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.     

Chromosome-specific control of chiasma formation

A desynaptic mutant of Hypochoeris radicata, 2n = 8, has been found in a population from France. The mutant is remarkable in that at metaphase-I over 90% of PMC's have a pair of univalents while two pairs are found in only 1% of cells. Only chromosome IV is affected by the desynapsis which is controlled by a single recessive gene. Bivalent chiasma frequency in cells with a pair of univalents is higher than in cells with complete bivalent formation indicating partial dependence of cell chiasma number on the availability of some factor within the anther. The F₂, produced by full-sib mating, has a raised frequency of univalents in the chromosomes other than pair IV. The higher level results from enforced sib-mating of an obligate outbreeder and is not related to the action of the major gene. It is suggested that chiasma formation is controlled both polygenically and by major genes which operate on a hierarchical system. Some genes affect the behaviour of the entire complement while the action of others is specific to individual chromosomes.     

Fertility and meiotic chromosome behaviour in autotetraploid pearl millet

Summary Tetraploidy was induced in outbred pearl millet and selection for high and low seed set was started in the C₁ generation. Segregation in the C₃ generation was observed for fertility and also for meiotic features: per cent seed set in selfed earhead, chiasma frequency, chromosome association and chromosome distribution in pollen mother cells were all affected. However, variation in seed set was observed even between samples not differing in meiotic features. It is apparent that factors regulating seed set in autotetraploid pearl millet were genic as well as chromosomal.A high frequency of univalents and trivalents was the main cause of sterility; quadrivalent misdisjunction was not a significant factor. As univalency decreased with increased chiasma formation, the gain was in the form of quadrivalents. However, individuals not differing in chiasma frequency did differ in chromosome association frequencies, indicating that the dependence of chromosome pairing behaviour on chiasmata was subject to genotypic influence.     

Chromosome-specific desynapsis in the n = 2 race of Haplopappus gracilis (Compositae)

During cytological screening for pollen sterility in a wild population of Haplopappus gracilis (n = 2), several partially sterile plants were found that had good pachytene pairing but varying numbers of univalents. Some plants had chromosome A bivalents or A univalents, while in the same cells chromosome B had only bivalents. In other plants the reverse condition occurred; the B chromosome had B bivalents or B univalents and only A bivalents. This demonstrates a chromosome-specific effect for the desynapsis genes. Hybridization between the two homozygous mutant genotypes produced only normal bivalents; this indicates the two mutants are not alleles and each is recessive. An F2 generation showed independent assortment of the desynaptic mutations. The chromosome A bivalent is the larger of the two and normally has one or two chiasmata; the B bivalent normally has a single chiasma. Chiasmata distribution was tested in the desynaptic mutant A bivalents and showed an acceptable fit to a binomial distribution. This occurs also in heterozygous, asynaptic pairing control gene mutations. Analysis of the NOR bivalent in two hologenomic desynaptic mutations in tomato also showed a good fit to a binomial distribution of chiasmata. This indicates the same methods are applicable to diverse species.     

Unstable B chromosomes in Silene maritima With. (Caryophyllaceae)

Unstable B chromosomes in Silene maritima With. (Carophyllaceae). B chromosomes have been found in 7 out of 39 populations of Silene maritima With. (Carophyllaceae) studied. These 7 populations are all from the Norfolk coast. The B chromosome is small, 0.8 μm long, telocentric and appears euchromatic. Within plants the B chromosome is highly unstable with different numbers in cells of the same root or pollen mother cells (pmcs) of an anther. The transmission of the Bs is efficient and in controlled crosses progeny plants usually have higher mean B frequencies than the B-containing parents. At metaphase I the pairing behaviour of the Bs is variable and they often appear as univalents. In general the presence of Bs in pmcs results in an increase in chiasma frequency/cell and populations with Bs have higher mean chiasma frequencies than those without Bs. Cloned plants grown under stress conditions induced by the addition of NaCl to a nutrient solution showed no difference in mean B number/cell or distribution when compared with controls grown in nutrient solution only.     

Genetic interactions between wheat and rye genomes in triticale

Summary Six primary triticale lines were produced from two advanced breeding lines of Triticum durum and three inbred genotypes of Secale cereale. The wheat and rye parents and the triticale derivatives were crossed in all possible combinations within each species group. Chiasma and univalent frequency of parents and hybrids were determined. The primary triticale lines had more univalents and less chiasmata per pollen mother cell than the corresponding wheat and rye parents together. The parental wheat F₁ exhibited negative heterosis for chiasma frequency whereas all rye hybrids had much higher chiasma frequencies than their inbred parents. Triticale F₁s generally showed lower chiasma frequencies and more univalents than their parents, but the degree of pairing failure was dependent upon which of the parental species within the triticale, wheat or rye, was in the heterozygous state. F₁s with heterozygous wheat genome only showed the least reduction in chiasma number (presumably caused by gene actions within the wheat genome), while F₁s with heterozygous rye genome showed high reduction in chiasma frequency and an increase in pairing failure (induced by negative interactions between the heterozygous rye and the wheat genome in triticale). A high correlation was found between the frequency of undisturbed pollen mother cells and the frequency of aneuploids in the subsequent generation. A higher number of aneuploids occurred in those populations which were heterozygous for the rye genome.     

木立芦荟小孢子母细胞减数分裂与花粉育性关系的初步研究

文章从减数分裂过程、小孢子发育两方面,探讨了木立芦荟（Aloe arboresens Mill.）花粉败育的原因。木立芦荟花粉母细胞染色体数目为2n=14,由四对长染色体和三对短染色体组成,属二型性核型。其减数分裂异常,发现存在单价体和多价体、染色体桥、落后染色体、不均等分离、微核等。同时观察到木立芦荟染色体具有极度的粘质性,使减数分裂各阶段的染色体不易散开。统计各种异常现象出现的频率并分析了这些异常现象形成的可能机制及对正常小孢子形成的影响,推测染色体间的丝状粘连可能是木立芦荟小孢子母细胞减数分裂异常并导致败育花粉产生的主要因素。成熟花粉粒中90%以上为败育花粉,属碘败型。     

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.     

Colchicine-induced heritable modification of chiasma frequency and distribution in perennial ryegrass (Lolium perenne).

One-week-old seedlings of inbred lines of perennial ryegrass (Lolium perenne L.) were given a 3-h period of treatment with 0.2% colchicine, and this treatment was found to have heritable effects on chromosome behaviour at meiosis. The mean pollen mother cell chiasma frequency was reduced, and there was a corresponding increase in the number of univalents per cell and also changes in cell and bivalent variances. Patterns of chiasma distribution within bivalents were also modified. The effects were seen in the generation in which the treatment was given and they were also observed in the following sexual generation.     

Chiasmata and variability in Lolium and Festuca populations

There are significant differences in mean pollen mother cell chiasma frequencies between populations within Lolium perenne, L. multiflorum and Festuca pratensis. The differences are genotypically controlled. With low chiasma frequencies the chiasmata are distally located. With increasing chiasma frequency the frequency of chiasmata in interstitial segments increases. Shorter lived populations have higher chiasma frequencies than the more perennial. — The higher the chiasma frequency of a population the lower the phenotypic and genetic variance for characters under polygenic control, such as flowering time, and the less effective also is the response to selection for such characters. These observations are interpreted on the premise that high chiasma frequencies are instrumental in the breaking up of supergene sequences in interstitial chromosome segments.     

Son-sire-regression based heritability estimates of chiasma frequency, using T70H mouse translocation heterozygotes, and the relation between univalence, chiasma frequency and sperm production.

T(1;13)70H/+ translocation heretozygous mice were used for assessing heritability values for chiasma frequencies and the epididymal sperm count. The chiasma frequency estimates were based on 15 son-sire pairs, the translocation heterozygotes being maintained in a Swiss random-bred genetic background. The chiasma frequencies were scored separately for the T70H/+ derived multivalent, specific pairing segments within the multivalent and the remaining bivalents. Chiasma counts within these specified parts of the genome were positively correlated. The heritability estimates, significantly greater than zero, ranged from 0.78-0.98, depending on the chromosome segments included. These results indicate a strong genetic control on a cellular basis for the formation of chiasmata in the mouse. Despite significantly positive correlations and regressions between the various chiasma frequencies and the sperm count (for which 29 pairs of observations were available), no significant heritability estimate for the sperm count was obtained. The relation between the chiasma frequency and the sperm count was weakest when the chiasma count was confined to a region of the translocation-caused multivalent in which the absence of a chiasma almost always resulted in the production of an univalent. This indicates that in the translocation heterozygotes used, the overall chiasma frequency has a greater predictive value for the sperm count than autosomal univalence alone.     

Studies on triploid Allium triquetrum

Changes in the frequency of univalents with increasing developmental age in pollen mother cells of triploid Allium triquetrum were studied. Univalent frequencies were compared in material grown in different places, in first and second inflorescences from plants grown in the same place, and in first inflorescences collected in different seasons. No variation in univalent frequencies (total, equatorial, and polar) was found with increasing developmental age. There were also no changes in the different dyad frequencies at anaphase I with increasing developmental age of the pollen mother cells.     

Disomic and ditelosomic alien chromosome additions in beet (Beta vulgaris), carrying an extra chromosome of B. procumbens or telosome of B. patellaris.

Alien chromosome transmission through the female germ line as well as meiosis in pollen mother cells were studied in disomic and ditelosomic alien chromosome additions of beet. Beta vulgaris, carrying an extra pair of chromosomes or telosomes of B. procumbens or B. patellaris, respectively. The alien chromosomes carried genes for resistance to the beet cyst nematode, Heterodera schachtii, and screening for this resistance was used to select plants with or without the alien chromosomes. A great variation for alien chromosome transmission was recorded and plants carrying two extra alien chromosomes were recovered in the backcross progenies of the disomic or ditelosomic additions. However, in these progenies the average frequencies of plants without alien chromosomes (86%) did not clearly differ from that in similar progenies of the original monosomic or monotelosomic chromosome additions, indicating that doubling the number of the alien chromosome did not enlarge their transmission to the next generation. The alien chromosomes fully paired at pachytene and desynapsed again before diakinesis, indicating decreased chiasma formation. At second metaphase nearly 60% of the cells had one extra chromosome, and the remaining cells carried two or no extra chromosomes in about equal proportions. The tetrads looked fully normal. The expected relation between the average number of alien chromosomes in the germ cells and in the plants of the progenies did not show up, indicating a strong selection favouring the female gametes without alien chromosomes.     

Variation in chiasma frequency among eight accessions of Arabidopsis thaliana

Natural variation in meiotic recombination frequency in Arabidopsis thaliana has been assessed by analyzing chiasma frequency variation among a range of geographically and ecologically diverse accessions. Fifty pollen mother cells at metaphase I of meiosis were analyzed from each of eight accessions and fluorescence in situ hybridization was applied to enable identification of all 10 chromosome arms. There was no significant variation in mean chiasma frequency between plants within accessions, but there was significant variation between accessions. Further analysis confirmed this finding and identified two particular accessions, Cvi and Ler, as having chiasma frequencies significantly lower than those of the other accessions. The analysis also revealed that the pattern of chiasma distribution between arms and among chromosomes is not consistent over accessions. Further detailed analyses were conducted on each individual chromosome (1-5) in turn, revealing that chromosome 4, one of the acrocentric chromosomes, is the least variable while the other acrocentric chromosome (2) is the most variable. These findings indicate the existence of recombination regulatory elements in Arabidopsis and we conclude that it may be possible in the future to identify these elements and determine their mode of action. The practical implications of such developments are considerable.     

矮牡丹小孢子母细胞减数分裂异常现象的观察

矮牡丹(Paeonia suffruticosa subsp. spontanea)(永济居群)存在多种结构杂合现象,减数分裂存在一些异常:如单价体、异形二价体、互锁四价体、六价体、后期I倒位桥、落后单价体、不均等分离、后期Ⅱ桥和微核等。统计了这些异常现象出现的频率,并对其形成的机制和对正常小孢子形成的影响进行了讨论。从细胞学水平上探讨了矮牡丹可能的濒危机制。同时结合前人的研究,对芍药属内3个组的结构杂合程度进行了比较。     

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.     

Meiotic Analysis of the Regenerated Plants Derived from a Young Spike of Octoploid Triticale ×Secale cereale F1 Hybrid

The regenerated plants had high frequencies of changes in chromosome number and the pairing variation. The chromosome number was more or less variably decreased in different pollen mother cells. Most of regenerated plants were mixoploids. Some had higher frequency of homolgous chromosome pairing because double doses of rye genome had inhibited the effect of Ph gene. But, there was obvious different of chromosome pairing among regenerted plants. The difference was related to numerical chromosome change. Meiosis in a regenerant was analysed with Giemsa-C banding technigue. Partial rye chromosomes did not pair homologously or only very loosely paired. There was moderate level of wheat-wheat homologous partial pairing, however, wheat-rye chromosome pairing also occurred.     

The influence of B-chromosomes on chiasma frequencies in Black Mexican Sweet Corn

The mean chiasma frequency at first metaphase of meiosis in pollen mother cells of a maize variety, Black Mexican Sweet Corn, increases with increasing number ofB-chromosomes. The increased chiasma frequency is also reflected by a decrease in the number of rod bivalents at first metaphase. There is some indication that theB-chromosomes also influence the distribution of chiasmata between pollen mother cells. The increase in chiasma frequency due toBs is closely analogous to the increase in crossing-over carlier attributed toBs between marker genes located on chromosome III. Together the evidence confirm the role ofBs in controlling genetic recombination at meiosis.     

Cytological studies on meiosis and male gametophyte development in autotetraploid cucumber

With improved staining and chromosome preparation techniques, meiosis of pollen mother cells (PMCs) and male gametophyte development in autotetraploid cucumber (Cucumis sativus L.) was studied to understand the correlation between chromosomes behaviour and fertility. Various chromosome configurations, e.g. multivalent, quadrivalents, trivalents, bivalents and univalents were observed in most PMCs at metaphase I. Lagging chromosomes were frequently observed at anaphase in both meiotic divisions. In addition, chromosomes segregations were not synchronous and equal in some PMCs during anaphase II and telophase II. Dyads, triads, tetrads with micronuclei and polyads were observed at tetrad stage, and the frequencies of normal tetrad with four microcytes were only 55.4 %. The frequency of abnormal behaviour in each stage of meiosis was counted, and the average value was 37.2 %. The normal meiotic process could be accomplished to form the microspore tetrads via simultaneous cytokinesis. Most microspores could develop into fertile gametophytes with 2 cells and 3 germ pores through the following stages: single-nucleus early stage, single-nucleus late stage and 2-celled stage. The frequency of abnormalities was low during the process of male gametophyte development. The germination rate of pollen grains was 46.9 %. These results suggested that abnormal meiosis in PMCs was the reason for low pollen fertility in the autotetraploid cucumber.     

Nucleotypic influences on chromosome-specific chiasma variation in Crepis capillaris. I. Responses to early colchicine treatment and chromosome doubling.

Additive and multiplicative effects of colchicine treatment at the seedling stage and of chromosome doubling on chromosome-specific chiasma frequency at metaphase I have been measured in comparisons between C0 and untreated diploids and between C0 autotetraploids and C0 diploids. Early colchicine treatment increases the frequency of chromosome C univalents to 1.8% but has no similar effect on chromosomes A and D. Colchicine treatment has little net effect on mean chiasma frequency, deducting an average of 0.204 chiasmata per set but otherwise multiplying the mean by a factor of 1.182. These additive and multiplicative effects represent averages of six phenotypes. Chromosome doubling in a tetraploid-diploid chimaera subtracts an average of 0.265 chiasmata per set but otherwise doubles the numbers of chiasmata at the diploid level (x 2.134). Comparison of six diploids and tetraploids reveals modest average additive (+ 1.103) and multiplicative effects (x 1.190). The implications of these findings are discussed in the light of new analyses of previously published data. Key words : chiasmata, Crepis, colchicine, meiosis, polyploidy.