Microtubule organization defects in Arabidopsis thaliana

Microtubules (MT) are critical cytoskeletal filaments that have several functions in cell morphogenesis, cell division, vesicle transport and cytoplasmic separation in the spatiotemporal regulation of eukaryotic cells. Formation of MT requires the co-interaction of MT nucleation and α-β-tubulins, as well as MT-associated proteins (MAP). Many key MAP contributing to MT nucleation and elongation are essential for MT nucleation and regulation of MT dynamics, and are conserved in the plant kingdom. Therefore, the deletion or decrease of γ-tubulin ring complex (γTuRC) components and related MAP, such as the augmin complex, NEDD1, MZT1, EB1, MAP65, etc., in Arabidopsis thaliana results in MT organizational defects in the spindle and phragmoplast MT, as well as in chromosome defects. In addition, similar defects in MT organization and chromosome structure have been observed in plants under abiotic stress conditions, such as under high UV-B radiation. The MT can sense the signal from UV-B radiation, resulting in abnormal MT arrangement. Further studies are required to determine whether the abnormal chromosomes induced by UV-B radiation can be attributed to the involvement of abnormal MT arrays in chromosome migration after DNA damage.

Rapid Birth or Death of Centromeres on Fragmented Chromosomes in Maize

Comparative genomics has revealed common occurrences in karyotype evolution such as chromosomal end-to-end fusions and insertions of one chromosome into another near the centromere, as well as many cases of de novo centromeres that generate positional polymorphisms. However, how rearrangements such as dicentrics and acentrics persist without being destroyed or lost remains unclear. Here, we sought experimental evidence for the frequency and timeframe for inactivation and de novo formation of centromeres in maize (Zea mays). The pollen from plants with supernumerary B chromosomes was gamma-irradiated and then applied to normal maize silks of a line without B chromosomes. In ∼8,000 first-generation seedlings, we found many B-A translocations, centromere expansions, and ring chromosomes. We also found many dicentric chromosomes, but a fraction of these show only a single primary constriction, which suggests inactivation of one centromere. Chromosomal fragments were found without canonical centromere sequences, revealing de novo centromere formation over unique sequences; these were validated by immunolocalization with Thr133-phosphorylated histone H2A, a marker of active centromeres, and chromatin immunoprecipitation-sequencing with the CENH3 antibody. These results illustrate the regular occurrence of centromere birth and death after chromosomal rearrangement during a narrow window of one to potentially only a few cell cycles for the rearranged chromosomes to be recognized in this experimental regime.

Cytological Effect of Gamma Radiation on Selected Mutants of Wheat <i>Triticum aestivum</i> L. in M3 Generation

BACKGROUND AND OBJECTIVE

Wheat (Triticum aestivum L.) offers some unique opportunities for the induction and exploitation of agronomic value. The use of gamma radiation has been proven to be an effective method to induce genetic variation in crops. We aimed to determine genetically stable mutants of wheat which could be utilized for breeding purposes.

MATERIALS AND METHODS

We did a cytological investigation of induced mutant's behavior and chiasma frequency. Selected mutant types induced in dry and soaked seeds were treated with different doses of gamma rays. Each treated sample and control were subjected to cytological examination of the fixed pollen mother cells in various meiotic stages.

RESULTS

The percentage of the total abnormal cells significantly increased in one mutant and significantly decreased in the other mutant. The percentage of total abnormal cells did not diminish from the first to the second meiotic division. The types of meiotic anomalies found included laggards (56.51%), univalent (9.43%), stickiness (45.45%) and bridges (19.32%). There were genotypic differences in the frequency of occurrence of multivalent (trivalent and quadrivalents). A marked reduction in the number of rod and ring bivalent/cell in some genotypes were noticed. The frequency of chiasmata per pollen mother cell was reduced subsequently. Depression index of mutants was negative compared with controls or treatments except for a few genotypes.

CONCLUSION

Selected mutants of wheat tend to be cytologically stable and can therefore, be utilized for breeding purposes.

Detecting Brachypodium distachyon Chromosomes Bd4 and Bd5 in MH- and X-Ray-Induced Micronuclei Using mcFISH

Micronuclei are biomarkers of genotoxic effects and chromosomal instability. They are formed when chromosome fragments or whole chromosomes fail to disjoin into daughter nuclei. We present qualitative and quantitative analyses of the involvement of specific chromosome regions of chromosomes Bd4 and Bd5 in the formation of micronuclei of Brachypodium distachyon root tip cells following maleic hydrazide (MH) treatment and X-radiation. This is visualised by cytomolecular approaches using bacterial artificial chromosome (BAC)-based multicolour fluorescence in situ hybridisation (mcFISH) in combination with 5S and 25S rDNA probes. The results showed that the long arm of submetacentric chromosome Bd4 forms micronuclei at twice the frequency of its short arm, suggesting that the former is more prone to double-strand breaks (DSBs). In contrast, no difference was observed in the frequency of micronuclei derived from the long and short arms of submetacentric chromosome Bd5. Interestingly, the proximal region of the short arm of Bd5 is more prone to DSBs than its distal part. This demonstrates that 5S rDNA and 35S rDNA loci are not "hot spots" for DNA breaks after the application of these mutagens.

INDUCED CYTOMICTIC VARIATIONS AND SYNCYTE FORMATION DURING MICROSPOROGENESIS IN PHASEOLUS VULGARIS L

The intercellular translocation of chromatin material along with other cytoplasmic contents among the proximate meiocytes lying in close contact with each other commonly referred as cytomixis was reported during microsporogenesis in Phaseolus vulgaris L., a member of the family Fabaceae. The phenomenon of cytomixis was observed at three administered doses of gamma rays viz. 100, 200, 300 Gy respectively in the diploid plants of Phaseolus vulgaris L. The gamma rays irradiated plants showed the characteristic feature of inter-meiocyte chromatin/chromosomes transmigration through various means.such as channel formation, beak formation or by direct adhesion between the PMC's (Pollen mother cells). The present study also reports the first instance of syncyte formation induced via cytomictic transmigration in Phaseolus vulgaris L. Though the frequency of syncyteformation was rather low yet these could play a significant role in plant evolution. It is speculated that syncyte enhances the ploidy level of plants by forming 2n gametes and may lead to the production ofpolyploid plants. The phenomenon of cytomixis shows a gradual inclination along with the increasing treatment doses of gamma rays. The preponderance of cytomixis was more frequent during meiosis I as compared to meiosis II. An interesting feature noticed during the present study was the channel formation among the microspores and fusion among the tetrads due to cell wall dissolution. The impact of this phenomenon is also visible on the development of post-meiotic products. The formation of heterosized pollen grains; a deviation from the normal pollen grains has also been reported. The production of gametes with unbalanced chromosomes is of utmost importance and should be given more attention in future studies as they possess the capability of inducing variations at the genomic level and can be further utilized in the improvement of germplasm.

Dissecting Plant Chromosomes by the Use of Ionizing Radiation

Radiation treatment of genomes is used to generate chromosome breaks for numerous applications. This protocol describes the preparation of seeds and the determination of the optimal level of irradiation dosage for the creation of a radiation hybrid (RH) population. These RH lines can be used to generate high-resolution physical maps for the assembly of sequenced genomes as well as the fine mapping of genes. This procedure can also be used for mutation breeding and forward/reverse genetics.

Identification of Novel Chromosomal Aberrations Induced by (60)Co-γ-Irradiation in Wheat-Dasypyrum villosum Lines

Mutations induced by radiation are widely used for developing new varieties of plants. To better understand the frequency and pattern of irradiation-induced chromosomal rearrangements, we irradiated the dry seeds of Chinese Spring (CS)-Dasypyrum villosum nullisomic-tetrasomic (6A/6D) addition (6V) line (2n = 44), WD14, with (60)Co-γ-rays at dosages of 100, 200, and 300 Gy. The M₀ and M₁ generations were analyzed using Feulgen staining and non-denaturing fluorescence in situ hybridization (ND-FISH) by using oligonucleotide probes. Abnormal mitotic behavior and chromosomes with structural changes were observed in the M₀ plants. In all, 39 M₁ plants had structurally changed chromosomes, with the B genome showing the highest frequency of aberrations and tendency to recombine with chromosomes of the D genome. In addition, 19 M₁ plants showed a variation in chromosome number. The frequency of chromosome loss was considerably higher for 6D than for the alien chromosome 6V, indicating that 6D is less stable after irradiation. Our findings suggested that the newly obtained γ-induced genetic materials might be beneficial for future wheat breeding programs and functional gene analyses.

Modulating crossover positioning by introducing large structural changes in chromosomes

BACKGROUND

Crossing over assures the correct segregation of the homologous chromosomes to both poles of the dividing meiocyte. This exchange of DNA creates new allelic combinations thus increasing the genetic variation present in offspring. Crossovers are not uniformly distributed along chromosomes; rather there are preferred locations where they may take place. The positioning of crossovers is known to be influenced by both exogenous and endogenous factors as well as structural features inherent to the chromosome itself. We have introduced large structural changes into Arabidopsis chromosomes and report their effects on crossover positioning.

RESULTS

The introduction of large deletions and putative inversions silenced recombination over the length of the structural change. In the majority of cases analyzed, the total recombination frequency over the chromosomes was unchanged. The loss of crossovers at the sites of structural change was compensated for by increases in recombination frequencies elsewhere on the chromosomes, mostly in single intervals of one to three megabases in size. Interestingly, two independent cases of induced structural changes in the same chromosomal interval were found on both chromosomes 1 and 2. In both cases, compensatory increases in recombination frequencies were of similar strength and took place in the same chromosome region. In contrast, deletions in chromosome arms carrying the nucleolar organizing region did not change recombination frequencies in the remainder of those chromosomes.

CONCLUSIONS

When taken together, these observations show that changes in the physical structure of the chromosome can have large effects on the positioning of COs within that chromosome. Moreover, different reactions to induced structural changes are observed between and within chromosomes. However, the similarity in reaction observed when looking at chromosomes carrying similar changes suggests a direct causal relation between induced change and observed reaction.

[Cytogenetic studies on submerged plants from the Yenisei river area in the zone of radioactive contamination]

Cytogenetic studies on three species of submerged plants from different parts of the Yenisei river area subjected to radioactive impact of the Krasnoyarsk Mining-and-Chemical Plant and the Electrochemical Factory have been conducted. A high level of irregularities in anatelophase and metaphase of mitoses has been revealed in test samples compared to the control: agglutination and fragmentation of chromosomes, lagging chromosomes, bridges, fragments, misdivisions, and others. The natuie of the disorders indicates that they are related in part to the direct damage to the chromosome structure and in part to damage to the spindle.

[Genetic effects of root extracts of Glycyrrhiza glabra L. on different test-systems]

The antimutagenic and geroprotective activities of root extracts of Glycyrrhiza glabra have been demonstrated both on plant test systems--Allium fistulosum L., Allium cepa L., Vicia faba L. and on animals--Vistar rats. The possibilities of the mobilization of Glycyrrhiza glabra root extracts as antimutagenic agents are discussed.

The rice RAD51C gene is required for the meiosis of both female and male gametocytes and the DNA repair of somatic cells

The RecA/RAD51 family of rice (Oryza sativa) consists of at least 13 members. However, the functions of most of these members are unknown. Here the functional characterization of one member of this family, RAD51C, is reported. Knockout (KO) of RAD51C resulted in both female and male sterility in rice. Transferring RAD51C to the RAD51C-KO line restored fertility. Cytological analyses showed that the sterility of RAD51C-KO plants was associated with abnormal early meiotic processes in both megasporocytes and pollen mother cells (PMCs). PMCs had an absence of normal pachytene chromosomes and had abnormal chromosome fragments. The RAD51C-KO line showed no obvious difference from wild-type plants in mitosis in the anther wall cells, which was consistent with the observation that the RAD51C-KO line did not have obviously abnormal morphology during vegetative development. However, the RAD51C-KO line was sensitive to different DNA-damaging agents. These results suggest that RAD51C is essential for reproductive development by regulating meiosis as well as for DNA damage repair in somatic cells.

Hybrid inflorescences derived from gamma-fusion of Arabidopsis thaliana with Bupleurum scorzonerifolium

In our early experiments, a variety of Bupleurum scorzonerifolium-like somatic hybrid plants were obtained from protoplast fusion between Arabidopsis thaliana and UV-treated/untreated B. scorzonerifolium. To compare the effects of UV and γ-ray irradiation on the B. scorzonerifolium partner and obtain Arabidopsis-like hybrids, we designed a novel combination of somatic hybridization between A. thaliana and B. scorzonerifolium. Before protoplast isolation and fusion, the suspension cells of B. scorzonerifolium were irradiated by gamma ray ((60)Co, 50 Gy with 1.3 Gy min(-1)). Both parental protoplasts lost regeneration capacity, but over 100 somatic hybrids restored the capacity and developed to Arabidopsis-like inflorescences and flowers with some characteristics of B. scorzonerifolium. Some hybrid flowers showed yellow sepal, petal, or carpel, whose color was similar to the petal of B. scorzonerifolium; the others had silique of Arabidopsis with angularity of B. scorzonerifolium, and their parts possessed five stamens, the same as B. scorzonerifolium. Cytological analysis showed that three hybrids had Arabidopsis-like karyotypes. Random Amplified Polymorphic DNA (RAPD) and Simple Sequence Repeats (SSR) profiles revealed that both parental fragments were amplified from these hybrids. These results indicated chromatin introgression from B. scorzonerifolium to A. thaliana, which may be related to the complementation of hybrid inflorescence and flower generation.

[The chiral mutagens: cytogenetic effects on higher plants]

The paper covers investigation of cytogenetic activity of chiral mutagens and their specific effects on the plant cells chromosomes of soft winter wheat (Triticum aestivum L.). Comparative analysis of cytogenetic activity of chiral NEU: S(+)1-N-nitroso- 1-N-methyl-3-N-sec-buthylureas (S(+)NMsBU) and R(-)1-N-nitroso- 1N-methyl-3-Nsec-buthylureas (R(-)NMsBU) on winter wheat was performed. As it was shown by the frequency of chromosomal aberrations the S(+) stereoisomer was twice more active than R(-). In addition to typical anaphase aberrations (fragments, bridges, lagging chromosomes) the numerous mitosis pathologies were revealed - K-mitoses, hyperspiralization and despiralization of chromosomes, unequal allocation of chromosomes between the daughter nuclei, mass fragmentation, nondisjunction and chromosome adhesion, three-pole mitoses, etc. Neither of the mentioned pathologies was observed under the action of NEU and gamma-rays.

[Remote consequences of a chronic radiation for the population of Hypochoeris maculata L growing on the territory of East-Ural radioactive trace]

The caryological study has been carried out on Hypochoeris maculata L. plants growing on the East-Urals radioactive trace. Two Hypochoeris maculata L. populations have been observed. The experimental population grows in contaminated area. 90Sr contamination density is 55 MBq/m2, 137Cs contamination density is 2.5 M Bq/m2. The control population grows in radionuclide-free area. Both in the experimental and in the control populations the plants have been detected bearing extra B-chromosomes in their karyotype. But their frequency was higher in the experimental population than in the control one. In the experimental population the plants with main A-chromosome set karyotype changes have been met in 9 families out of 30 families observed. In the control population one such family has been detected out of 27 families observed. Two plants with karyotype changes in both chromosome sets have been detected in one family of the experimental population, which indicates a possibility of sibling species appearance in the experimental population.

[Some radiobiological effects in higher plants growing at the territory of the East Ural radioactive trace]

The spontaneous level of cytogenetic damage in three plant species (Achyrophorus maculatus (Scop.) L., Plantago lanceolata L., Plantago media L.) growing at the territory of East Ural radioactive trace was studied. The radiation resistance of plants from radioactive and control nonpolluted sites was determined. The effects of additional fractionated irradiation by different doses and the role of antioxidant systems in the formation of radioprotector effect were examined. It was shown that the level of mutation process in the plant populations growing at the radiation polluted sites is increased compared to the control populations from the pure territory. The additional acute gamma-irradiation of seeds collected from the polluted and pure territories showed the improved radiation resistance of the plants from the polluted territory. In the control population of A. maculatus in the versions with a one-hour interval between fractions, the radiation effect follows the additivity principle; in the same time, at a one-day interval between fractions, a highly significant radioprotective effect manifested most clearly in the experimental population is induced. For higher plants, the enhanced effectiveness of the functioning of antioxidant systems in plants growing at radiation polluted territories was first shown. Thus, the radioprotector mechanisms of low-dose chronic and preliminary irradiation are similar and one of these mechanisms is the activation of antioxidant systems in plants growing under conditions of chronic low-intensity irradiation for long periods of time.

Replication protein A (RPA1a) is required for meiotic and somatic DNA repair but is dispensable for DNA replication and homologous recombination in rice

Replication protein A (RPA), a highly conserved single-stranded DNA-binding protein in eukaryotes, is a stable complex comprising three subunits termed RPA1, RPA2, and RPA3. RPA is required for multiple processes in DNA metabolism such as replication, repair, and homologous recombination in yeast (Saccharomyces cerevisiae) and human. Most eukaryotic organisms, including fungi, insects, and vertebrates, have only a single RPA gene that encodes each RPA subunit. Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), however, possess multiple copies of an RPA gene. Rice has three paralogs each of RPA1 and RPA2, and one for RPA3. Previous studies have established their biochemical interactions in vitro and in vivo, but little is known about their exact function in rice. We examined the function of OsRPA1a in rice using a T-DNA insertional mutant. The osrpa1a mutants had a normal phenotype during vegetative growth but were sterile at the reproductive stage. Cytological examination confirmed that no embryo sac formed in female meiocytes and that abnormal chromosomal fragmentation occurred in male meiocytes after anaphase I. Compared with wild type, the osrpa1a mutant showed no visible defects in mitosis and chromosome pairing and synapsis during meiosis. In addition, the osrpa1a mutant was hypersensitive to ultraviolet-C irradiation and the DNA-damaging agents mitomycin C and methyl methanesulfonate. Thus, our data suggest that OsRPA1a plays an essential role in DNA repair but may not participate in, or at least is dispensable for, DNA replication and homologous recombination in rice.

Replication protein A (RPA1a) is required for meiotic and somatic DNA repair but is dispensable for DNA replication and homologous recombination in rice

Replication protein A (RPA), a highly conserved single-stranded DNA-binding protein in eukaryotes, is a stable complex comprising three subunits termed RPA1, RPA2, and RPA3. RPA is required for multiple processes in DNA metabolism such as replication, repair, and homologous recombination in yeast (Saccharomyces cerevisiae) and human. Most eukaryotic organisms, including fungi, insects, and vertebrates, have only a single RPA gene that encodes each RPA subunit. Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa), however, possess multiple copies of an RPA gene. Rice has three paralogs each of RPA1 and RPA2, and one for RPA3. Previous studies have established their biochemical interactions in vitro and in vivo, but little is known about their exact function in rice. We examined the function of OsRPA1a in rice using a T-DNA insertional mutant. The osrpa1a mutants had a normal phenotype during vegetative growth but were sterile at the reproductive stage. Cytological examination confirmed that no embryo sac formed in female meiocytes and that abnormal chromosomal fragmentation occurred in male meiocytes after anaphase I. Compared with wild type, the osrpa1a mutant showed no visible defects in mitosis and chromosome pairing and synapsis during meiosis. In addition, the osrpa1a mutant was hypersensitive to ultraviolet-C irradiation and the DNA-damaging agents mitomycin C and methyl methanesulfonate. Thus, our data suggest that OsRPA1a plays an essential role in DNA repair but may not participate in, or at least is dispensable for, DNA replication and homologous recombination in rice.

Effects of heavy-ion beams on chromosomes of common wheat, Triticum aestivum

To investigate the nature of plant chromosomes irradiated by heavy-ion beams, the effects of nitrogen (N) and neon (Ne) ion beams on hexaploid wheat chromosomes were compared with those of X-ray. Chromosome aberrations, such as short, ring and dicentric chromosomes appeared in high frequency. The average numbers of chromosome breaks at LD-50 by irradiation with X-ray, N and Ne ion beams were 32, 20 and 20, respectively. These values may be underestimated because chromosome rearrangement without change in chromosome morphology was not counted. Thus, we subsequently used a wheat line with a pair of extra chromosomes from an alien species (Leymus racemosus) and observed the fate of the irradiated marker chromosomes by genomic in situ hybridization. This analysis revealed that 50Gy of neon beam induced about eight times more breaks than those induced by X-ray. This result suggests that heavy-ion beams induce chromosome rearrangement in high frequency rather than loss of gene function. This suggests further that most of the novel mutations produced by ion beam irradiation, which have been used in plant breeding, may not be caused by ordinary gene disruption but by chromosome rearrangements.

[Cytological and SSR analysis of irradiation-induced progenies of Trititrigia substitution line SN0095]

Pollens of Trititrigia substitution line SN0095 were radiated with 60Co-gamma ray and cytological characters of the progenies M1 and M2 were studied. The results showed that variations with different frequencies and chromosome numbers (2n=41, 2n=40 and 2n=39) were observed in both M1 and M2. Abnormal phenomena, such as univalents, multivalents, chromosome fragments, laggard chromosomes, chromosome bridges and micronucleus, appeared in high frequencies during the meiosis. It suggested that irradiation could promote changes of chromosome number and configuration efficiently, and could lead to rearrangements or translocations between chromosomes. The M2 progenies were examined using BARC159(240), a Thinopyrum intermedium-specific SSR marker of SN0095. Most M2 plants carried the locus BARC159(240), which indicated that the specific segment involving the locus BARC159(240) of Thinopyrum intermedium existed in these plants. However, a band of common wheat YN15 disappeared in a few plants although the locus BARC159(240) existed, which implied that chromosome rearrangements may have occurred in these plants.

[Cytogenetic effects in Scots pine populations from the Briansk region contaminated by radioactive pollutants as a result of the Chernobyl NPP accident]

Aberrant cell frequency in root meristem of germinated seeds collected from four populations of Scots pine in the Bryansk Region that was radioactively contaminated as a result of the accident at the Chernobyl NPP in 1986 significantly exceeded the control level durring all three years of study (2003-2005). An analysis of cytogenetic disturbances occurrence in dependence on radiation situation characteristics such as 137Cs and 90Sr content in pine cones, 137Cs specific activity in soil, and calculated doses absorbed by pine tree generative organs shows an increase in biological effect with dose burden increasing. Findings obtained are in agreement with the results of our previous studies on cytogenetic effects induction in Scots pine populations experiencing chronic radiation (the 30-km zone of the ChNPP) and technogenic (a radioactive waste reprocessing facility) impact.

[The estimation of ionizing irradiation impact on Vicia cracca cenopopulations growing on the territory contaminated with the wastes of radium production]

The radiation exposure on cenopopulations Vicea cracca L. growing on the territory contaminated with the wastes of radium production was estimated. The relationship between the chromosome aberration in seedlings root tip cells and irradiation dose was found to be linear. The significant cytogenetic effects in chronically irradiated Vicia cracca cenopopulation are observed at doses (equal to 0.006-0.7 Gy) 10 times upward the natural radiation background level. The reduced reproductive success (significantly increased embryonic lethal mutation level) is observed at weighted absorbed doses (equal to 0.2-0.7 Gy) up to 200-700 times higher than the natural radiation background level. It is showh, that the radiation hygienic standards of permissible exposure are noticeably stricter that the radioecological limits.

[Effect of antioxidant-ambiol on the radiation adaptive response]

For studying of the mechanism of adaptive response of plants the seeds of soft wheat Triticum aestivum cultivar Moscovskaya 39 were irradiated in doses 0.25, 50 and 0.25 + 50 Gy and the frequency of cells with aberrations and the mitotic activity in the meristem of seedlings were scored. The pre- and post-treatments of seeds with antioxidant--ambiol were also used. It was found that the exposure of seeds to 0.25 Gy reduce the effects of challenge dose of 50 Gy: the mitotic index increases and the frequency of cells with aberrations decreases--the adaptive response appear. It was also found that the pretreatment with ambiol reduce the effects of the irradiation in the dose of 50 Gy. Post-treatment was less efficiently. Both treatments raise the adaptive response. The correlation between the frequency of aberrant cells and the mitotic index was found and, regardless of the type of treatment all points of experiment fall on the common regression line with the regression coefficient -0.85 (p < 0.01). These facts serve as evidence (1) that the radioprotective effect by the pre- and post-treatment occurs by a common mechanism and (2) that the in the exhausted concentration antioxidant does not change the extent of genome damage inflicted by irradiation. The evidence is consistent with the hypothesis that a nonspecific inducible process of stimulated repopulation was a mechanism of adaptive response of plants.

Analysis of cytogenetic damage in rice seeds induced by energetic heavy ions on-ground and after spaceflight

To analyze the biological effects of the space environment, we flew nine lines of rice seeds on the Chinese 20(th) recoverable satellite for a duration of 18 days. The same lines of seeds were also irradiated to low doses (2.0 mGy) of Carbon, or Neon or Iron ions (with different LET value of 13.3 keV/microm, 31 keV/microm and 500 keV/microm respectively) at National Institute of Radiological Sciences in Chiba, Japan. The total number of mitotic cells and chromosomal aberrations were analyzed. The mitotic index (MI) and the frequency of chromosomal aberration were evaluated in order to compare the cytogenetic damages from spaceflight and from exposure to similar doses of charged particle on the ground. The results of the present study show that the space environment and heavy energy ion can alter cell growth, and induce various chromosome aberrations including micronuclei, chromosomal bridges, fragments and laggards. With all the lines combined, the frequency of chromosome aberrations and MI in seeds flown in space are the highest. The effectiveness of cytogenetic damage from spaceflight (SP) and the heavy ion irradiations is SP > Fe > Ne > C.

[The influence of cadmium and potassium on the level of cytogenetic effects induced by thorium-232 in Allium cepa root meristem]

The influence of different concentrations of cadmium and potassium on the levels of mutagenic and cytotoxic effects induced by thallium-232 in Allium cepa root meristem has been studied. The combined action of 232Th (0.8 microM) with cadmium in non-toxic (0.009 microM) and toxic (5 microM) concentrations resulted in sinergetic increase of the frequency of aberrant cells in Allium cepa root meristem. Decrease of the mutagenic effect to the additive level and antagonism with respect to the cytotoxic one was observed only at the certain concentrations of 232Th (0.8 microM) and Cd (0.09 microM) and the time of impact 30 h. In contrast to the heavy metal cadmium the essential for plants potassium at all studied concentrations (0.008, 6, 13 mM) decreased the number of cytogenetic aberrations in control experiments and under the effect of 232Th. The maximum protective effect of potassium was detected at the concentration 13 mM.

[Dynamics of the induced chromosomal instability in welsh onion (Allium fistulosum L.): gamma irradiation of the seeds of different storage periods]

The chromosome aberrations in root meristem cells of welsh onion (Allium fistulosum L.) seeds after gamma-irradiation (5 and 10 Gy) of different-aged seeds (7, 19, 31, 43 and 55 months of storage) were studied. The irradiation dose of 5 Gy significantly increased the frequency of aberrant anaphases (FAA) for 31- and 43-months seeds; the dose of 10 Gy significantly increased the FAA in seeds of all age groups. The irradiation of young (7 months) seeds resulted in decreasing of the fraction of bridges to the control level of the old (55-months) seeds for the dose of 5 Gy and below the control level of the old seeds--for the dose of 10 Gy. Some peculiarities of cytogenetic parameters of genome instability and the germinating capacity of the seeds made it possible to suppose that the third year of storage is a critical period for the welsh onion seeds.

Joint effects of microwave and chromium trioxide on root tip cells of Vicia faba

The mutagenic effects of microwave and chromium trioxide (CrO(3)) on Vicia faba root tip were studied. Micronucleus assay and chromosomal aberration assay were used to determine the mitotic index, the micronucleus frequency and chromosomal aberration frequency of Vicia faba root tip cells induced by microwave and CrO(3). The results showed that the micronucleus frequency decreased, and that the mitotic index and chromosomal aberration frequency showed linear dose responses to CrO(3), in treatment of microwave for 5 s. In microwave of 25 s, the mitotic index decreased, the micronucleus frequency and chromosomal aberration frequency increased with increase of CrO(3) concentration. We concluded that microwave and CrO(3) had antagonistic effect on the mitotic index of Vicia faba root tip cells, but had synergetic effect on micronucleus frequency and chromosomal aberration frequency of Vicia faba root tip cells.

[Radiation and genetic consequences of ionizing irradiation on population of Pinus sylvestris L. within the zone of the Chernobyl NPP]

Main results of the nineteen year radiation-genetic monitoring of radiobiological, of cytogenetic, and of genetic parameters of Pinus sylvestris L. of forest plantation from the Chernobyl meltdown area are presented. The acute ionizing irradiation at doses >1 Gy induces the formation of morphoses and of depressed growth; at doses >2 Gy, the reproductive ability of the trees declined. The radiobiological parameters shown linear or close to it dose-dependence relationship. The acute irradiation at dose of 0.5 Gy induces cytogenetic and genetic effects that were significantly higher than the corresponding control values. The relationship between the cytogenetic effects and the absorbed dose was exponential. The dependence of mutation frequency at specific loci on the absorbed dose was described by a nonlinear curve. The results of the cytogenetic analysis of seedlings obtained from seeds annually collected in zones of slight, of moderate, and of strong damage of Pinus sylvestris L. are presented.

[Genetic effects in plant populations in the zone of the Chernobyl accident]

During 6 years, starting from 1986, the monitoring of the dynamics of the frequency of embryo lethal and of chlorophyll mutations was carried out in arabidopsis populations in areas with different levels of radioactive contamination by the Muller embryo-test in the 30 km of ChNPP. The dose rate of chronic irradiation in the examined areas varied from 0.014 to 17 nA/Kg. Monitoring of the dynamics of the mutation process in natural arabidopsis populations showed the correlation between the level of the mutation process and the dose rate of chronic irradiation. The genetic effects of different levels of radioactive contamination were estimated by determining the frequency of mutations occurred in this generation and by calculating the dose of irradiation of one was found. That the dependence of the mutation frequency on the dose of irradiation presents a power function with a power index less 1, which suggests a higher efficiency of low radiation doses per unit dose. Possible explanations of this phenomenon are considered in the work. The studies of cytogenetic effects in chronically exposed Crepis tectorum populations in the zones of the Chernobyl accident showed that starting from the second year after the Chernobyl disaster there appeared plants with an altered karyotype and their frequency of chromosome aberrations correlates in root meristem cells.

Wide-cross whole-genome radiation hybrid mapping of the cotton (Gossypium barbadense L.) genome

Whole-genome radiation hybrid mapping has been applied extensively to human and certain animal species, but little to plants. We recently demonstrated an alternative mapping approach in cotton (Gossypium hirsutum L.), based on segmentation by 5-krad gamma-irradiation and derivation of wide-cross whole-genome radiation hybrids (WWRHs). However, limitations observed at the 5-krad level suggested that higher doses might be advantageous. Here, we describe the development of an improved second-generation WWRH panel after higher dose irradiation and compare the resulting map to the 5-krad map. The genome of G. hirsutum (n = 26) was used to rescue the radiation-segmented genome of G. barbadense (n = 26) introduced via 8- and 12-krad gamma-irradiated pollen. Viable seedlings were not recovered after 12-krad irradiation, but 8-krad irradiation permitted plant recovery and construction of a 92-member WWRH mapping panel. Assessment of 31 SSR marker loci from four chromosomes revealed that the 8-krad panel has a marker retention frequency of ca. 76%, which is approximately equivalent to the rate of loss in a low-dose animal radiation hybrid panel. Retention frequencies of loci did not depart significantly from independence when compared between the A and D subgenomes, or according to positions along individual chromosomes. WWRH maps of chromosomes 10 and 17 were generated by the maximum likelihood RHMAP program and the general retention model. The resulting maps bolster evidence that WWRH mapping complements traditional linkage mapping and works in cotton, and that the 8-krad panel complements the 5-krad panel by offering higher rates of chromosome breakages, lower marker retention frequency, and more retention patterns.

[Chronic irradiation with low doses can be characterized by significant biological efficacy]

Chromosomal aberrations (ChA) level was analyzed in the onion root meristem after the chronic irradiation with different dose capacities. It was shown that after the chronic irradiation with doses of 0.87 cGy, 2.61 cGy and 4.35 cGy the level of chromosomal aberrations depended on the dose capacity. Its value may also correspond to those which have been induced with accute irradiation. Biological efficacy of chronic irradiation may be from 20 to 1000 time folder in order to compare it with accute irradiation and this value depends on the irradiation regime.

DNA damage processing and aberration formation in plants

Various types of DNA damage, induced by endo- and exogenous genotoxic impacts, may become processed into structural chromosome changes such as sister chromatid exchanges (SCEs) and chromosomal aberrations. Chromosomal aberrations occur preferentially within heterochromatic regions composed mainly of repetitive sequences. Most of the preclastogenic damage is correctly repaired by different repair mechanisms. For instance, after N-methyl-N-nitrosourea treatment one SCE is formed per >40,000 and one chromatid-type aberration per approximately 25 million primarily induced O6-methylguanine residues in Vicia faba. Double-strand breaks (DSBs) apparently represent the critical lesions for the generation of chromosome structural changes by erroneous reciprocal recombination repair. Usually two DSBs have to interact in cis or trans to form a chromosomal aberration. Indirect evidence is at hand for plants indicating that chromatid-type aberrations mediated by S phase-dependent mutagens are generated by post-replication (mis)repair of DSBs resulting from (rare) interference of repair and replication processes at the sites of lesions, mainly within repetitive sequences of heterochromatic regions. The proportion of DSBs yielding structural changes via misrepair has still to be established when DSBs, induced at predetermined positions, can be quantified and related to the number of SCEs and chromosomal aberrations that appear at these loci after DSB induction. Recording the degree of association of homologous chromosome territories (by chromosome painting) and of punctual homologous pairing frequency along these territories during and after mutagen treatment of wild-type versus hyperrecombination mutants of Arabidopsis thaliana, it will be elucidated as to what extent the interphase arrangement of chromosome territories becomes modified by critical lesions and contributes to homologous reciprocal recombination. This paper reviews the state of the art with respect to DNA damage processing in the course of aberration formation and the interphase arrangement of homologous chromosome territories as a structural prerequisite for homologous rearrangements in plants.

Molecular targets and mechanisms in formation of chromosomal aberrations: contributions of Soviet scientists

Studies of mechanisms for formation of chromosomal aberrations (CAs) with special emphasis on data from Soviet/Russian investigations are reviewed that argue in favor of a minor fraction of genomic DNA that forms specific molecular targets/contacts for the formation of chromosomal exchanges. This DNA is presumably associated with matrix attachment sites of DNA loops, enriched with AT base pairs and repetitive DNA sequences. It is assumed that there are two main mechanisms in formation of chromosome aberrations: 1) mutually reciprocal recombination, resulting in formation of all kinds of chromosome exchanges; 2) the process of telomere formation, resulting in the generation of true deletions. A significant part of chromosomal breaks and apparently unrejoined ends in incomplete exchanges as seen with cytogenetic techniques reflect decondensation in the discrete units of chromatin organization such as the megabase-size DNA domains. The possible ways for further analysis of alternative theories with emerging technologies are also discussed.

On the nature of visible chromosomal gaps and breaks

From the earliest days of chromosomal aberration studies, the distinction, nature and origin of light-microscope observed "gaps" and "breaks" have been topics for debate and controversy. In this paper we survey, briefly, the various ideas that have appeared in the very extensive literature, and attempt to evaluate them in the light of our current understanding of chromosome structure and aberration formation. Attention is drawn to the problems of interpretation caused by G2/S cell imprecision.

[Evaluation of the effect of pollen irradiation on karyotype variability in M2 cotton plants]

The karyotypes of biomorphologically abnormal cotton (Gossypium hirsutum L.) plants obtained in M2 after pollination with pollen irradiated at dose rates 10, 15, 20, and 25 Gy were studied. Various genomic and chromosomal mutations were detected in 57 M2 families. The primary monosomics isolated in M2 were found to be cytologically more stable and more viable, since they had higher meiotic indices, pollen fertility, and seed formation. In M2, a decrease in the number of plants with multiple karyotype aberrations and interchromosomal exchanges with high frequency of multivalent formation was observed. The multivalents had diverse patterns and types of chromosome segregation and translocation complexes. Their pollen fertility was higher than in translocants found in M1. Desynapsis often occurred in M2, including plants with chromosome deficiency or rearrangements. The variation in the number of univalents in various cells was found to result from different expression of synaptic genes. The results indicate stabilization of karyotypes, increase in cytologic stability and viability, and the absence of sterility in aberrant plants.

Rapid detection of aneuploidy in Musa using flow cytometry

We report a procedure for the rapid and convenient detection of aneuploidy in triploid Musa using DNA flow cytometry. From a population of plants derived from gamma-irradiated shoot tips, plants were selected based on aberrant morphology and their chromosome numbers were counted. Aneuploids plants with chromosome numbers 2n=31 or 32 were found as well as the expected triploid plants (2n=3x=33). At the same time, the nuclear DNA content of all plants was measured using flow cytometry. The flow cytometric assay involved the use of nuclei isolated from chicken red blood cells (CRBC), which served as an internal reference standard. The relative DNA content of individual plants was expressed as a ratio of DNA content of CRBC and Musa (DNA index). In order to estimate the chromosome number using flow cytometry, the relative DNA content of plants with unknown ploidy was expressed as a percentage of the DNA content of triploid plants. The classification based on flow cytometry fully agreed with the results obtained by chromosome counting. The results indicated that flow cytometry is a convenient and rapid method for the detection of aneuploidy in Musa.