Mean Chiasma Frequency Research Articles

B-chromosome Effects at Meiosis in <i>Crepis capillaris</i>

Analysis of chromosome behaviour at meiosis in Crepis capillaris, 2n=2x=6+B-chromosomes, shows that the B-chromosomes influence the frequency and distri-bution of chiasmata in the A-chromosomes. The mean chiasma frequency of the A's is increased by the presence of one to four B's, so too is the variance of the chiasma frequency mean between the pollen mother cells within plants. The A-chromosome bivalents differ in length, but all have similar mean chiasma frequencies, and they are all influenced to the same extent. Some data are also presented on the meiotic behaviour of the B-chromosomes themselves. Many of them get eliminated as micronuclei.

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.

Influence of B-chromosomes on meiosis in pearl millet

B-chromosome behaviour and the effects of B's on the endophenotype were studied in the third back-cross progeny of a cross between non-B and B-carrying parents ofPennisetum typhoides Stapf et Hubb. In the experimental material a regular increase in B-chromosome chiasma frequency and an increase in the variation of mean chiasma frequency with increased number of B's was observed. When four or more B-chromosomes were present quadrivalents occurred more frequently. In these respects the experimental material differed from the B-carrying parent. B's had no effect on the mean chiasma frequency of A-chromosomes when present in numbers of up to and including four but when present in numbers of more than four had a depressing effect. The variation of the mean chiasma frequency of the A-chromosomes increased with increasing number of B's. No correlation was observed between A-chromosome chiasma frequencies and B-chromosome chiasma frequencies. In the experimental material B's when present in larger numbers i.e. more than four had deleterious effects on A-chromosome behaviour and on fertility. In the effects of B's on the endophenotypic characters the experimental material differed from the B-carrying parent. It is suggested that the behaviour and effects of B's on the endophenotypic characters are the result of interaction between the A-chromosome genotype and the B-chromosomes.

Variation of Chiasma Formation in Populations of <i>Aegilops spoltoides</i>

Chiasma frequency at PMCs was studied in two successive generations of Aegilops speltoides, the one consisting of six populations, the other of four families. Each family was the progeny of a single plant taken from the corresponding population of the first generation.The analysis of data showed significant differences to exist between populations and between families, as well as between plants within populations and families. Among the overall material three subgroups were recognised.The first one, including two populations and the corresponding families, displayed the lowest inter- and intraplant variation of chiasma frequency in both generations, but a more asymmetrical pattern of chiasma distribution was found in families. The next group, represented by one population-family system, dimorphic by constitution, showed similar variability in population and family, with regards to chiasma frequency. The distribution of chiasmata has proven partly similar, including the same proportion of the predominant bivalent type.The third category was the most variable, containing a B-carrying class, in which mean chiasma frequency differed significantly between the two generations, being lower in B-family. The suggestion is that B chromosomes, when accumulated, exert a reducing effect on chiasma formation.

Chiasma frequency in homozygotes and heterozygotes of pearl millet

Four inbreds of pearl millet and their F1 hybrids, obtained by crossing the inbreds in all possible combinations, were grown in two seasons—the stress season and the non-stress season—to study whether there are any differences in the genotype environment interactions between the inbreds and their F1 hybrids. The mean chiasma frequency of the inbreds was lower and the variation of the mean chiasma frequency was more in the stress conditions than in the non-stress conditions. In both seasons the heterozygotes showed limited variation among their means, and larger mean chiasma frequencies when compared to the inbreds. The differences between the inbreds in their interaction with the seasons were significant. Different hybrids interacted with the seasons in a significantly different manner from each other. However, the differences between the inbreds as a group and hybrids as a group interacted with the environment in a large measure. When examined with the means, the significant differences between hybrids and inbreds in their interaction with the seasons showed that the hybrids were relatively stable over seasons as compared to the inbreds. The intercell variation within each plant was also examined as a variable to detect differences if any between genotypes, particularly to find out if the hybrids have a different type of variation as compared to the inbreds. The analysis revealed that the inbreds as a group are having larger means for this variable over hybrids. The hybrids have much less intercell variation within individual plants and nearly twice the buffering capacity of the inbreds. The hybrids and inbreds interacted similarly to the environment for the intercell variation. Diallel analysis carried out to study the genetic nature of heterosis for chiasma frequency revealed average overdominance for this character and the overdominance was shown to be at least partly due to non-allelic gene action.

THE EFFECTS OF SUPERNUMERARY CHROMATIN IN THREE SPECIES OF GRASSHOPPER

The chiasma frequencies in males of three species of grasshopper with and without B-chromosomes and supernumerary segments were scored at diplotene. The partially heterochromatic B's found in some individuals of Chimarocephala pacifica pacifica have no effect on chiasma frequencies. On the other hand, heterochromatic supernumerary segments in Oedaleonotus phryneicus males significantly increase the mean chiasma frequencies over individuals without them but not the between-cell variances. In Camnula pellucida neither the mitotically unstable B's nor supernumerary segments have any apparent effect on chiasma.frequencies. The supernumerary elements in Chimarocephala pacifica and Camnula pellucida may, however, produce other effects during the life cycle These and previous observations suggest that polymorphism involving extra genetic material in grasshoppers can result in increased population variability via two different means: (1) by producing variation in chiasma frequency or (2) through direct effects in the individuals possessing it. The importance of such a mechanism for producing variation is discussed.

Meiosis in the grasshopper. 3. Chiasma frequencies in females after elevated temperature.

The effect of high temperature on chiasma formation during oogenesis has been studied in the grasshopper Melanoplus femur-rubrum. Prolonged heat treatment (40° C.) during mid-prophase of meiosis causes a reduction in the mean chiasma frequency per cell. Only those bivalents in which more than one chiasma occurs are affected by the heat. The pattern of chiasma frequency response to heat is similar to that which occurs in males of the same species.

Male and female meiosis in grasshoppers. I. Stethophyma grossum.

Male meiosis in the grasshopper Stethophyma grossum is well known as an example of proximal chiasma localisation. An investigation of female meiosis in oocytes of this species shows that both the frequency and distribution of chiasmata are quite different from the male situation. Mean chiasma frequency per cell (14.98) in considerably higher in females than in males (11.28) which agrees with the trend established in other comparative studies of male and female meiosis. More strikingly, males and females also show not only different but quite opposite patterns of chiasma distribution. In spermatocytes of males, chiasmata are strictly localised proximally in most bivalents, but in oocytes of females very few chiasmata form in proximal regions and nearly all chiasmata form either in distal or interstitial regions. The genetical significance of these findings is considered.

Genetic and environmental components of chiasma control. 3. Genetic analysis of chiasma frequency variation in two selected lines of Schistocerca gregaria Forsk.

Two selected lines of Schistocerca gregaria with mean chiasma frequencies of 17.68 and 21.66 were crossed in all possible combinations up to the F2 generation. A genetic analysis of both generation means and within family variances reveals that the control of chiasma frequency in these lines is governed by a large additive genetic component (d) and a small dominance component (h) directed towards high frequency types. 40% of the total variance can be explained in terms of environmentally induced variation. It is argued that such significant and readily induced changes in the chiasma pattern, associated with a mean cellular difference of 4 chiasmata, can be expected to lead to an alteration in the linkage relationships within the genomes of each line. Unfortunately the intensity of the effect upon linked non-allelic associations can only be assumed since, at the present time, there is no evidence to correlate chiasma frequency variation in the parent with ensuing exophenotypic variation among the progeny.

Meiosis of Coronilla varia L.1

Meiosis in microsporocytes was studied in nine ‘Chemung’ plants, 14 ‘Penngift’ plants, and one S1 plant of crownvetch (Coronilla varia L.). All plants were tetraploid (2n=24), and 41% of the diakinesis cells had one or more multivalents (III's or IV's). Averages of pairing configurations were 0.8 I's, 10.7 II's, 0.1 III's, and 0.4 IV's per cell. Quadrivalents ranged from 0 to 5 per cell, compared to a theoretical maximum of 6. We concluded that C. varia had autotetraploid‐like meiotic chromosome behavior.No differences were found between cultivars in mean frequencies of configurations per cell. However, plants within cultivars differed significantly in their frequencies of I's per cell (0.2 to 1.8), and in their proportions of cells free of I's or III's (33 to 94%). The mean chiasma frequency among all plants was 1.1 chiasmata per biva‐lent‐equivalent (range 1.0 to 1.3).Laggards were present at meiotic anaphases. Chromosome numbers at TI were largely euploid (n = 12). No micronuclei occurred in pollen quartets, thus laggard chromosomes may have been included in TII nuclei. A first generation inbred plant was much more irregular in meiosis than other plants studied.Cytomixis was a sporadic occurrence in microsporocytes of a few collections.

GENETIC VARIABILITY INAEGILOPS SPELTOIDESAFFECTING HOMOEOLOGOUS PAIRING IN WHEAT

Three Chinese Spring ditelocentric lines and four Agropyron telocentric addition lines were crossed with a single population of Aegilops speltoides. Chromosome pairing was studied in the hybrid plants. The mean chiasma frequencies per cell in different hybrid plants indicate the existence of three different genotypes in the population of Ae. speltoides each differing significantly in ability to suppress the diploidizing mechanism in polyploid wheat. Plants at the three levels of pairing showed means of 6.4, 10.4 and 15.0 chiasmata per cell, respectively.Agropyron telocentric chromosomes did not pair with their wheat homoeolgues at the lowest pairing level. At intermediate and high pairing levels they did pair with wheat chromosomes. The increase in pairing of Agropyron telocentrics paralleled the increase in chiasma frequency per cell.The action of genes controlling homoeologous synapsis is discussed and it is suggested that pairing affinity between chromosomes may result from the interaction between two factors: homology in nucleotide sequences at specific sites in chromosomes, and degree of activation of these sequences.

Cytology of inbreds and f 1 hybrids of pearl millet

The meiotic behaviour of chromosomes was studied in four inbred lines and their F 1 hybrids of P. typhoides. The inbred lines showed a decrease in mean chiasma frequency when compared with the population plants, but differed from one another in their mean chiasma frequencies. Together with the decrease in mean chiasma frequency the inbreds showed variation in mean chiasma frequencies. The inbred lines showed a number of meiotic abnormalities such as extra chromosomes, extra fragments, desynapsis, persistent nucleoli and differential condensation of chromosomes. The F 1 hybrids of these inbreds exhibited heterosis for chiasma frequency. All the F 1's had mean chiasma frequencies higher than the means of the respective participating parents. The F 1's, however, differed in the degree of heterosis exhibited. In the F 1 hybrids, the variation in mean chiasma frequency, both between plants and between PMC's within plants, was significantly lower than that of the inbred lines. The effect of environment was studied in the inbred lines and their F 1 hybrids. The mean chiasma frequencies of the inbred lines were significantly lower, and the variation in mean chiasma frequencies was greater, in the stress season. The mean chiasma frequencies of F 1's did not show any significant differences between the two seasons. The F 1's exhibited less variation in mean chiasma frequency than the inbred lines, showing that F 1's were developmentally more stable. The F 1's did not show any meiotic abnormalities in either season.

Fractional Diallel Analysis of Regulation of Recombination in <i>Triticum aestivum</i> L.

Combining ability analyses were made for average chiasma frequency, pollen mother cell and bivalent variances for chiasma frequency from a fractional diallel set involving seventeen varieties of Triticum aestivum L.Varieties differed significantly in their means for average chiasma frequency and its associated variances. Mean chiasma frequency in F1's exhibited a decline over that of parents. Heterosis was positive for pollen mother cell variance and negative for bivalent variance. The fractional diallel analysis of combining ability indicated predominantly nonadditive gene action for chiasma frequency and its variances attributable to pollen mother cell and bivalent. Pronounced complementary epistasis was indicated for chiasma frequency while both duplicate and complementary epistasis appeared to be equally important for the variances. From the occurrence of high chiasma frequency in some varieties and low chiasma fre-quency in others it was concluded that chromosomal mechanism counteracts the restrictive effects of the breeding system in the former and reinforces in the latter. Genotypic control of chiasma frequency at plant, pollen mother cell and bivalent level with a compensatory mechanism between the different levels was also indicated. The role of cytological restriction of recombination in the conservation of variability, exploitation of additive and nonadditive components of genetic variance and its implications in adaptation are discussed.

The supernumerary segment system of Stethophyma. II. Heterochromatin polymorphism and chiasma variation.

Three species of the genus Stethophyma have been cytologically examined and all three show variation both for supernumerary heterochromatic segments and for the distribution of standard heterochromatin among the autosomes. The European species, S. grossum, for example, shows considerable interpopulation variation for standard heterochromatin while two of the populations, from Spain and Austria, show supernumerary segment polymorphism. The segments are located interstitially on the S11 chromosome but occupy different positions in the different populations. — In all species, the presence of the extra heterochromatic segments increases the mean chiasma frequency. Moreover, the influence of the segments upon mean chiasma frequency is different in different populations and in different species. In the Spanish population, the increase is both intra- and interchromosomal whereas in Austria the influence of the segment is completely interchromosomal. — In the American species, S. gracile and S. lineatum, where supernumerary heterochromatic segments are carried on both S10 and S11 chromosomes, the effect on chiasma frequency shows a dosage relationship, an increase in the number of segments per individual being correlated with an increase in mean chiasma frequency. It is suggested that the interstitial segments found in all species have originated by direct duplication of chromosome material. By contrast the terminal segments in S. lineatum and S. gracile may be derived by translocation from a B-chromosome since such a chromosome has been found in one individual of the former species. — The variation in segment structure and the distribution of standard heterochromatin, among the European species of S. grossum suggests that these systems have evolved independently in different populations.

Cytological Regulation of Recombination in <i>Pennisetum</i> and <i>Brassica</i>

The restriction of recombination through chiasma frequency was investigated in Pennisetum and Brassica where outbreeding is controlled by two different genetic mechanisms. Lines with known inbreeding coefficients were utilised for this purpose. Self-compatible and self-incompatible Brassica material subjected to five generations of disruptive selection for flowering was also included.A consistent increase was observed in the mean chiasma frequency per cell of Pennisetum and Brassica inbreds as compared to the outbred populations. The increase was parallel with the degree of inbreeding in Pennisetum. Disruptive selection for flowering increased chiasma frequency in Brassica populations with the exception of late self-incompatible group.The buffering mechanism of chiasma frequency under inbreeding, referred to as cytological homeostasis, was examined from the viewpoint of the genetic diversity of the populations, the past history of selection, the ecological conditions under which the material is grown, the nature of the breeding system and the fitness-flexibility axis. It is concluded that a compromise between fitness and flexibility is arrived essentially by regulation of recombination through cytological homeostasis and the breeding system.

Induced variation in chiasma frequency in Rye in response to phosphate treatments

SUMMARYPlants of two rye genotypes, one highly inbred, the other not, were grown with varying amounts of available mineral phosphate. In two experiments, one using culture solutions, the other a field experiment, the results show an effect of phosphate treatments on mean chiasma frequency at first metaphase in meiocytes. Plants given increased amounts of phosphate showed an increase in chiasma frequency. A similar effect of phosphate on chromosome size and mass at mitotic metaphase in meristems is known and there may be a direct link between chromosome size and chiasma frequency.

Meiosis in the grasshopper: chiasma frequency after elevated temperature and x-rays.

Grasshoppers were given an acute dose of X-rays, injected with H3-thymidine and thereafter maintained at 42 °C. Autoradiographic analysis of testicular tubules taken at daily intervals after injection gave meiotic stage durations. The mean chiasma frequency was reduced 4 days after the hoppers were placed at 42°, in both X-rayed and non-irradiated animals, and remained low in the non-irradiated animals throughout the experiment. In the X-rayed animals the chiasma frequency rose to control levels on die 6th day and dropped again thereafter. Analysis of the data indicates a period ending approximately at zygotene during which chiasma frequency can be reduced by high temperature. The chiasma frequency can be increased by irradiating leptotene-zygotene. The end of the heat and X-ray sensitive stages are temporally distinct. Univalents were never produced by the heat treatment. Changes in chiasma frequency are restricted to the four pairs of larger chromosomes where multiple chiasmata are commonly found. Hence, the presence or absence of chiasmata shows a complex interaction with the physical environment.

Persistent heterochromatic association in Metrioptera brachyptera. I. Variation in the frequency of multiple formation, chiasma production and chromosome morphology.

The non-homologous and achiasmate association of L3 and L4 chromosomes leading to the formation of persistent pseudo-multiples has been detected in each of the natural populations sampled. Populations exhibit different mean frequencies and within a given sample a considerable range may be found. The mean chiasma frequencies of populations vary significantly but there is no correlation between these and the level of multiple formation. It is proposed that the mechanism controlling chiasma production and that determining the frequency of multiples act independently of one another. Individuals have been discovered where either one or both members of the L3 or L4 pair have been modified structurally. An analysis of these entire germ line conditions indicates that not only is heterochromatin primarily concerned in the formation and maintenance of multiples, but also, that their inception is correlated with the distal position of these segments on L3 and L4 chromosomes.

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.

A Karyotype Study ofVaranus MonitorLinn

SUMMARY1. Karyotype in Varanus comprises of 40 chromosomes; 8 Vs, 8 rods and 24 dots. The fundamental number of arms according to present investigations is 48.2. One pair of rods, in the late mitotic prophase and at metaphase II has a knob like structure at one of the ends. Second pair of V in order of size has a constriction and appears to be nucleolar chromosome.3. Mean chiasma frequencies, cell variance and bivalent variance for the macrochromosomes have been calculated.4. One pair of microchromosomes is very small and is hardly visible under the oil immersion. This pair stains poorly even at metaphase I and is labelled as « m » chromosome.5. Bone marrow and the spleen of the female show variation in chromosome number ranging from 32 to 40, but most of the plates have the number as 32±1,35±1,38±1 and 40 ±1.6. Morphologically differentiated sex pair seems to be lacking.