Cytomixis-like chromosomes/chromatin elimination from pollen mother cells (PMCs) in wheat-rye allopolyploids

Unusual nuclear structures in male meiocytes of wild-type rye as revealed by volume microscopy

BACKGROUND AND AIMS

During the analysis of plant male meiocytes coming from destroyed meiocyte columns (united multicellular structures formed by male meiocytes in each anther locule), a considerable amount of information becomes unavailable. Therefore, in this study intact meiocyte columns were studied by volume microscopy in wild-type rye for the most relevant presentation of 3-D structure of rye meiocytes throughout meiosis.

METHODS

We used two types of volume light microscopy: confocal laser scanning microscopy and non-confocal bright-field scanning microscopy combined with alcohol and aldehyde fixation, as well as serial block-face scanning electron microscopy.

KEY RESULTS

Unusual structures, called nuclear protuberances, were detected. At certain meiotic stages, nuclei formed protuberances that crossed the cell wall through intercellular channels and extended into the cytoplasm of neighbouring cells, while all other aspects of cell structure appeared to be normal. This phenomenon of intercellular nuclear migration (INM) was detected in most meiocytes at leptotene/zygotene. No cases of micronucleus formation or appearance of binucleated meiocytes were noticed. There were instances of direct contact between two nuclei during INM. No influence of fixation or of mechanical impact on the induction of INM was detected.

CONCLUSIONS

Intercellular nuclear migration in rye may be a programmed process (a normal part of rye male meiosis) or a tricky artefact that cannot be avoided in any way no matter which approach to meiocyte imaging is used. In both cases, INM seems to be an obligatory phenomenon that has previously been hidden by limitations of common microscopic techniques and by 2-D perception of plant male meiocytes. Intercellular nuclear migration cannot be ignored in any studies involving manipulations of rye anthers.

Intercellular nuclear migration in cryofixed tobacco male meiocytes

In this study, intercellular nuclear migration (INM), also known as cytomixis, was documented in cryofixed plant meiocytes for the first time. Intact tobacco inflorescences and flower buds as well as dissected individual anthers were cryofixed in liquid nitrogen by plunge freezing. Cryosubstituted and cryosectioned male meiocytes were analyzed by light microscopy. For cryosubstitution, the frozen material was kept in acetic alcohol at - 70 °C for 1 week. For cryosectioning, the frozen material was sectioned at - 20 °C, and fixed with precooled acetic alcohol. Fixation of the intact tobacco inflorescences in Carnoy's solution was used as a control. Microscopy revealed good preservation of cell structure in the cryofixed anthers, flower buds, and inflorescences. INM was detectable in all the studied cryofixed and chemically fixed samples. The cytological picture of INM observed in the cryofixed meiocytes did not noticeably differ from the picture obtained with the chemically fixed cells. These results indicate that INM is observable irrespective of whether a physical or chemical fixation method is employed, with minimal damage from handling. Our results contradict the notion that INM is a phenomenon caused by mechanical, osmotic, or chemical artifacts during sample preparation.

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.

Cytomixis in plants: facts and doubts

The migration of nuclei between plant cells (cytomixis) is a mysterious cellular phenomenon frequently observable in the male meiosis of higher plants. Cytomixis attracts attention because of unknown cellular mechanisms underlying migration of nuclei and its potential evolutionary significance, since the genetic material is transferred between the cells that form pollen. Although cytomixis was discovered over a century ago, the advance in our understanding of this process has been rather insignificant because of methodological difficulties. The data that allowed for a new insight into this phenomenon were obtained by examining the migrating nuclei with electron and confocal laser microscopy, immunostaining, and fluorescence in situ hybridization. As has been shown, the chromatin migrating between cells is surrounded by an undamaged nuclear membrane. Such chromatin does not undergo heterochromatization and contains normal euchromatin markers. The condensation degree of the migrating chromatin corresponds to the current meiotic stage, and normal structures of synaptonemal complex are present in the migrating part of the nucleus. The cells involved in cytomixis lack any detectable morphological and molecular markers of programmed cell death. It has been shown that individual chromosomes and genomes (in the case of allopolyploids) have no predisposition to the migration between cells, i.e., parts of the nucleus are involved in cytomixis in a random manner. However, the fate of migrating chromatin after it has entered the recipient cell is still vague. A huge amount of indirect data suggests that migrating chromatin is incorporated into the nucleus of the recipient cell; nonetheless, the corresponding direct evidences are still absent. No specific markers of cytomictic chromatin have been yet discovered. Thus, the causes and consequences of cytomixis are still disputable. This review briefs the recent data on the relevant issues, describes the classical and modern methodological approaches to analysis of the intercellular migration of nuclei, and discusses the problems in cytomixis research and its prospects.

Reorganization of wheat and rye genomes in octoploid triticale (× Triticosecale)

The analysis of early generations of triticale showed numerous rearrangements of the genome. Complexed transformation included loss of chromosomes, t-heterochromatin content changes and the emergence of retrotransposons in new locations. This study investigated certain aspects of genomic transformations in the early generations (F5 and F8) of the primary octoploid triticale derived from the cross of hexaploid wheat with the diploid rye. Most of the plants tested were hypoploid; among eliminated chromosomes were rye chromosomes 4R and 5R and variable number of wheat chromosomes. Wheat chromosomes were eliminated to a higher extent. The lower content of telomeric heterochromatin was also found in rye chromosomes in comparison with parental rye. Studying the location of selected retrotransposons from Ty1-copia and Ty3-gypsy families using fluorescence in situ hybridization revealed additional locations of these retrotransposons that were not present in chromosomes of parental species. ISSR, IRAP and REMAP analyses showed significant changes at the level of specific DNA nucleotide sequences. In most cases, the disappearance of certain types of bands was observed, less frequently new types of bands appeared, not present in parental species. This demonstrates the scale of genome rearrangement and, above all, the elimination of wheat and rye sequences, largely due to the reduction of chromosome number. With regard to the proportion of wheat to rye genome, the rye genome was more affected by the changes, thus this study was focused more on the rye genome. Observations suggest that genome reorganization is not finished in the F5 generation but is still ongoing in the F8 generation.

Cytomixis in tobacco microsporogenesis: are there any genome parts predisposed to migration?

Cytomixis is a poorly studied process of nuclear migration between plant cells, discovered in microsporogenesis of several hundreds of plant species. The chromosomes that migrate between tobacco microsporocytes have been for the first time identified using fluorescence in situ hybridization (FISH), and the question whether cytomixis is a random or a targeted process is answered. The distribution of four repetitive sequences used for identifying the tobacco chromosomes-NTRS, 5S rDNA, GRS, and HSR60-has been examined in the migrating chromatin, and the micronuclei formed after cytomixis. The distribution of tobacco S and T genomes has been analyzed in the cytomictic chromatin using genomic in situ hybridization (GISH). As has been shown (χ 2 test), the labeled DNA probes marking the listed sequences in tobacco genome are observed in the micronuclei formed after cytomixis with the probability not exceeding the theoretically expected value if cytomixis considered as a random process. Thus, it is shown that cytomixis is not a targeted process, and the chromosomes migrate between microsporocytes in a random manner.

Analysis of cytomixis in tobacco microsporocytes with confocal laser scanning microscopy

Confocal laser scanning microscopy for the first time is used to examine the structure of the tobacco microsporocytes involved in the intercellular migration of nuclei (cytomixis). As is observed, the cytomictic channels are distributed over the surface of tobacco microsporocytes in a non-random manner and their number depends on the meiotic stage. Analysis of non-squash cells demonstrates the differences in cytological patterns of cytomixis in a normal meiosis of control tobacco plants (SR1 line) and the abnormal meiosis of polyploids. As a rule, two to three adjacent cells are involved in cytomixis during meiosis of control tobacco plants; after cytomixis, several micronuclei are formed in recipient cells; cytoplasts (enucleated cells) are rare; and polyads are undetectable. In the meiosis of polyploids, cytomixis is massive, with a larger number of cells (sometimes, over ten) involved in nuclear migration simultaneously; recipient cells on completion of cytomixis develop tens of micronuclei; cytoplasts and polyads are frequently detectable.

A rise of ploidy level influences the rate of cytomixis in tobacco male meiosis

The effect of plant ploidy level on the rate of cytomixis in microsporogenesis has been analyzed with the help of a unique model, the collection of tobacco plants of different ploidies (2n = 2x = 24, 4x = 48, 6x = 72, and 8x = 96). As has been shown, the rate of cytomixis proportionally increases in 6x and 8x cytotypes, being rather similar in 2x and 4x plants. The rate of cytomixis is highly variable, differing even in the genetically identical plants grown under the same conditions. The cytological pattern of cytomixis in the microsporogenesis of control 4x plants has been compared with the corresponding patterns of 2x, 6x, and 8x plants. Involvement of cytomixis in production of unreduced gametes and stabilization of the newly formed hybrid and polyploidy genomes is discussed.

Divergent Development of Hexaploid Triticale by a Wheat - Rye -Psathyrostachys huashanica Trigeneric Hybrid Method

Hexaploid triticale is an important forage crop and a promising energy plant. Some forms were previously reported for developing the hexaploid triticale, such as crossing tetraploid wheat or hexaploid wheat with rye, crossing hexaploid triticale and/or hexaploid wheat with octoploid triticale, and spontaneously appearing in the selfed progenies of octoploid triticale. In the present study, we developed an effective method for production of diverse types of hexaploid triticale via wheat—rye—Psathyrostachys huashanica trigeneric hybrid. Genomic in situ hybridization (GISH) and fluorescence in situ hybridization (FISH) karyotyping revealed that D genome chromosomes were completely eliminated and the whole A, B, and R genome chromosomes were retained in three lines. More interestingly, the composite genome of the line K14-489-2 consisted of complete A and B genomes and chromosomes 1D, 2R, 3R, 4R, 5R, 6R, and 7R, that of line K14-491-2 was 12 A-genome (1A-6A), 14 B-genome (1B-7B), 12 R-genome (1R-3R, 5R-7R), and chromosomes 1D and 3D, and that of the line K14-547-1 had 26A/B and 14R chromosomes, plus one pair of centric 6BL/2DS translocations. This finding implies that some of D genome chromosomes can be spontaneously and stably incorporated into the hexaploid triticale. Additionally, a variety of high-molecular-weight glutenin subunits (HMW-GS) compositions were detected in the six hexaploid triticale lines, respectively. Besides, compared with its recurrent triticale parent Zhongsi828, these lines showed high level of resistance to stripe rust (Puccinia striiformis f. sp. tritici, Pst) pathogens prevalent in China, including V26/Gui 22. These new hexaploid triticales not only enhanced diversification of triticale but also could be utilized as valuable germplasm for wheat improvement.

Cytomixis in the cereal (Gramineae) microsporogenesis

The specific features in behavior of the nuclei and chromatin migrating through cytomictic channels as well as in formation of micronuclei in the cereal microsporogenesis have been studied. Immunofluorescence microscopy has allowed for demonstration that the tubulin cytoskeleton does not play a significant role in the intercellular migration of nuclei. Potential involvement of the actin cytoskeleton and SUN-KASH linker complexes in cytomixis is discussed. Comparative analysis of the published and own data suggests that the cytological patterns of cytomixis in monocots and dicots are conserved. As has been shown, each higher ploidy level in the polyploid series of the family Gramineae is accompanied by an increase in the rate of cytomixis independently of individual species. The results confirm the assumption on a correlation between the rate of cytomixis, ploidy level, and genome balance.

Spontaneous and divergent hexaploid triticales derived from common wheat × rye by complete elimination of D-genome chromosomes

BACKGROUND

Hexaploid triticale could be either synthesized by crossing tetraploid wheat with rye, or developed by crossing hexaploid wheat with a hexaploid triticale or an octoploid triticale.

METHODOLOGY/PRINCIPAL FINDINGS

Here two hexaploid triticales with great morphologic divergence derived from common wheat cultivar M8003 (Triticum aestivum L.) × Austrian rye (Secale cereale L.) were reported, exhibiting high resistance for powdery mildew and stripe rust and potential for wheat improvement. Sequential fluorescence in situ hybridization (FISH) and genomic in situ hybridization (GISH) karyotyping revealed that D-genome chromosomes were completely eliminated and the whole A-genome, B-genome and R-genome chromosomes were retained in both lines. Furthermore, plentiful alterations of wheat chromosomes including 5A and 7B were detected in both triticales and additionally altered 5B, 7A chromosome and restructured chromosome 2A was assayed in N9116H and N9116M, respectively, even after selfing for several decades. Besides, meiotic asynchrony was displayed and a variety of storage protein variations were assayed, especially in the HMW/LMW-GS region and secalins region in both triticales.

CONCLUSION

This study confirms that whole D-genome chromosomes could be preferentially eliminated in the hybrid of common wheat × rye, "genome shock" was accompanying the allopolyploidization of nascent triticales, and great morphologic divergence might result from the genetic variations. Moreover, new hexaploid triticale lines contributing potential resistance resources for wheat improvement were produced.

New insights into cytomixis: specific cellular features and prevalence in higher plants

The phenomenon of intercellular migration of nuclei in plant tissues (cytomixis) was discovered over a century ago, which has been followed by numerous attempts to clarify the essence of this process as well as to determine its causes and consequences. Most attention of researchers has been paid to cytomixis in microsporogenesis, since the transfer of part of genetic material between microsporocytes may influence the ploidy level of the produced pollen and, presumably, have an evolutionary significance. This review compiles the data on cytological pattern of cytomixis and proposes a scheme as to how cytomictic channels are formed and function in angiosperms. The prevalence of cytomixis in different plant taxa is analyzed using the published data. The causes, mechanisms, and consequences of the nuclear migration between cells in plant tissues are discussed.