Sergey Mursalimov

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.

An ultrastructural study of cytomixis in tobacco pollen mother cells

Intercellular chromatin migration (cytomixis) in the pollen mother cells of two tobacco (Nicotiana tabacum L.) lines was analyzed by electron microscopy during the first meiotic prophase. The maximal manifestation of cytomixis was observed in the pachytene. As a rule, several cells connected with one another by cytomictic channels wherein the nuclei migrated were observable at this stage. In the majority of cases, nuclei passed from cell to cell concurrently through several closely located cytomictic channels. Chromatin migrated between cells within the nuclear envelope, and its disintegration was unobservable. The nucleus, after passing through cytomictic channels into another cell, can be divided into individual micronuclei or, in the case of a direct contact with another nucleus, can form a nuclear bridge. It has been demonstrated that the chromatin structure after intracellular migration visually matches the chromatin structure before it passed through the cytomictic channel. No signs of pyknosis were observable in the chromatin of the micronuclei formed after cytomixis, and the synaptonemal complex was distinctly seen. The dynamics of changes in the nucleoli during cytomixis was for the first time monitored on an ultrastructural level. Possible mechanisms determining cytomixis are discussed and the significance of this process in plant development is considered.

How cytomixis can form unreduced gametes in tobacco

Cytomixis is migration of the nuclei between cells, widespread in various higher plant species. Most frequently, cytomixis takes place in microsporogenesis and is assumed to be a possible cause of unreduced gamete generation. In the present work the cytological mechanisms leading to a change in the cell chromosome number via cytomixis in tobacco microsporocytes are described. The amount of chromatin migrating between cells in cytomixis may be different, varying from a single bivalent to the whole nucleus. It is shown that a whole nucleus when migrating from one cell to another displays no signs of injury or degradation and the formed binucleated microsporocyte continues its meiotic division. If individual nuclear fragments rather than the whole nucleus migrate between cells, one or several micronuclei are formed in the recipient cell, which may contact the recipient cell nucleus. The most probable consequence of these events in tobacco microsporogenesis is the unreduced pollen formation.

An ultrastructural study of microsporogenesis in tobacco line SR1

This article provides an ultrastructural atlas of microsporogenesis in the tobacco model line SR1. The stages of cell-wall remodeling and reorganization of the intercellular channels, accompanying this process, are reported for the microspore mother cells. The meiotic changes in the cell nucleus and cytoplasm are traced. The appearance of single-, double-, or multi-membrane nuclear vacuoles in microspore mother cells and their further elimination from the nucleus are for the first time described for the genus Nicotiana as well as deviations from a normal course for this process. Intercellular chromatin migration (cytomixis) was observed in the microsporogenesis of the line SR1 and behavior of the nuclear vacuoles within the cytomictic nucleus was described for the first time. The enzymatic activity of spherosome-like vesicles in the tobacco microsporogenesis is discussed. The features of microsporogenesis in the tobacco line SR1 are compared with those of other plant species and its association with the transition from a diploid to a haploid phase of the life cycle is discussed.

Cytomixis doesn’t induce obvious changes in chromatin modifications and programmed cell death in tobacco male meiocytes

Cytomixis is a poorly studied process of nuclear migration between plant cells. It is so far unknown what drives cytomixis and what is the functional state of the chromatin migrating between cells. Using immunostaining, we have analyzed the distribution of posttranslational histone modifications (methylation, acetylation, and phosphorylation) that reflect the functional state of chromatin in the tobacco microsporocytes involved in cytomixis. We demonstrate that the chromatin in the cytomictic cells does not differ from the chromatin in intact microsporocytes according to all 14 analyzed histone modification types. We have also for the first time demonstrated that the migrating chromatin contains normal structures of the synaptonemal complex (SC) and lacks any signs of apoptosis. As has been shown, the chromatin migrating between cells in cytomixis is neither selectively heterochromatized nor degraded both before its migration to another cell and after it enters a recipient cell as micronuclei. We also showed that cytomictic chromatin contains marks typical for transcriptionally active chromatin as well as heterochromatin. Moreover, marks typical for chromosome condensation, SC formation and key proteins required for the formation of bivalents were also detected at migrated chromatin.

The role of spherosome-like vesicles in formation of cytomictic channels between tobacco microsporocytes

The formation of cytomictic channels (CCs) during the tobacco microsporogenesis has been analyzed by microscopy and cytochemical methods. Starting from the pachytene stage, CCs were formed between microsporocytes with involvement of specific organelles, the so-called spherosome-like vesicles. The presence of the enzyme callase, able to degrade callose and form CCs in the cell wall of microsporocytes, has been demonstrated for the first time in the spherosome-like vesicles. An active form of callase was detectable in the spherosome-like vesicles and cell wall but not in the endoplasmic reticulum and Golgi apparatus. The release of callase from spherosome-like vesicles into the cell wall was described. Two ways in formation of the CCs in the tobacco microsporogenesis, the primary formation in the cell wall composed of pectins and cellulose (leptotene-zygotene) and secondary formation in the cell wall of callose (after the pachytene stage), were compared.

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.

Distribution of telomeres in the tobacco meiotic nuclei during cytomixis.

Cytomixis is the migration of nuclei from one cell to another in higher plants, most frequently observable during microsporogenesis, which has a potential evolutionary significance. Currently, a major challenge is to label the chromatin migrating between cells to clarify its further fate. We have for the first time succeeded in visualizing the telomeric chromatin regions in the nuclei migrating between cells using fluorescent in situ hybridization. It has been shown that the telomeric signals in tobacco microsporocytes are randomly distributed in migrating nuclei without any deviations from their normal meiotic dynamics. According to our data, the chromatin migrating during cytomixis always contains telomeres and the telomeric signals are retained in the micronuclei formed after cytomixis.

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 (χ2 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.

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.