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

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.

Methomyl, imbraclaobrid and clethodim induced cytomixis and syncytes behaviors in PMCs of Pisum sativum L: Causes and outcomes

Cytomixis is a common phenomenon observed in meiotic cells such as anther which is influenced by various factors. Use of pesticides is a common practice in agriculture. However, it is not known whether pesticides can induce cytomixis in plant cells and induce genetic variation. To understand this, the present study was planned to assess the cytomixis and syncytes behaviors in PMCs of Pisum sativum L. Seeds of P. sativum (Family: Fabaceae) were treated with different concentrations of commonly used pesticides methomyl (ME), imbraclaobrid (IM) and clethodim (CL). Seeds were treated with various concentrations (0.1, 0.2, 0.3, 0.4 and 0.5% of ME, IM and CL prepared in water) for 1 and 3 h. Effect of pesticides on pollen fertility, frequency of cytomixis, and kind of cytomixis cells was assessed. In the cytomixis cells, the cytomictic channel (CC) and direct fusion (DF), and various stages of meiosis (PI, MI, AI and TI) with cytomixis cells were observed. In addition, frequency of syncytes cell and their various stages of meiosis I (PI, MI, AI and TI) in pollen mother cells (PMCs) was assessed. During the microsporogenesis in P. sativum, the occurrence of cytomixis and syncytes at various stages of meiosis I were seen. The formation of cytoplasmic channels and direct fusing of pollen mother cells (PMCs) were both seen to cause cytomixis, with the former being more common than the latter. The percentage of PMCs with cytomixis and syncytes cells increased with increase in the concentration of pesticides. The result of the present investigation indicates that commonly used pesticides ME, IM, and CL have a significant effect on pollen fertility, frequency of cytomixis, and kind of cytomixis cells, the cytomictic channel (CC) and direct fusion (DF), in addition, frequency of syncytes cell and their various stages of meiosis I (PI, MI, AI and TI) in pollen mother cells (PMCs) on P. sativum.

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.

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.

FlowerMorphology: fully automatic flower morphometry software

MAIN CONCLUSION

The software FlowerMorphology is designed for automatic morphometry of actinomorphic flowers. The novel complex parameters of flowers calculated by FlowerMorphology allowed us to quantitatively characterize a polyploid series of tobacco. Morphological differences of plants representing closely related lineages or mutants are mostly quantitative. Very often, there are only very fine variations in plant morphology. Therefore, accurate and high-throughput methods are needed for their quantification. In addition, new characteristics are necessary for reliable detection of subtle changes in morphology. FlowerMorphology is an all-in-one software package to automatically image and analyze five-petal actinomorphic flowers of the dicotyledonous plants. Sixteen directly measured parameters and ten calculated complex parameters of a flower allow us to characterize variations with high accuracy. The program was developed for the needs of automatic characterization of Nicotiana tabacum flowers, but is applicable to many other plants with five-petal actinomorphic flowers and can be adopted for flowers of other merosity. A genetically similar polyploid series of N. tabacum plants was used to investigate differences in flower morphology. For the first time, we could quantify the dependence between ploidy and size and form of the tobacco flowers. We found that the radius of inner petal incisions shows a persistent positive correlation with the chromosome number. In contrast, a commonly used parameter-radius of outer corolla-does not discriminate 2n and 4n plants. Other parameters show that polyploidy leads to significant aberrations in flower symmetry and are also positively correlated with chromosome number. Executables of FlowerMorphology, source code, documentation, and examples are available at the program website: https://github.com/Deyneko/FlowerMorphology .

Evaluation of DNA damage in tobacco male meiocytes involved in cytomixis using comet assay

Cytomixis is a process of nuclear migration between plant cells. As a rule, it is detectable in male meiocytes and gives rise to the cells with micronuclei. Examination of the integrity and functional state of migrating chromatin is of great interest, since cytomixis is assumed to change the gamete karyotype. We, for the first time, used comet assay to assess the DNA integrity in the chromatin that migrates between plant meiocytes. As was shown, the cells involved in cytomixis are viable and display no signs of DNA damage. Any comet tails are undetectable in both the main nuclei of the cells involved in cytomixis and cytomictic micronuclei. On the other hand, the cytomictic micronuclei after heat shock (positive control) form typical comet tails.

New chromosome reports in Lamiaceae of Kashmir (Northwest Himalaya), India

Meiotic studies and chromosome data are imperative in order to have an overall germplasm evaluation of a taxon. In the present effort, the meiotic study is carried out in 48 populations belonging to 26 species of Lamiaceae collected from their natural habitats in Kashmir Himalaya, which forms an important part of Northwest Himalaya. Chromosome counts in the five species viz. Dracocephalum nutans (2n = 10), Lycopus europaeus (2n = 22), Marrubium vulgare (2n = 54), Nepeta nervosa (2n = 18) and Salvia sclarea (2n = 22) are first time reported from India. Besides, 17 species are cytologically evaluated for the first time from the study area-Kashmir Himalaya. In Marrubium vulgare, hexaploid cytotype (2n = 6 × =54) is reported for the first time. Also, diploid and tetraploid cytomorphovariants are observed in Calamintha vulgaris (2n = 20, 40), Elsholtzia ciliata (2n = 16, 32) and Mentha longifolia (2n = 20, 40). Various meiotic abnormalities like chromatin stickiness, cytomixis, nonsynchronous disjunction, laggards, chromatin bridges, etc. leading to pollen abnormalities have been documented for the first time in some species. The worldwide status of chromosome number data in each genus is presented.

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.