participation of exogenous DNA in the repair processes of meristematicVicia faba cells injured by monofunctional alkylating agens ethylmethane sulphonate

The effect of exogenous DNA of syngeneic origin on the course of reparation of meristematic cells ofVicia faba primary roots followed after treatment with monofunctional alkylating agent, ethyl methanesulphonate (EMS), was tested. Time course of alternations in mitotic activity of investigated cellular population and the dynamics of formation of postmetaphase chromosomal aberrations was evaluated. A reparation of damaged cells was significantly supported by syngeneic DNA; its application induced an increased incidence of cellular division already in the early intervals of the repair which was accompanied by concomitant decrease of microscopically detectable rupture in the chromosomes. The study performed on the localization of induced damages occurring in metaphase chromosomes pointed out an increased sensitivity of small chromosomes ofVicia faba to EMS. Similarly, a reparative action of syngeneic DNA was exhibited by significant decrease of aberrations frequency, predominantly in the same chromosomal group. Per cent representation of individual types, not affected by the action of syngeneic DNA, was established by detailed classification of induced aberrations. In both cases, isochromatide breaks were found of greatest predominance.     

Unusual chromosome pairing and B chromosomes inBriza spicata (Poaceae)

One plant from a population ofBriza spicata (Poaceae) was found to have highly irregular meiotic behaviour. It is characterized by having a reduced chiasma frequency, a large between cell variance in chiasma frequency and the formation of multivalents involving pairs of A chromosomes. The B chromosome present in this plant also forms multivalents with a pair of A chromosomes. It is suggested that the normal control of strict bivalent pairing has broken down and homoeologous chromosomes are associating as multivalents. Furthermore, the partial homology of the B chromosome with a pair of A chromosomes is revealed.     

The use of translocation-derived "marker-bivalents" for studying the origin of meiotic instability in female mice

Female mice of two age groups, 3--4 and 11--14 months old, homozygous for the T(1;13)70H reciprocal mouse translocation were used for cytological observations of bivalents (in primary oocytes) and metaphase II chromosomes (in secondary oocytes). Special attention was given to the behavior of the long (131) and short (113) marker chromosomes. In primary oocytes, univalents were considered "true" or "opposite". The aged females showed an eight-folded increase in "true" univalent frequency for chromosomes 113 over the young ones. A nine-fold rise for nondisjunction with regard to this chromosome was observed. For the other chromosomes, these factors were 2 and 1.7, respectively. The absolute levels of nondisjunction remained low at old age (1.42% for chromosome 113, 1.22% for all other chromosomes). The long marker bivalent 131 was used for chiasma counts. No change in chiasma number with age was observed. It is argued that poorer physiological conditions within the maturing oocytes of older females are the major cause for both the increasing frequencies of "true" and "opposite" univalents and the increased incidence for nondisjunction.     

Studies on triploid Allium triquetrum

This work describes the relationship between the univalents seen at metaphase I and the distribution of dyads at anaphase I in the pollen mother cells of triploid Allium triquetrum. The orientation of the centromeres within the trivalents and bivalents at metaphase I towards the two poles of the pollen mother cells is random. The distribution of polar univalents towards the two poles at metaphase I is also random, as is the distribution of dyads at anaphase I in low univalent frequency collections. However, in a high univalent frequency collection, the distribution of dyads at anaphase I is non-random. There is an excess of cells with the most equal dyad distribution (13–14) and a paucity of cells with a 12–15 distribution. In low univalent frequency collections, the equatorial univalents are believed to remain in the equatorial region during anaphase I and are seen as laggards at late anaphase I. The remaining chromosomes move according to the metaphase I orientation of their centromeres to give a random distribution of dyads at anaphase I. In high univalent frequency collections it is argued that the non-random dyad distribution seen at anaphase I is the result of non-random movement of some of the equatorial univalents away from the equatorial region during anaphase I. The remaining equatorial univalents remain in the equatorial region and are seen as laggards at late anaphase I.