Frequency Of Aneuploids Research Articles

Production of somatic hybrids between frost-tolerant Solanum commersonii and S. tuberosum: characterization of hybrid plants

Somatic fusion of mesophyll protoplasts was used to produce hybrids between the frost-tolerant species Solanum commersonii (2n=2x=24) and dihaploid S. tuberosum (2n=2x=24). This is a sexually incompatible combination due to the difference in EBN (Endosperm Balance Number, Johnston et al. 1980). Species with different EBNs as a rule are sexually incompatible. Fifty-seven hybrids were analysed for variation in chromosome number, morphological traits, fertility and frost tolerance. About 70% of the hybrids were tetraploid, and 30% hexaploid. Chloroplast counts in stomatal guard cells revealed a low frequency of cytochimeras. The frequency of aneuploids was relatively higher at the hexaploid level (hypohexaploids) than at the tetraploid level (hypotetraploids). The somatic hybrids were much more vigorous than the parents, and showed an intermediate phenotype for several morphological traits and moderate to profuse flowering. Hexaploid hybrid clones were less vigorous and had a lower degree of flowering than the tetraploid hybrid clones. All of the hybrids were female fertile but male sterile except for one, which was fully fertile and self-compatible. Many seeds were produced on the latter clone by selfing and on the male-sterile clones by crossing. The somatic hybrid plants showed an introgression of genes for frost tolerance and an adaptability to cold from S. commersonii. Therefore, the use of these somatic hybrids in breeding for and in genetic esearch on frost tolerance and cold-hardening is suggested.

Flow cytometric analysis of posterior oropharyngeal tumors

To date, few studies have evaluated the diagnostic and prognostic potential of flow cytometry in oropharyngeal tumors. The purpose of this study was to evaluate the interrelationship of anatomic location with the cytogenetic and cytokinetic characteristics of squamous cell carcinomas of the posterior oropharynx. Forty-eight paraffin-embedded squamous cell carcinoma specimens from the posterior oropharynx in clinical stages I to IV were analyzed with an Epics-C flow cytometer (Coulter Diagnostics, Hialeah, FL). Histopathologic grading was performed by the same pathologist, based on tumor differentiation. Of the 38 specimens suitable for analysis, several correlations were noted. First, as the %S-phase increased, the clinical stage increased. Second, as the histologic grade increased, the aneuploid frequency increased. Finally, as the tumor size increased, the histologic grade increased. From the results of this study, it was concluded that %S-phase may be useful in assessing the prognosis of squamous cell carcinomas of the posterior oropharynx. DNA index or aneuploid frequency, also may have value as a prognostic indicator. Finally, the lack of correlation between TNM staging and histologic grading stems not from tumor size but from nodal and metastatic involvement. Further studies are warranted to better define the usefulness of flow cytometric analysis in squamous cell carcinomas of the oropharyngeal region.

Effect of DNA ploidy classification on prognosis in breast cancer.

A series of 327 breast cancers was analyzed for DNA ploidy by flow cytometry from paraffin-embedded tissue, and the resulting DNA histograms were classified independently by 6 researchers in the field as DNA diploid (Di), aneuploid (An), tetraploid (Te), multiploid (Mu), or technically uninterpretable. The frequency of diploid, aneuploid, tetraploid and multiploid cancers varied from 28 to 41%, 33 to 49%, 8 to 21% and 2 to 6%. According to the scale Di-An-Te-Mu, DNA ploidy was not significantly associated with breast-cancer mortality by 2 classifiers, but if DNA euploid cancers (Di+Te) were tested against non-euploid, or diploid cancers against non-diploid, all classifiers found DNA euploid and diploid cancers to have better prognosis. Mortality associated with diploid or tetraploid cancers decreased with improving histogram quality and increasing uniformity of classification, whereas that associated with aneuploid cancers remained unaltered. Among the cases where all classifiers agreed on ploidy, tetraploid, diploid and aneuploid cancers were associated with 100%, 88% and 68% 5-year survival rates. In this sub-set the S-phase fraction and possibly DNA ploidy were independent prognostic factors, together with histological grade, axillary node status, and primary tumor size.

Rye C-banding patterns and meiotic stability of hexaploid triticale (× Triticosecale) selections differing in kernel shriveling

Two winter hexaploid triticale populations derived from the same cross were selected on the basis of grain appearance and agronomic performance. The five lines from 84LT402 showed more kernel shriveling than the four lines from 84LT401. The derived lines were analyzed for aneuploid frequencies, rye chromosome banding patterns, and meiotic stability to detect associations with kernel development. The aneuploid frequencies were 16% in 84LT401 and 18% in 84LT402. C-banding showed that both selection groups had all the rye chromosomes except 2R. The two groups had similar telomeric patterns but differed in the long-arm interstitial patterns of 4R and 5R. Compared with lines from 84LT402, those from 84LT401 had significantly fewer univalents and rod bivalents, and more paired arms at metaphase I; fewer laggards and bridges at anaphase I; and a higher frequency of normal tetrads. There were no significant differences among lines within each group for any meiotic character. Since there were no differences within or between groups in telomeric banding patterns, the differences in kernel shriveling and meiotic stability might be due to genotypic factors and (or) differences in the interstitial patterns of 4R and 5R. By selecting plump grains, lines with improved kernel characteristics along with improved meiotic stability are obtainable.Key words: triticale, meiotic stability, C-banding, Secale cereale, heterochromatin.

Chromosomal analysis of cultured cells of barley(Hordeum vulgare L.): Chromosome number variation.

A study on the variation in chromosome number of the cells from a nodular morphogenic callus (Mohanty and Ghosh 1987), derived from embryo or mesocotyl, or leaf, was undertaken. This study also covered the extent of variation in relation to repeated subculturing for 6 months. The chromosome number showed wide variations (2n=5 to 100); the diploid cells (2n=2x=14), however, were dominant in all the cultures obtained from each type of explant. In an embryo callus, the tetraploid (2n=4x=28) cells increased significantly after 90 days of culturing, but no significant rhythmic increase of tetraploid cells with age could be observed in mesocotyl and leaf calli. The frequency of aneuploid and high polyploid cells decreased with increase in age of the culture.

Cytology of Bilaterally Derived Tetraploid Red Clover

An interest in increasing heterozygosity in tetraploid (2n = 4x = 28) red clover (Trifolium pratense L.) has led to the use of 2x‐2x (bilateral) crosses for tetraploid production. This study cytologically examined F2 bilaterally derived tetraploid red clover plants (Bi4x) for aneuploid frequency and meiotic chromosome pairing. Cytological results on Bi4x were compared to colchicine derived, N2O derived, and 2x‐4x (unilateral) derived tetraploid populations. In addition, dry pollen morphology of 90 Bi4x plants was examined. The frequency of aneuploids in Bi4x was 23%, with chromosome numbers of 25, 26, 27, and 30. Hypoploids (2n < 28; frequency = 90.48%) were more frequent than hyperploids (2n > 28; frequency = 9.52%). Chromosome associations in euploid pollen mother cells averaged 0.35 univalents, 5.96 bivalents, 0.14 trivalents, and 3.80 quadrivalents. The difference between euploid chromosome associations and aneuploids was nonsignificant (x2 = 2.36; P > 0.50). Observations on dry pollen morphology indicated 78.6% of the Bi4x population produced more than 80% tetrahedron shaped pollen (typical of n = 2x = 14 pollen). The frequency of cylindrical shaped pollen (typical of n = x = 7 pollen) ranged from 1% to 90%. The Bi4* plants with a high frequency of cylindrical pollen were crossed to diploid and tetraploid plants. The 2x ✕ Bi4x crosses produced no seed, whereas, the 4x ✕ Bi4x crosses produced seed (18.13% fertility) suggesting cylindrically shaped pollen in the Bi4x population reverted to morphology typical of pollen from diploid plants (n = 2x = 14). Fiftyfour Bi4x plants were selfed and seven seeds were obtained from 6177 hand pollinations indicating a high level of self incompatability.

Chromosome pairing and aneuploidy in tetraploid triticale. II. Unstabilized karyotypes

In the progeny of tetraploid triticale plants that segregated for either one, two, or three pairs of homoeologous wheat chromosomes, plants were selected that had 13 pairs of homologues and 1 pair of presumed wheat homoeologues. Segregation ratios of "homoeologues" were close to 1:2:1 except for group 4, where no 4B homozygotes were recovered. Aneuploid frequency among 190 progeny of the segregating plants was 4.74%. C-banding at meiosis showed that "homoeologues" paired with frequencies ranging from 0.1 to 1.74 paired arms per chromosome. The only exception was the 4A–4B pair, which did not synapse. It was concluded that the high pairing frequencies between "homoeologous" chromosomes were due to translocations that had accumulated during line development. With few exceptions, translocations could not be detected by C-banding. The results demonstrate that genome recombination in polyploid species may occur at two levels simultaneously: by segregation of complete chromosomes and by translocations between homoeologues. Key words: C-banding, homoeologous pairing, translocations.

Chromosome pairing and aneuploidy in tetraploid triticale. I. Stabilized karyotypes

Among 38 lines of tetraploid triticale analyzed at meiosis, the number of paired arms per rye chromosome ranged from 1.14 to 1.76 and from 1.46 to 1.96 per wheat chromosome. The frequency of cells without univalents ranged from 22 to 90%. Pairing frequencies within rye and wheat genomes were correlated in all groups of lines. Lines without wheat chromosome 3B showed reduced pairing in both genomes, while lines with an additional pair of 5R chromosomes substituted for group-5 wheat chromosomes showed improved pairing of the rye genome but not of the wheat genome. In the rye genome, the chromosome arms that carry major blocks of telomeric heterochromatin paired with an average of 25.1% lower frequency than the arms without the telomeric heterochromatin, the difference being attributed to the difference in arm length and not to the presence of heterochromatic blocks. In the wheat genome, chromosome arms IBS, 5AS, and 5BS and chromosomes 4A and 7B paired with significantly lower frequency than the remaining arms of wheat chromosomes. Average aneuploid frequency in a sample of 1383 plants was 4.55%, with the mean value of 5.77% in lines with 14 wheat and 14 rye chromosomes, and 2.01% in lines with an extra pair of 5R chromosomes. The results indicate that while the meiotic pairing is poorer and aneuploid frequency is higher than previously believed, tetraploid triticales are nevertheless chromosomally much more stable than hexaploid or octoploid triticales. Key words: C-banding, heterochromatin, paired arms, chromosome substitution.

The induction of aneuploidy by nitrogen mustard in a human lymphoblastoid cell line

We report that the presence of an extra Y chromosome can be used as a marker for the induction of aneuploidy (mitotic non-disjunction) in a huan lymphoblastoid cell line. This endpoint is easily visualized in metaphase chromosome preparations after staining with quinacrine mustard. The induction of cells with two Y chromosomes by nitrogen mustard (NM) was examined. Exposure to 150 ng/ml nitrogen mustard induced a 6-fold increase in aneuploid frequency relative to untreated control levels; maximal induction of aneuploidy was observed 2 days after treatment. Lower concentrations of nitrogen mustard (36 and 75 ng/ml) induced smaller increases in aneuploid frequency, with maximal induction observed 1 day after treatment. This system has the potential to be used as an assay for the induction of aneuploidy in cultured human cells.

Distribution of heterozygous translocations and aneuploid spermatocyte frequency in domestic sheep.

In an analysis of the chromosomes in 1556 metaphase II figures from 26 translocation-carrying rams and 84 figures from 3 normal rams (54, XY), there were no hypermodal cells in the normal rams but 35 such cells in the translocation rams, giving mean aneuploid frequencies of 3.92 percent, 4.29 percent, and 5.29 percent for the rams heterozygous for 1, 2, and 3 translocations, respectively. The translocation chromosome in the heterozygous state was found to segregate evenly during meiosis. Although the effect of heterozygosity was to increase the aneuploid frequency of the ram it is seen that the frequency in all translocation-carrying rams is low and it is thought that these cells are eliminated during spermatogenesis and therefore do not reduce the fertility of the sheep.

Aneuploidy and inbreeding depression in random mating and self-fertilizing autotetraploid populations

Almost all autotetraploids produce aneuploid progeny because of irregularities at meiosis. Aneuploid plants produce high frequencies of aneuploids. If it were not for selection against aneuploid gametes and sporophytes the amount of aneuploidy would increase every generation. Most experimental and theoretical studies on population genetics and heterosis in autotetraploids have neglected aneuploidy as a factor. To take aneuploidy into account experimentally requires the cytological identification of all chromosomes and to consider it theoretically requires a huge amount of computations. Consequently, microcomputer programs have been devised to show the effects of random mating and self-fertilization in autotetraploid populations. According to the model aneuploidy rapidly increases in randomly mated and self-fertilized autotetraploid populations until they achieve an equilibrium where the amount of aneuploidy introduced into the population is balanced by the amount of aneuploidy removed from the population by selection. The model suggests that self-fertilized populations have greater frequencies of aneuploid gametes and zygotes than do randomly mated populations and therefore aneuploidy may be a significant cause of the great inbreeding depressions found in autotetraploids.

In vitro culture ofBellevalia romana (L.) Rchb.

Bellevalia romana (L.) Rchb., a monocotyledonous plant characterized by few (2 n=2 x=8) and very large chromosomes, is a useful subject for studying developmental problemsin vitro. Cytological analysis of callus revealed that the majority of cells were diploid, but the remaining cells had aneuploid nuclei with a wide range of chromosome numbers, tetraploid and haploid nuclei. The frequency of aneuploid and polyploid cells was higher in callus grown in the presence of 2,4-D than in callus grown in NAA plus BAP. These nuclei seemed to increase with the duration of culture. The chromosome number distribution as determined by chromosome counts in calli at different culture times was confirmed by DNA cytophotometry. Chromosome number mosaicism (mixoploidy and aneusomaty) also occurred in all root apices of 9 out of 46 plantlets regenerated from callusvia adventitious shoots.

Genetic interactions between wheat and rye genomes in triticale

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.

Mechanisms and detection of chromosome malsegregation using Drosophila and the yeast Saccharomyces cerevisiae.

Chromosome aneuploidy in humans may have severe consequences in the individuals in which it occurs or in the following generation (35; Evans, this Volume). Approximately 15% of recognized pregnancies result in spontaneous abortion (78). Approximately half of these result from chromosome abnormalities (13,67). The rate of aneuploidy may actually be much higher than this because early fetal losses may go undetected (12) and certain types of aneuploidy are underrepresented in this sample (5). In fact, the frequency of aneuploids among human sperm is 1.5–5% (47; Martin, this Volume). Among livebirths the frequency of aneuploidy is roughly 0.3% (4). Most of these result from trisomy of one of three autosomes, primarily chromosome 21 (Down syndrome), or from trisomy or monosomy for the sex chromosomes. Although it is generally assumed that meiotic nondisjunction is responsible for the aneuploidy in these spontaneous abortions and birth defects, mitotic aneuploidy may contribute substantially to these incidences. Mason (49) and Vig (77) have pointed out that mitotic nondisjunction in the germline will mimic meiotic nondisjunction, which may serve to explain some otherwise anomalous observations such as the paucity of monosomies among early abortuses and the high frequency of apparent meiosis 2 errors.

Isolation of aneuploid-generating mutants of Aspergillus nidulans, one of which is defective in interphase of the cell cycle.

A method is described for isolating mutants potentially defective in loci involved in mitotic chromosome segregation. Conditional lethal, heat-sensitive (42 degrees ) mutants were assayed at a subrestrictive temperature of 37 degrees for an inflated production of colonies displaying phenotypes and behavior patterns of whole chromosome aneuploids. Of 14 mutants, three showed specificity for one disomic phenotype, whereas 11 generated colonies mosaic for different aneuploid phenotypes. This latter group is designated hfa (high frequency of aneuploid). For ten of the 11 mutants temperature sensitivity and aneuploid production cosegregated, indicating a single mutation in each. These mutations were recessive and nonallelic. Analysis was concentrated on the hfaB3 mutation which is mapped to chromosome VI tightly linked to the methB and tsB loci. The disruptive influence of hfaB3 on mitosis at 37 degrees was shown by (1) ploidy and whole chromosome-type segregation of markers in the breakdown sectors of phenotypically aneuploid colonies obtained from multiply marked homozygous hfaB3 disploids; (2) a high frequency of haploid and nondisjunctional diploid segregants among spontaneous yellow-spored parasexual recombinants taken from green-spored homozygous hfaB3 diploids. The mutation had no effect on meiotic chromosome segregation at 37 degrees. The single interphase nucleus in germlings at 42 degrees, coupled with changes in the mitotic index in temperature exchange experiments, showed hfaB3 to arrest the cell cycle in interphase at restrictive temperature. A conclusion drawn is that the hfaB gene product is required both for entry into mitosis and for normal chromosome segregation in dividing nuclei.

The effect of telomeric heterochromatin on the frequency of aneuploids in triticale (× Triticosecale)

The relationship between the frequency of aneuploids and blocks of telomeric heterochromatin on rye (Secale cereale L.) chromosomes in a triticale (× Triticosecale Wittmack) background was investigated. Lines with or without blocks of telomeric heterochromatin were examined to determine the percentage of aneuploid progeny. The results showed that the amount of telomeric heterochromatin present on rye chromosomes 4R and 6R had little effect (2%) on lowering the proportion of aneuploids. When rye chromosome pair 2R was replaced by wheat chromosome 2D in the presence of chromosomes 4R and 6R lacking heterochromatin, the percentage of aneuploids increased slightly (2.7%) over that found in the control population.

An investigation into the origins of meiotic aneuploidy using ascus analysis

SUMMARYAneuploidy can result from a variety of defects at meiosis. Results are presented of crosses ofSordaria brevicollisin which aneuploid spores are detected through complementation of spore colour mutants at thebufforgrey-6 loci on linkage groups II and IV respectively. By using ascus analysis, the underlying cause of the aneuploidy can be deduced in many cases. Thus non-conjuction (pairing failure) and non-disjunction at the first meiotic division, premature centromere division, non-disjunction at the second division, and pre-meiotic errors such as extra replication of the chromosomes can be distinguished. Both linkage groups were found to give a similar proportion of the different errors. Non-conjunction and first-division non-disjunction formed 60–70% of detectable cases, whilst premature centromere division and second-division non-disjunction comprised 10% and 5% of aneuploids respectively. However, only a small proportion of second-division errors are detected.It is proposed that the systems described in this paper can form the basis of a valuable screening method for detecting agents which increase aneuploid frequency. The advantages and disadvantages of using lower eukaryotes in this way are discussed.

SPONTANEOUS ANEUPLOIDS OF MAIZE (ZEA MAYS) IN A SELECTED SAMPLE

A maize (Zea mays L.) sample with a high frequency of spontaneous aneuploids was prepared by selecting seeds and plants. The study was carried out on seeds discarded during the mechanical processing of a commercial hybrid. Starting from a discarded bulk of 20,000, a selected fraction of about 4,000 small and malformed kernels was planted in the field. Further selection was made by examining the developing plants and their pollen. A group of 46 plants was selected because of relatively high pollen sterility or abnormal plant growth and subjected to genetic investigation. After crossing these plants to inbred lines, 14 of them (about 30%) produced aneuploids among the offspring; a few of these aneuploids were cytologically identified as primary trisomics. The high pollen sterility was not associated with the aneuploid condition. This finding suggests that selection was made for structurally altered chromosomes whose behavior during the meiotic process gave rise to aneuploids.

A THEORETICAL ANALYSIS OF THE EFFECTS OF SEX CHROMOSOME ANEUPLOIDY ON X AND Y CHROMOSOME MEIOTIC DRIVE.

Extra sex chromosomes are normally detrimental to the individual carrying them. In XY (or WZ) sex determining systems, an extra X chromosome in the homogametic sex generates enough X-autosome imbalance to usually cause inviability, or at least sterility. On the other hand, extra Y chromosomes are sometimes tolerated, although in mammals and other higher organisms where the Y is actively involved in sexual development, XXY individuals are generally sterile. In Drosophila and perhaps other invertebrates where the Y is largely genetically inert, a single extra Y may be permitted in both sexes, while two extra copies is only fertile in XXYY females (Cooper, 1956), if at all. Even if XXY and XYY genotypes were perfectly viable and fertile, abnormal meiotic segregation and the consequent production of unbalanced gametes would lead to enough zygotic lethality to reduce their overall fitness (see Fig. 1). Thus we expect sex chromosome aneuploids in nature to be rare, primarily arising from recurrent spontaneous nondisjunction events. However, the literature records many cases of unusual accessory or sex chromosome constitutions found in both plants and animals (particularly insects, see White, 1973) which must have faced at least a transient period of aneuploidy during the process of their establishment. The genetic forces which might be responsible for this form of karyotype evolution are at present largely in the realm of speculation (White, 1978). I recently (Lyttle, 1981a) reported an experimental population of Drosophila melanogaster in which such aneuploids were present at high frequency. This population had been segregating for a type of Y chromosome meiotic drive (pseudo-Y or pY drive, Lyttle, 1977), which operates by coupling the Y chromosome to the autosomal drive element Segregation distorter (SD) by reciprocal translocation. Normally, pY drive males produce only male offspring, as potential daughters are removed both by the SD mediated dysfunction of X;SD+ gametes and the concomitant lethality of the hypoploid zygotes arising from fertilization events involving X;SD sperm. Since the pY drive was expected to push the male frequency to near 100% of the population, the observed high frequency of aneuploids was interpreted as a strategy for reducing this deviation of the population sex ratio from the optimal 1:1 value. This would occur because XYY males and XXY females, owing to their production of gametes with unusual sex chromosome combinations, have proportionately more daughters than do their XY and XX counterparts under the conditions of pY drive (Lyttle, 1981a). That is, since the y2 portion of the translocation carries the large missing piece of the SD chromosome, its added presence in XY;SD sperm can rescue usually hypoploid zygotes to give autosomally euploid XXY females. To investigate this apparent selection for aneuploidy, I constructed a simple discrete generation model which included effects of chromosomal segregation, fitness, and meiotic drive, the latter defined as the excess recovery in the gametes of one member of a pair of heterozygous alleles or heteromorphic chromosomes (cf. Sandler and Novitski, 1957; Zimmering et al., 1970). The resulting equations gave pre1 This research was supported by Public Health Service Grant GM 24083.

Aneuploid spermatocyte frequency in domestic sheep heterozygous for three Robertsonian translocations.

In an analysis of the chromosomes in 332 metaphase II figures from 3 triple heterozygous rams (51,xy,t1t2t3) and a further 84 figures from 3 normal rams (54,xy), there were no hypermodal cells in the normal rams but 9 such cells were found in the triple heterozygotes, giving a mean aneuploid frequency of 5.42% which was similar to the levels previously found in the single heterozygotes. Forty hypomodal cells were found in the 6 rams of which a number would have lost chromosomes due to lagging at anaphase I. Individual variation in the aneuploid metaphase II frequency was observed in the triple heterozygotes. A significant surplus of secondary spermatocytes of normal karyotype and a deficit of 25,t1t3 were found in the metaphase II figures from the triple heterozygotes. There was also a significant increase of normal progeny and a deficit of 52,t1t3 and 53,t2 progeny from the matings of triple heterozygous rams and normal ewes. It is possible that the significantly uneven distribution of segregation products in the triple heterozygotes might have been the result of either cell selection or degeneration during spermatogenesis.