Heterozygous Effects on Fitness of Ems-Treated Chromosomes in DROSOPHILA MELANOGASTER

The heterozygous effects on fitness of second chromosomes carrying mutants induced with different doses of EMS were ascertained by monitoring changes in chromosome frequencies over time. These changes were observed in populations in which the treated chromosomes, as well as untreated competitors, remained heterozygous in males generation after generation. This situation was achieved by using a translocation which links the second chromosome to the X chromosome; however, only untranslocated second chromosomes were mutagenized. Chromosomes were classified according to their effects on viability in homozygous condition. A preliminary homozygosis identified completely lethal chromosomes; secondary tests distinguished between drastic (viability index < 0.1) and nondrastic chromosomes. Chromosomes that were nondrastic after treatment were found to reduce the fitness of their heterozygous carriers by 3-5%. The data show that flies homozygous for these chromosomes were about 2.7% less viable per treatment with 1 mm EMS than flies homozygous for untreated chromosomes. By comparing the fitness-depressing effects of nondrastic EMS-induced mutants in heterozygous condition with the corresponding viability-depressing effects measured by Temin, it is apparent that the total fitness effects are several times larger than the viability effects alone. Completely lethal chromosomes derived from the most heavily treated material reduced fitness by 11% in heterozygous condition; approximately half of this reduction was due to the lethal mutations themselves.     

The genetic conditions in heterozygous and homozygous populations of Drosophila. II. X-ray induced lethals in a homozygous and a heterozygous population

Strains of Drosophila melanogaster were made isogenic for their second chromosomes by means of the marker strain LCy/Pm. One of these strains was used as a founder for a homozygous experimental population (W). All other strains were mixed and established a heterozygous population (LKW). Both populations were free of lethals in the beginning with respect to their second chromosomes. After they had been exposed to an X-ray irradiation of 7000 r they contained about 26 per cent newly induced lethal chromosomes. Whereas in the heterozygous population the lethal frequency decreased rather fast to 10 per cent, that of the homozygous population remained rather constant at 25 per cent during a period of 135 days. After a year of continuation, however, both populations reached the same lethal frequency of about 10 per cent. Allelism tests carried out after 10 generations revealed that there was a highly heterotic lethal factor in the homozygous population. After excluding this heterotic lethal from the calculations, the lethal frequencies of the two populations remained significantly different. It was assumed that the relative mean fitness of lethal heterozygotes was generally higher in the homo-than in the heterozygous populations. The results indicate that homozygous populations are much more capable of incorporation new mutations than heterozygous.     

Correlation between recombination frequency and fitness in Drosophila melanogaster

The origin and maintenance of genetic recombination are unsettled evolutionary issues. Genetic variation affecting recombination frequency appears to be pervasive in nature, suggesting that natural selection must increase recombination frequency under some circumstances. However, theoretical arguments and experimental evidence indicate that the frequency of recombination should be reduced by natural selection.A hypothesis not previously explored is that recombination modifiers may directly affect the fitness of their carriers; rather than only indirectly (through the production of recombinant progeny) as generally assumed. We have tested this hypothesis by examining three fitness components (viability, male fertility, and female fecundity) in Drosophila melanogaster homozygous for second chromosomes isolated from a natural population. Then, we have measured the frequency of recombination in flies heterozygous for each wild second chromosome and a chromosome carrying five recessive alleles.The results indicate that genes modulating the frequency of recombination have direct effects on fitness as proposed by the hypothesis. However, the correlation between frequency of recombination and fitness is negative. Thus, the riddle of recombination remains unexplained and, in fact, more puzzling that ever.     

Recessive genetic variants affecting mating activity of males in natural populations ofDrosophila melanogaster

Mating activity of 115 wild males was compared with 88 homozygotes and 42 heterozygotes for their second chromosomes. Wild males, 48–96 hours old, inseminated on the average, 4.4±0.1 females per 24 hours. The hetero- and homozygotes for their second chromosomes (other chromosomes being randomly combined with those from the laboratory strain), inseminated on the average 2.8±0.2 and 2.0±0.2 females/24 h. respectively. There is no correlation between homozygotes and heterozygotes for the second chromosome and their wild ancestors which carried these chromosomes. Wild second chromosomes which in homozygous condition produced total sterility of their carriers, and some others which made for an unusually high activity in homozygous males, had on an average similar effects in wild carriers.This ariicle is warmly dedicated to Professor Theodosius Dobzhansky.     

A mouse translocation associated with Caspr5-2 disruption and perinatal lethality

We have previously described the paralogous mouse genes Caspr5-1, -2, and -3 of the neurexin gene family. Here we present the cytogenetic and molecular mapping of a null mutation of Caspr5-2 which was caused by reciprocal translocation between chromosomes 1 and 8 with breakpoints at bands 1E2.1 and 8B2.1, respectively. The translocation disrupts Caspr5-2 between exons 1 and 2 and causes stillbirth or early postnatal lethality of homozygous carriers. Because no other candidate genes were found, the disruption of Caspr5-2 is most likely the cause of lethality. Only rarely do homozygotes survive the critical stage, reach fertility, and are then apparently normal. They may be rescued by one of the two other Caspr5 paralogs. Caspr5-2 is expressed in spinal cord and brain tissues. Despite giving special attention to regions where in wild-type fetuses maximum expression was found, no malformation that might have caused death could be detected in fetal homozygous carriers of the translocation. We, therefore, suspect that Caspr5-2 disruption leads to dysfunction at the cellular level rather than at the level of organ development.

     

Low-Moisture Food: A Physicochemical Approach to Investigate the Origin of Their Physical Instability versus Water or Sucrose

Low-moisture biopolymer-based systems are commonly encountered in food. Obviously, understanding the physical basis of their quality [texture, or performance over time or as a function of their composition (water or other added solutes)] is of primary importance. A polymer science approach using physical chemistry concepts based on physical state, phase transitions and molecular mobility can be applied to investigate the performances of food in particular versus moisture. Based on the example of starch-based samples and their texture property changes versus composition, the role of water and sucrose is considered through different aspects. The relations existing between the observed changes and physical state are investigated. While the motions associated with the glass transition were observed at high temperature, secondary relaxations are observed below Tg (at T _β): T _β decreased with water content and increased with increasing sucrose content. These local motions are suggested to contribute to the observed texture modifications versus water. Moreover, the stability of the glassy state was investigated by differential scanning calorimetry through the study of enthalpy relaxation (physical ageing). The amplitude of enthalpy relaxation decreased with both increasing sucrose and water content. All in all, this study strengthened the hypotheses that sub-Tg mobility could contribute to texture instability versus moisture or sugar content.     

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.     

Viabilities of Third Chromosomes of DROSOPHILA PSEUDOOBSCURA Differing in Relative Competitive Fitness

Arrowhead (AR) third chromosome arrangements of Drosophila pseudoobscura, whose competitive fitnesses had been determined in population cages, were tested for their genetic loads in homozygous, heterozygous (homokaryotypic), and heterokaryotypic (AR/CH) combinations. The results showed that their competitive population cage performances were correlated to their viabilities as homozygotes but were not correlated to their viabilities as heterozygotes or as heterokaryotypes. However, the results do not fit in too simply with the mutational model of population structure, since the improvement of homozygous viability with increased competitive fitness was not accompanied by a significant degree of dominance as measured by the regression of viabilities of heterozygotes on homozygotes. Only the AR chromosomes derived from the population with poorest competitive fitness showed marked partial dominance (h=.35). The viabilities of heterokaryotypes were markedly uniform for all chromosomes tested and produced significantly greater numbers of flies per culture than the homokaryotypes. In general, the results show that the ranking of relative competitive fitnesses of these chromosomes is not a simple extrapolation of their viabilities, although marked changes in the populations tested have occurred. It is proposed that the differences in competitive fitness, homozygous viability, and degree of dominance observed among these chromosomes, arise from differences in genetic variability which enable different linkage relationships to be established for genes affecting these attributes.     

Genetic differentiation in populations of the yellow-necked mouse,Apodemus flavicollis,harbouring B chromosomes in different frequencies

Two alternative models are used to explain maintenance of polymorphism of B chromosomes (Bs) in populations of a great number of species. The parasitic model suggests deleterious effects of Bs on fitness of carriers, while the heterotic model assumes that, in the absence of drive, equilibrium is produced by beneficial effects of Bs at low numbers. In order to determine the potential contribution of Bs to genetic differentiation and diversity, four populations of Apodemus flavicollis, differing in frequency of Bs (from 0.23 to 0.38) and settled in ecologically different habitats, were analyzed by 471 AFLP markers. Although numerous loci were demonstrated to be population specific, none of them was associated with individuals with Bs. AMOVA showed that the presence of Bs does not affect population differentiation, pointing to greater genetic similarity of Bs to A chromosomes. The greatest genetic diversity (0.241) was found in the population settled in optimal conditions for this species featured by the lowest frequency of animals with Bs (0.23). We found that the majority of loci marked as loci under directional selection, are characteristic of either a population with lower or one with a higher frequency of Bs. Several loci detected as outliers were associated with environmental variables that could directly and/or indirectly influence population dynamics of A. flavicollis. Thus, we suggest that the different frequency of Bs carriers in populations is related to adaptive differentiation to diverse habitats, which is in accordance with the heterotic model of Bs maintenance.     

TRUE-BREEDING COLCHICINE-INDUCED MUTANTS FROM SORGHUM HYBRIDS

Colchicine treatment of sorghum seedlings of both F₁ hybrids and of their pollen parent lines resulted in complex mutant plants the majority of which were true-breeding. Recessive irradiation-induced characters originally homozygous in the pollen parent lines and heterozygous in the hybrids did not appear in any of the mutants or their self progenies. Recessive normal characters apparently absent in the pollen parent lines and heterozygous in the hybrids appeared in some mutants and bred true in their self progenies. These results are consistent with the hypothesis that colchicine-induced mutants arise through chromosome substitution (Sanders and Franzke, 1964, Jour. Arnold Arb. 45: 36). Since sorghums with 2n = 20 are believed to be of polyploid derivation, interchangeable chromosomes carrying different alleles could be present in true-breeding lines. There is evidence that diploid mutant complements may result from reduction of polyploid nuclei. Since the proportion of true-breeding mutants was as large from the highly heterozygous seedlings as from the homozygous seedlings, segregation of chromosomes during reduction must be primarily by c-pairs which later separate as the homologues of a new cell. Plants from such cells would be equivalent to doubled haploids. With colchicine treatment, true-breeding lines have been obtained from multiple hybrids in a single step rather than after many generations of selling.     

Cytogenetic characteristics of a population of Chironomus riparius Meigen 1804 (Diptera, Chironomidae) from a polluted Po river station

A population of Chironomus riparius from a Po river station near Moncalieri (a trace-metal polluted station) was studied. In this population was established a great variability of band structure of polytene chromosomes as well as paracentric heterozygous inversions, deletions, deficiencies, partial breaks, diploid chromosome fragments, and changes in functional activity and appearance of heterochromatin. In arms A through F, some bands had an increased size compared to the standard chromosomic map. Some bands appeared in a heterozygous or normal homozygous state or were amplified. In all arms, many condensed stable bands appeared in the decondensed state when compared to the standard map. Asynaptic zones in arms E and G as well as heterozygous Balbiani rings and NORs were established. Very often the 4th chromosome was almost completely heteropycnotic and looded like a pompon chromosome. For the first time in this species, a high frequency of ectopic pairings of different arms was observed. Telomeric regions involved in ectopic pairings had a granular appearance, as did some centromeres. The hypothesis is advanced that such a high frequency of structural rearrangements could be correlated with genomic distribution of specific mobile elements.     

Fitness Effects of Ems-Induced Mutations on the X Chromosome of DROSOPHILA MELANOGASTER. II. Hemizygous Fitness Effects

X chromosomes mutagenized with EMS were tested for their effects on the fitness of hemizygous carriers. The tests were carried out in populations in which treated and untreated X chromosomes segregated from matings between males and attached-X females; the populations were maintained for several generations, during which time changes in the frequencies of the treated and untreated chromosomes were observed. From the rates at which the frequencies changed, the fitness effects of the treated chromosomes were determined. It was found that flies hemizygous for a mutagenized chromosome were 1.7% less fit per mM EMS treatment than those hemizygous for an untreated chromosome. Since the same flies were only 0.5% per mM less viable than their untreated counterparts, the total fitness effect of an X chromosome carrying EMS-induced mutants is three to four times greater than its viability effect. By comparing the heterozygous effect of a mutagenized X chromosome on fitness with the corresponding hemizygous effect, the dominance value for the chromosome is estimated to be about 0.25.     

AGE-RELATED ALTERATIONS IN HUMAN CHROMOSOME COMPOSITION AND DNA CONTENT IN VITRO DURING SENESCENCE

Different theories of ageing involving somatic mutations, error catastrophe, compensation and repair, and programmed ageing were subjected to analysis for their feasibility from experimental data. Due to the relative difficulty of carrying out longitudinal studies in human subjects in vivo and the long periods involved, most of these experiments dealt with in vitro systems. I. Fibroblast cells in culture were found to be ideal materials for demonstrating senescence for a particular species and at the terminal end the cultures showed certain changes associated with age. II. It appears that the ideas regarding the alterations in DNA content at the terminal stages of the culture or otherwise are related to the tissues concerned and the duration of the cultured condition. III. The importance of DNA content specially related to the concept of stability of the DNA strand supports indirectly the error catastrophe theories. The rate of net DNA synthesis, the size of the replicon and duration of S phase is reported not to change during in vitro ageing. The regulation system of DNA replication may however change, but this alteration does not affect the DNA replication machinery. Following cell fusion studies it has been hypothesized that senescent human diploid fibroblasts contain a diffusible inhibitor which blocks cells at G₁ phase. Some immortal cell lines like HeLa and SV40-transformed cells would contain a dominant inducer that could override or inactivate the putative inhibitor, the nature of which is not yet clear. IV. DNA repair competence of fibroblast cultures is reported to decline near the end of in vitro life-span. However, it has been noted that the human skin fibroblasts from both young and old donors are equally proficient in repairing damage by UV light. V. The replication patterns of chromosomes from both senescent embryonic fibroblasts and early-passage adult skin fibroblasts were essentially identical. There were very few differences between the early-passage embryonic and adult skin cells. It was concluded that the terminal replication pattern of fibroblasts changes very little with cellular ageing. VI. A statistically significant increase in sister chromatid exchange frequency has been reported during the terminal part of fibroblast cultures. VII. Electron-microscopic studies have confirmed the increased organization of microfillaments into bundles in senescent cells. The presence of a rigid cytoskeletal structure may contribute in part to the inability of such cells to replicate. VIII. Age-related increase in nuclear proteins was attributed to accumulation of residual acid proteins. Densitometric analysis showed that histone HI was low in late-passage cells and H₄ fraction increased relatively at the terminal phase. IX. In contrast to age-matched controls, fibroblasts cultured from progeria and Werner's syndrome undergo significantly low population doubling. Metaphase plates from these patients demonstrated a much higher frequency of chromosomal abnor malities than normal fibroblasts. Frequency of sister chromatid exchange in cells from Fanconi's anaemia did not show any significant change as compared with control sets. X. Significantly lower Feulgen DNA values have been recorded from lymphocytes of the elderly as compared with younger ones, indicated by hypodiploidy as well as by the individual amounts of euchromatin and heterochromatin. However, later data from flow-cytometric measurements indicated that DNA content was the same for all age groups. XI. The UV-induced unscheduled DNA synthesis in lymphocytes of 80 to 90 year-old individuals was reduced as compared to younger persons. However, the rate of repair of DNA strand breaks is apparently constant in all the age groups. XII. Increase in aneuploidy from lymphocyte cultures of aged individuals has been recorded by many workers. XIII. Significantly lower titres of serine, threonine, histidine, ornithine and lysine have been observed in aged persons, and only the first three decreased equally in both sexes. Some of the amino acids were influenced by sex hormones. XIV. Study of the frequency of spontaneous sister chromatid exchanges showed that neither intra-individual variation between replicate cultures established from the same blood sample nor variation among samples from the same individual initiated at different times was significant. However, sensitivity to induced sister chromatid exchange is reported to increase with age.     

<Emphasis Type="Italic">Arctica islandica:</Emphasis> the longest lived non colonial animal known to science

The ocean quahog, Arctica islandica is not just the longest living bivalve, it is also the longest lived, non-colonial animal known to science. With the maximum life span potential ever increasing and currently standing in excess of 400 years the clam has recently gained interest as a potential model organism for ageing research. This review details what is known about the biology of A. islandica, it discusses observed age-associated changes and reviews previous ageing research undertaken on the species and other long-lived bivalves which may be applicable to future ageing research and discusses future directions for ageing research with A. islandica. Historically much of the research on bivalves has been targeted at their utilization as a food source, environmental sentinels and more recently the use of their shells as archives of environmental change. The result of this has been an abundance of knowledge on bivalve life strategies, and a limited amount of information on the physiological changes in the cells and tissues of bivalves during the ageing process. However, research into the mechanisms of senescence of long-lived bivalves from a biogerontological perspective has advanced only recently. The research undertaken thus far has documented age-related differences in anti-oxidant defences and accumulation of oxidative products but despite the recent attention into ageing of A. islandica it is still to be ascertained if the species experiences senescence. Future directions for ageing research using A. islandica are discussed.     

Manifestations of ageing at the cytogenetic level

The effects of ageing in humans appear to be a combination of influence of genetically programmed phenomena and exogenous environmental factors, and take place at the cellular level (senescence), rather than at the level of the organism. There are many processes, which occur in somatic cells as a consequence of DNA replication (accumulation of DNA errors or mutations that outstrip repair processes, telomere shortening, deregulation of apoptosis, etc.) and which drive replicative senescence in human cells. DNA errors are considered to be critical primary lesions in the formation of chromosomal aberrations. It can be concluded that the chromosome aberrations are biomarkers of ageing in human cells. Studies of human metaphases, interphase nuclei and micronuclei showed the increase in loss of chromosomes and the increase in frequency of stable chromosome aberrations as a function of age.     

Genetic load and viability studies in Korean natural populations of Drosophila melanogaster

Further studies about the amount of genetic load in the Korean Anyang and Cheju (Sughipo) Island populations of Drosophila melanogaster were performed. In total 1630 second chromosomes were extracted from the Anyang opulation between 1983 and 1985; 19.0 % of the chromosomes proved lethal, 8.8 % semilethal in homozygous condition. From the island oulation, 504 wild second chromosomes were analysed in 1986; 24.2 % were lethal, 4.6 % subleiaf A slight increase of lethal and semilethal frequencies between 1976 and 1986 can be observed. Mean viabilities of “all homozy-gotes”, “quasinormal-homozygotes”, and “random heterozygotes” were estimated from crossin experiments with marker strains. Random heterozygotes were always more viable than quasinormal homozygotes. An analysis for correlation between random heterozgous and homozygous viabilities gave values significantly different from zero only for the 1985 kyang sample (r =—0.4625, P < 0.01), but no significances could be observed for all other Anyang samples from 1983, 1984, and 1986, respectively. he effective population sizes were estimated to be between 2000 and 6300 individuals for the Anyan and 4200 individuals for the island population, using Nel's formula (1968). It is sugested that baknced natural selection is mainly responsible for the maintenance of genetic load in the Anyang natural populations of D. melanogaster.     

Frequency of sister chromatid exchanges in a balanced reciprocal whole-arm translocation

Summary The frequency of sister chromatid exchanges (SCEs) in the centromere of chromosomes involved in a whole-arm translocation t(1;19) was evaluated in altogether 911 metaphases of translocation carriers (n=5) and of normal controls (n=6). Comparison of the two groups reveals no significant differences in the SCE rate (x ²=3.06, n _f =1). The question as to whether the possible increase of the SCE rate at the translocation point could be detected by light microscopy is discussed. Parameters included in the discussion are the ratio of the SCE frequency at the translocation point to the SCE frequency at any of the possible breakage points in the centromeric region and the number of possible breakage points in the centromeric region.     

CHROMOSOMAL DIFFERENTIATION IN CLARKIA DUDLEYANA

Snow , Richard . (U. California, Davis.) Chromosomal differentiation in Clarkia dudleyana. Amer. Jour. Bot. 47 (4) : 302—309. Illus. 1960.—Clarkia dudleyana (n=9) is a common, colonial annual of the early-summer California flora. Of 275 individuals, derived from 9 natural populations and their garden-grown representatives, 17.1% were heterozygous for reciprocal translocations. Supernumerary chromosomes were also found in about 2% of the plants examined. The translocation heterozygotes are not distributed regularly over the species range but are concentrated near the geographical center of distribution. Most of the populations contained none or only a few heterozygotes, but in one colony 69% of 42 plants sampled were heterozygous. Judging from the meiotic metaphase associations observed, at least 5 different chromosome arrangements are present at this locality. Hybrids between colonies have invariably been translocation heterozygotes, the largest association found in such hybrids being a chain of all 18 chromosomes (a potential ring of 18). No correlation is evident between geographical separation and degree of cytological differentiation. Heterozygotes with smaller rings of 4 or 6 chromosomes, whether from natural populations or resulting from interpopulation hybridization, are highly fertile owing to the regular alternate disjunction of the chromosomes of the rings. In the larger rings of 12 to 18 chromosomes, derived from interpopulation crosses, segregation is much more irregular and leads to high sterility. It is possible that at least in some localities the heterozygotes enjoy a selective advantage over their homozygous sibs. It is also postulated that homozygosity for a particular chromosome arrangement may be selectively favored in a certain habitat, as a result of a position effect attendant upon placing formerly non-linked genes in the same linkage group through reciprocal translocation. The high degree of chromosomal differentiation between some populations of this species suggests that the complex heterozygotes of Oenothera have arisen as a result of hybridization of cytologically differentiated races.     

Ultrastructure of meiotic pairing in B chromosomes of Crepis capillaris

The meiotic pairing behaviour of four B isochromosomes of Crepis capillaris was studied by synaptonemal complex (SC) surface spreading of pollen mother cells. The four B chromosomes form a tightly associated group, separate from the standard chromosomes, throughout zygotene and pachytene. All four B chromosomes are also folded around their axis of symmetry, the centromere, and the eight homologous arms are closely aligned from the earliest prophase I stages. A high frequency of multivalent pairing of the four B chromosomes is observed at pachytene, in excess of 90%, mirroring the situation observed at metaphase I but exceeding the frequency expected (76.2%) on the assumption of random pairing among the eight B isochromosome arms with a single distal pairing initiation site per arm. The higher than expected frequency of multivalents is due to the occurrence of multiple pairing initiations along the B isochromosome arms, resulting in high frequencies of pairing partner switches. Pairing of the standard chromosome set is frequently incomplete in the presence of four B chromosomes, and abnormalities of SC structure such as thickening and splitting of axes and lateral elements are also frequently seen. Similarly, B chromosomes show partial pairing failure, the extent of which is correlated with pairing failure in the standard chromosome set. The B chromosomes themselves also show abnormalities of SC structure. Both standard and B chromosomes show non-homologous foldback pairing of regions that have failed to pair homologously.by D. Schweizer     

The control of chiasma frequency in Vicia faba L.

Summary InVicia faba two groups of chromosomes can be distinguished and consequently correlations of chiasma frequency can be calculated between the groups and also within the group of small chromosomes. A negative correlation was demonstrated during some analyses while positive ones occurred in others, in fact a wide range of coefficients could be calculated by both methods. It is suggested that negative correlation of chiasma frequency occurs only when reproduction of the chromosomes is precipitated or pairing delayed, so that the degree of pairing may be affected in plants where many chromosomes are present or where the chromosomes are large in size. Such correlations can have no influence on recombination, but rather they are the occasional outcome of the mechanism controlling chiasma frequency.