Asynapsis and meiotic restitution in tomato male meiosis induced by heat stress

Susceptibility of the reproductive system to temperature fluctuations is a recurrent problem for crop production under a changing climate. The damage is complex as multiple processes in male and female gamete formation are affected, but in general, particularly pollen production is impaired. Here, the impact of short periods of elevated temperature on male meiosis of tomato (Solanum lycopersicon L.) is reported. Meiocytes in early stage flower buds exposed to heat stress (>35°C) exhibit impaired homolog synapsis resulting in partial to complete omission of chiasmata formation. In the absence of chiasmata, univalents segregate randomly developing unbalanced tetrads and polyads resulting in aneuploid spores. However, most heat-stressed meiotic buds primarily contain balanced dyads, indicating a propensity to execute meiotic restitution. With most meiocytes exhibiting a complete loss of chiasma formation and concomitantly showing a mitotic-like division, heat stress triggers first division restitution resulting in clonal spores. These findings corroborate with the plasticity of male meiosis under heat and establish a natural route for the induction of sexual polyploidization in plants and the engineering of clonal seed.

Genomic and Meiotic Changes Accompanying Polyploidization

Hybridization and polyploidy have been considered as significant evolutionary forces in adaptation and speciation, especially among plants. Interspecific gene flow generates novel genetic variants adaptable to different environments, but it is also a gene introgression mechanism in crops to increase their agronomical yield. An estimate of 9% of interspecific hybridization has been reported although the frequency varies among taxa. Homoploid hybrid speciation is rare compared to allopolyploidy. Chromosome doubling after hybridization is the result of cellular defects produced mainly during meiosis. Unreduced gametes, which are formed at an average frequency of 2.52% across species, are the result of altered spindle organization or orientation, disturbed kinetochore functioning, abnormal cytokinesis, or loss of any meiotic division. Meiotic changes and their genetic basis, leading to the cytological diploidization of allopolyploids, are just beginning to be understood especially in wheat. However, the nature and mode of action of homoeologous recombination suppressor genes are poorly understood in other allopolyploids. The merger of two independent genomes causes a deep modification of their architecture, gene expression, and molecular interactions leading to the phenotype. We provide an overview of genomic changes and transcriptomic modifications that particularly occur at the early stages of allopolyploid formation.

Hazelnut Pollen Phenotyping Using Label-Free Impedance Flow Cytometry

Impedance flow cytometry (IFC) is a versatile lab-on-chip technology which enables fast and label-free analysis of pollen grains in various plant species, promising new research possibilities in agriculture and plant breeding. Hazelnut is a monoecious, anemophilous species, exhibiting sporophytic self-incompatibility. Its pollen is dispersed by wind in midwinter when temperatures are still low and relative humidity is usually high. Previous research found that hazelnut can be characterized by high degrees of pollen sterility following a reciprocal chromosome translocation occurring in some cultivated genotypes. In this study, IFC was used for the first time to characterize hazelnut pollen biology. IFC was validated via dye exclusion in microscopy and employed to (i) follow pollen hydration over time to define the best pre-hydration treatment for pollen viability evaluation; (ii) test hazelnut pollen viability and sterility on 33 cultivars grown in a collection field located in central Italy, and two wild hazelnuts. The accessions were also characterized by their amount and distribution of catkins in the tree canopy. Pollen sterility rate greatly varied among hazelnut accessions, with one main group of highly sterile cultivars and a second group, comprising wild genotypes and the remaining cultivars, producing good quality pollen. The results support the hypothesis of recurring reciprocal translocation events in Corylus avellana cultivars, leading to the observed gametic semi-sterility. The measured hazelnut pollen viability was also strongly influenced by pollen hydration (R adj 2 = 0.83, P ≤ 0.0001) and reached its maximum at around 6 h of pre-hydration in humid chambers. Viable and dead pollen were best discriminated at around the same time of pollen pre-hydration, suggesting that high humidity levels are required for hazelnut pollen to maintain its functionality. Altogether, our results detail the value of impedance flow cytometry for high throughput phenotyping of hazelnut pollen. Further research is required to clarify the causes of pollen sterility in hazelnut, to confirm the role of reciprocal chromosome translocations and to investigate its effects on plant productivity.

Meiotic chromosome behavior of the male-fertile allotriploid lily cultivar 'Cocossa'

Cytological observations of microsporogenesis in the allotriploid lily cultivar 'Cocossa' showed that viable pollen production could be attributed mainly to disoriented spindles, abnormal cytokinesis, and cytomixis during male meiosis. To identify the reasons why the allotriploid lily cultivar 'Cocossa' can produce aneuploid and euploid functional male gametes and can be used as the paternal parent in lily introgression breeding, we performed a detailed investigation of microsporogenesis using the conventional cytological methods. The allotriploid not only produced single pollen grains with variable sizes but also produced adherent pollen grains. Pollen viability was estimated at 50.1% based on staining and 30.8% based on germination. Based on the chromosomal analysis of BC₂ plants derived from Oriental cultivars (♀) crossed with the OOT cultivar 'Cocossa' (♂), it was concluded that the objective allotriploid contributed haploid (x), diploid (2x), and aneuploid chromosome complements. Common meiotic abnormalities were observed, indicating the high genetic imbalance of this allotriploid. In addition to normally oriented metaphase II spindles (linear and perpendicular), abnormal spindles, such as parallel, tripolar, fused, and multiple spindles, accounted for 6.21, 6.41, 14.27, and 1.17%, respectively. Tripolar and fused spindles resulted in the production of triads and dyads, which contributed to unreduced pollen production. Some microsporocytes exhibited complete or partial absence of cytokinesis, which led to relatively high frequencies of monads, dyads, and triads. Furthermore, the phenomenon of cytomixis during microsporogenesis occurred mainly in the first meiotic prophase and early development of pollen grains, which we assume is a possible cause of unreduced gamete generation. Our study offers a new resource for lily introgression breeding.

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.

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.

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.

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 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.

Cytomictic anomalous male meiosis and 2n pollen grain formation in Mertensia echioides Benth. (Boraginaceae) from Kashmir Himalaya

Presently Mertensia echioides Benth. (Boraginaceae) collected from Kashmir Himalaya, India, is cytologically analyzed for the first time revealing 2n = 2x = 24 (diploid). Interestingly we found 4.3-6.2% syncytic meiocytes/PMCs with 2n = 4x = 48 (tetraploid) in addition to normal meiocytes (2n = 24) during male meiosis. These comparatively larger PMCs (pollen mother cells) lead to the formation of fertile giant 2n pollen grains. A frequency of 6.4-13.3% PMCs shows transfer of chromatin material at prophase-I and, therefore, results in aneuploid meiocytes. Whole chromatin transfer by the process of cytomixis could also have led to the formation of tetraploid cells. Translocation heterozygosity is also evident in the form of multivalents in 12-17% diploid (2x) meiocytes at diakinesis and metaphase-I and is reported for the first time in this species. The syncytes formed depict open chain hexavalent and quadrivalent formation in the three populations with different frequencies. Moreover chromatin stickiness at metaphase-I is observed in 45% of PMCs in population-1 (P-1). Syncyte or unreduced PMC formation leading to unreduced fertile gametes is here speculated to act as a possible way out for infraspecific polyploidization in the species.

Meiotic studies in some species of tribe Cichorieae (Asteraceae) from Western Himalayas

The present paper deals with meiotic studies in 15 species belonging to 6 genera of the tribe Cichorieae from various localities of Western Himalayas. The chromosome number has been reported for the first time in Hieracium crocatum (2n = 10) and Lactuca lessertiana (2n = 2x = 16). Further, intraspecific variability has been reported for the first time in H. umbellatum (2n = 2x = 10 and 2n = 6x = 54), Tragopogon dubius (2n = 2x = 14 and 2n = 4x = 28), and T. gracilis (2n = 2x = 14). The chromosome report of 2n = 2x = 10 in Youngia tenuifolia is made for the first time in India. Maximum numbers of the populations show laggards, chromosome stickiness, and cytomixis from early prophase to telophase-II, leading to the formation of aneuploid cells or meiocytes with double chromosome number. Such meiotic abnormalities produce unreduced pollen grains and the reduced pollen viability.

Cytomixis--a unique phenomenon in animal and plant

Cytomixis is reported to be a uniform phenomenon in the context of fertilization during spermatogenesis of animals and in some lower groups of plants where oogamous reproduction prevails. However, the phenomenon is versatile in flowering taxa as it lacks uniformity in occurrences, causes, formation of intercellular bridges, involvement of number of cells in a cluster, evolutionary significance among others. A review on cytomixis is conducted with an objective that it may offer a scope to unravel some of the ambiguities associated with it and provide further information on cell, reproductive, structural and evolutionary biology.

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.

Pre-meiotic endomitosis in the cytokinesis-defective tomato mutant pmcd1 generates tetraploid meiocytes and diploid gametes

Sexual polyploidization through the formation and functioning of 2n gametes is considered a major route for plant speciation and diversification. The cellular mechanism underlying 2n gamete formation mostly involves a restitution of the meiotic cell cycle, generating dyads and triads instead of tetrad meiotic end-products. As an alternative mechanism, the tomato mutant pmcd1 (for pre-meiotic cytokinesis defect 1), which generates diploid gametes through the ectopic induction of pre-meiotic endomitosis, is presented here. Using cytological approaches, it is demonstrated that male pmcd1 meiocyte initials exhibit clear alterations in cell cycle progression and cell plate formation, and consequently form syncytial cells that display different grades of cellular and/or nuclear fusion. In addition, it was found that other somatic tissue types (e.g. cotyledons and petals) also display occasional defects in cell wall formation and exhibit alterations in callose deposition, indicating that pmcd1 has a general defect in cell plate formation, most probably caused by alterations in callose biosynthesis. In a broader perspective, these findings demonstrate that defects in cytokinesis and cell plate formation may constitute a putative route for diplogamete formation and sexual polyploidization in plants.

Cytology of five species of subfamily Papaveroideae from the Western Himalayas

During the present course, population-based meiotic studies were carried out on five species of subfamily Papaveroideae from selected localities of Kashmir and Himachal Pradesh in the Western Himalayas (India). Varied intraspecific chromosome counts were reported for the first time in Argemone mexicana and Meconopsis latifolia, both existing on 2n = 2x = 14. The x = 7, confirmed for the first time from the newly found diploid cytotype, is suggested to be the primary chromosomal basic number for the Meconopsis. Furthermore, meiotic course was noted to be normal in Argemone ochroleuca, it varied from normal to abnormal in the populations of A. mexicana and Papaver dubium whereas it was invariably found to be abnormal in all the populations of Meconopsis aculeata and M. latifolia. These anomalous taxa were marked with meiotic abnormalities in the form of cytomixis, chromosomal stickiness, unoriented bivalents, formation of laggards and bridges resulting in abnormal microsporogenesis, and production of heterogeneous-sized fertile pollen grains along with reduced pollen fertility.

Abnormal male meiosis explains pollen sterility in the polyploid medicinal plant Pinellia ternata (Araceae)

Pinellia ternata is an important traditional Chinese medicinal plant. Its different populations in China have various ploidy levels, based on x = 13, as well as extensive aneuploid series. The microsporogenesis process was observed in specimens from three populations from three regions of Hubei Province; they were characterized by normal and abnormal meiotic divisions in pollen mother cells (PMCs) at all stages simultaneously. Meiotic abnormalities including univalents/multivalents, chromosomal laggards/bridges and micronuclei appeared in about 50% of the PMCs, together with abnormal cytokinesis. Chromatin/chromosome transfer between meiocytes occurred only during the first division, at low frequency; this might contribute to these meiotic abnormalities. Although the remaining 50% of the PMCs presented normal cytological behavior, pollen fertility was only about 2%. These results provide cytological explanations for its low seed-set and the general use of asexual reproduction through tubers and bulbils; it also explains the wide variations in chromosome number.

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.

Male meiosis, morphometric analysis and distribution pattern of 2× and 4× cytotypes of Ranunculus hirtellus Royle, 1834 (Ranunculaceae) from the cold regions of northwest Himalayas (India)

In this study, we examined the chromosome number, detailed male meiosis, microsporogenesis, pollen fertility and morphological features and distribution of 2× and 4× cytotypes of Ranunculus hirtellus Royle, 1834. The majority of the populations scored now from cold regions of the northwest Himalayas showed tetraploid (n=16) meiotic chromosome count and one of the populations studied from the Manimahesh hills existed at diploid level (n=8). The individuals of diploid cytotype exhibited perfectly normal meiotic course resulting in 100% pollen fertility and pollen grains of uniform sizes. On the other hand, the plants of the tetraploid cytotype from all the populations in spite of showing normal bivalent formation and equal distribution to the opposite poles at anaphases showed various meiotic abnormalities. The most prominent among these meiotic abnormalities was the cytomixis which involved inter PMC (pollen mother cell) chromatin material transfer at different stages of meiosis-I. The phenomenon of cytomixis induced various meiotic abnormalities which include chromatin stickiness, pycnotic chromatin, laggards and chromatin bridges, out of plate bivalents at metaphase-I, disoriented chromatin material at anaphase/telophase and micronuclei. Consequently, these populations exhibited varying percentages of pollen sterility (24 - 77 %) and pollen grains of heterogeneous sizes. Analysis of various morphometric features including the stomata in 2× and 4× cytotypes showed that increase in ploidy level in the species is correlated with gigantism of vegetative and floral characters and the two cytotypes can be distinguished from each other on the basis of morphological characters. The distribution patterns of the 2× and 4× cytotypes now detected and 2×, 3×, 4× cytotypes detected earlier by workers from other regions of the Indian Himalayas have also been discussed.

Origin of triploid Arachis pintoi (Leguminosae) by autopolyploidy evidenced by FISH and meiotic behaviour

BACKGROUND AND AIMS

Polyploidy is a dominant feature of flowering-plant genomes, including those of many important crop species. Arachis is a largely diploid genus with just four polyploid species. Two of them are economically important: the cultivated peanut and A. glabrata, a tropical forage crop. Even though it is usually accepted that polyploids within papilionoid legumes have arisen via hybridization and further chromosome doubling, it has been recently suggested that peanut arose through bilateral sexual polyploidization. In this paper, the polyploid nature of the recent, spontaneously originated triploid cytotype of the tropical lucerne, A. pintoi, was analysed, and thereby the mechanism by which polyploids may arise in the genus.

METHODS

Chromosome morphology of 2x and 3x A. pintoi was determined by the Feulgeńs technique and the rDNA sites were mapped by FISH. To investigate whether polyploidization occurred by means of unreduced gametes, a detailed analysis of the microsporogenesis and pollen grains was made.

KEY RESULTS

The 2x and 3x plants presented 9m + 1sm and a satellited chromosome type 2 in each haploid genome. Physical mapping revealed a cluster of 18S-26S rDNA, proximally located on chromosome 6, and two 5S rDNA loci on chromosomes 3 and 5. Diploid plants presented 10II in meiosis while trivalents were observed in all triploids, with a maximum of 10III by cell. Diploid A. pintoi produced normal tetrads, but also triads, dyads and monads. Two types of pollen grains were detected: (1) normal-sized with a prolate shape and (2) large ones with a tetrahedral morphology.

CONCLUSIONS

Karyotype and meiotic analysis demonstrate that the 3x clone of A. pintoi arose by autopolyploidy. The occurrence of unreduced gametes strongly supports unilateral sexual polyploidization as the most probable mechanism that could have led to the origin of the triploid cytotype. This mechanism of polyploidization would probably be one of the most important mechanisms involved in the origin of economically important species of Arachis, either by triploid bridge or bilateral sexual polyploidization.

Polyploidization mechanisms: temperature environment can induce diploid gamete formation in Rosa sp

Polyploidy is an important evolutionary phenomenon but the mechanisms by which polyploidy arises still remain underexplored. There may be an environmental component to polyploidization. This study aimed to clarify how temperature may promote diploid gamete formation considered an essential element for sexual polyploidization. First of all, a detailed cytological analysis of microsporogenesis and microgametogenesis was performed to target precisely the key developmental stages which are the most sensitive to temperature. Then, heat-induced modifications in sporad and pollen characteristics were analysed through an exposition of high temperature gradient. Rosa plants are sensitive to high temperatures with a developmental sensitivity window limited to meiosis. Moreover, the range of efficient temperatures is actually narrow. 36 °C at early meiosis led to a decrease in pollen viability, pollen ectexine defects but especially the appearance of numerous diploid pollen grains. They resulted from dyads or triads mainly formed following heat-induced spindle misorientations in telophase II. A high temperature environment has the potential to increase gamete ploidy level. The high frequencies of diplogametes obtained at some extreme temperatures support the hypothesis that polyploidization events could have occurred in adverse conditions and suggest polyploidization facilitating in a global change context.

Meiotic studies of some Aegilops (Poaceae) species and populations in Iran

Thirteen populations of three Aegilops (Poaceae) species were analysed for meiotic characters including chiasma frequency and distribution, as well as chromosomal association and segregation. Populations of A. triuncialis and A. cylindrica possessed n= 14 chromosome number (tetraploid) while populations of A. umbellulata possessed n = 7 (diploid). Tetraploid species showed diplontic behavior and formed bivalents only. Cluster analysis of meiotic data showed distinctness of the species, although variations were observed in the chiasma frequency and distribution among different populations of each species. Cytomixis and chromosome elimination led to ancuploid and unreduced pollen mother cell formation in the species studied.