Cytological Mechanisms Research Articles

Evaluation for chipping and tuber soft rot (Erwinia carotovora) resistance in potato clones from unilateral sexual polyploidization (2x × 4x)

The aim of this work is the evaluation of tetraploid potato clones with germplasm fromS. chacoense andS. phureja for chipping and resistance to tuber soft rot (Erwinia carotovora subsp.carotovora). Tetraploid potato clones were obtained via 2n eggs through 2x–4x crosses. Cytological mechanisms were investigated in the diploid hybrid 2n-egg producer used as parent in the unilateral sexual polyploidization scheme; the omission of the second meiotic division and cytokinesis failure at the end of meiosis were identified with a genetic consequence equivalent to second division restitution (SDR). Tuber marketable yield, specific gravity and chip-processing ability evaluated at harvest and after cold storage with and without reconditioning were higher in some clones as compared to theS. tuberosum tetraploid parent. Tuber soft rot resistance, contributed byS. chacoense, was successfully transferred to the4x level. After inoculation withErwinia carotovora subsp.carotovora and estimation of the rotted area, one tetraploid clone proved to be resistant and three partially resistant. The genetic value of exotic germplasm and sexual polyploidization strategy are discussed in relation to potato improvement.

Automictic parthenogenesis in the parasitoid Venturia canescens (Hymenoptera: Ichneumonidae) revisited.

Both arrhenotokous and thelytokous reproduction are known to occur in the parasitoid wasp Venturia canescens. The cytological mechanism of thelytoky was previously reported to involve the formation of a restitution metaphase after the reduction division, but the exact nature of the subsequent divisions, whether reductional or equational, remained unclear. We reinvestigated the cytological mechanisms in a thelytokous strain collected in France. Our observations confirm previous results, but an equational and not a reduction division was observed after restitution. This type of reproduction can be classified as central fusion automictic parthenogenesis. In two arrhenotokous strains the normal pattern of oogenesis and syngamy of Hymenoptera was observed. In addition, we used PCR amplification to show that thelytoky in V. canescens is not caused by Wolbachia bacteria. The results are discussed in relation to maintenance of heterozygosity and female sex.

Production of Diploid Eggs through Premeiotic Endomitosis in the Hybrid Medaka between Oryzias latipes and O. curvinotus

A hybrid medaka between Oryzias latipes and O. curvinotus spawns diploid eggs. We examined the cytological mechanisms of diploid egg formation in this hybrid. Oocytes in the hybrid passed through the first and second meiotic divisions and excluded two polar bodies, associated with reduction of the DNA content in oocytes. Each germinal vesicle in vitellogenic oocytes of the hybrid had 48 chromosomes with bivalent chiasmata, precisely twice the number of chromosomes in normal oocytes. These results suggest that before meiosis the chromosomes are doubled by mitosis without cytokinesis, that is, endomitosis, and that the resulting tetraploid oogonia undergo normal meiosis to produce diploid eggs. Except for a few vitellogenic oocytes that are probably derived from endomitotic oogonia, most oocytes were arrested at the zygotene stage in the hybrid ovary, suggesting the existence of checkpoint control that ensures the pairing of homologous chromosomes at prophase I, a situation differing from that in the male in which the checkpoint is at metaphase I. A sac-like structure was characteristic of the hybrid ovary. Although this structure was observed only in the hybrid, it may be a native component of the medaka ovary but difficult to observe because of its deformed outer structure due to enlargement of the inside oocytes. The present study confirms that this hybrid medaka provides a useful experimental system for investigations into the mechanisms of oogenesis and basic architecture of the ovary, which are difficult to analyze by using normal medaka.

Cytological Mechanism of Cytoplasmic Inheritance in Pinus tabulaeformis: II. Transmission of Male and Female Organelles During Fertilization and Proembryo Development

In an earlier report the ultrastructure and nucleoid organelles of male gamete in Pinus tabulaeformis Carr. have been described. Presently, the ultrastructure of the cytoplasm of the egg cell and pollen tube—immediately before fertilization and during cytoplasmic transmission of male gametophyte—has been described for the same species. The fate of parental plastids and mitochondria in the proembryo has also been followed. The mature egg cell contains a large amount of mitochondria, but seems to lack normal plastids. Most plastids have transformed into large inclusions. Apart from the large inclusions, there are abundant small inclusions and other organelles in the egg cell. During fertilization, pollen tube penetrates into the egg cell at the micropylar end and thereafter the contents are released. Plastid and mitochondrion of male origin are lacking near the fusing sperm_egg nuclei. The second sperm nucleus—not involved in karyogamy—remains at a site near the receptive vacuole. This nucleus is surrounded by large amount of male cytoplasm containing mixed organelles from the sperm cell, tube cell, and egg cell. At the free nuclear proembryo stage, organelles of male and female origin are visible in the perinucleus_cytoplasmic zone. Most of the mitochondria have the same morphological features as those in the egg cell. Some of the mitochondria appear to have originated from the sperm and tube cells. Plastids are most likely of male gametophyte origin because they have similar appearance as those of the sperm and tube cell. Large inclusions in the egg cell become vacuole_like. Paternal plastids have been incorporated into the neocytoplasm of the proembryo. In the cellular proembryo, maternal mitochondria are more abundant. Plastids resembling those of the sperm and tube cell are still present. These cytological results clearly show that in P. tabulaeformis, plastids are inherited paternally and mitochondria bipaternally. The cytological mechanism of plastid and mitochondrion inheritance in gymnosperm is discussed.

Cytological Mechanism of Cytoplasmic Inheritance in Pinus tabulaeformis: I. Ultrastructural Aspects and Nucleoids of the Male Gametes

The cytological mechanism of plastid and mitochondrion inheritance in Pinus is an interesting research topic with only a limited number of published articles. The results indicate that the sperms of Pinus tabulaeformis Carr. contain abundant plastids, mitochondria and organelle DNA. These data provide reliable cytological evidence of paternal plastid and mitochondrion inheritance in Pinus . The results are in line with the confirmed general rule of paternal plastid inheritance in Pinaceae. But whether mitochondria in sperm cells can be transmitted into the embryos is an issue needs further developmental studies. Another important finding is that contrary to earlier classification of the male gamete of Pinaceae into the male nuclei type, the results reveal that male gametes in Pinus tabulaeformis are actually cells. However, the sperm cells are only surrounded by plasma membranes without cell walls. The larger leading sperm cell in a pollen tube section is long in shape, with a large amount of cytoplasm; while the second sperm cell is smaller, round in shape and contains less cytoplasm. Whether this feature of the male gamete type could be considered as a representative characteristic of the family is discussed and further conclusions await more experimental evidences from studies on plants from different species.

Production and cytogenetics of intergeneric hybrids between the three cultivated Brassica diploids and Orychophragmusviolaceus

It has been proposed that both complete and partial separation of the parental genomes during mitosis and meiosis occurs in the intergeneric hybrids between Orychophragmus violaceus (2n=24) and the three cultivated Brassica tetraploids (B. napus, B. carinata and B. juncea). The hypothesis has been that this and the variations in chromosome numbers of these hybrids and their progenies result from the different roles of the A, B and C genomes originating from Brassica. To test this hypothesis, we produced hybrids between O. violaceus and the cultivated Brassica diploids. The hybrids with B. oleracea (2n=18, CC) had an intermediate morphology, but their petals were purple like those of O. violaceus. They were sterile and had the expected chromosome number (2n=21) in their mitotic and meiotic cells. The hybrid with B. campestris (2n=20, AA) was morphologically intermediate, except for its partial fertility and its yellow petals, which were similar to those of B. campestris. It was mixoploid (2n=23-42), and cells with 2n=34 were most frequent. Partial separation of parental genomes during mitosis, leading to the addition of O. violaceus chromosomes to the B. campestris complement, was proposed to explain the findings in the mitotic and meiotic cells of the hybrid and its progeny. In crosses with B. nigra (2n=16, BB), the majority of the F(1) plants were of the maternal type (2n=16), a small fraction had B. nigra morphology but were mixoploids (2n=16-18), predominantly with 2n=16 cells and three plants, each with a specific morphology, were mixoploids consisting of cells with varying ranges of chromosome numbers (2n=17-26, 11-17 and 14-17). The origin of these different types of plants was inferred to be a result of the complete and partial separation of parental genomes and the loss of O. violaceus chromosomes. Our findings in the three crosses suggest that the A genome was more influential than the C genome with respect to complete genome separation during mitosis and meiosis of the hybrids with B. napus. Possible complete and partial genome separation during mitotic divisions of the hybrids with B. carinata was mainly attributed to the role of the B genome. The combined roles of the A and B genomes would thus contribute to the most variable chromosome numbers of mitotic and meiotic cells in the hybrids with B. juncea and their progenies. The possible cytological mechanisms pertaining to these hybrids and the potential of genome separation in the production of Brassica aneuploids and homozygous plants are discussed.

Mannose-Specific Recognition Mediates Two Aspects of Synaptic Growth of Leech Sensory Afferents: Collateral Branching and Proliferation of Synaptic Vesicle Clusters

The developmental role of carbohydrate markers in the genesis of neuronal networks was studied using leech sensory afferents as a model. Leech sensory afferents express a mannose-containing epitope on their cell surface that is recognized by monoclonal antibody Lan3-2. Previously, the elaboration of sensory arbors in the synaptic neuropil of CNS ganglia was experimentally shown to depend on this mannose marker. Sensory arbors were abolished by perturbing sensory afferents in the intact nervous system with Lan3-2 Fab fragments, a glycosidase, or mannose–BSA. To understand the cytological mechanisms underlying mannose-specific recognition for synaptogenesis, we have now studied the effects of antibody perturbation at the ultrastructural level in the sensory afferent target region. A characteristic signature of a normal sensory afferent is its profuse collateral branching, which, with ongoing development, is replaced by a single widened process, the sensory trunk, which possesses numerous synaptic vesicle clusters. The inhibition of mannose-specific recognition leads to a rapid, major reorganization of different stages of sensory afferent growth. Collateral branches at the distal growing region are reduced three- to fourfold. The pruned axons grow at an accelerated rate. Developmentally older sensory trunks experience a threefold reduction in synaptic vesicle clusters. These responses suggest that depriving sensory afferents of mannose-specific recognition aborts their synaptogenesis and causes them to resume behavior typical of tracking through axonal tracts. The current findings also suggest that the mannose marker, by promoting both collateral branching andthe proliferation of synaptic vesicle clusters, plays a critical role in two stages of sensory afferent synaptogenesis.

P VIII.18 - P VIII.18 Ectopic conjugation of chromosome heterochromatic regions as cytological mechanism of instability

P VIII.18 - P VIII.18 Ectopic conjugation of chromosome heterochromatic regions as cytological mechanism of instability

Effects of Thyroid Status on Cold-Adaptive Thermogenesis in Brandt's Vole, Microtus brandti

Hyper- and hypothyroidism were induced by subcutaneous injection of thyroxine and by oral administration of methimazol in Brandt's voles. The effects of the two treatments on metabolic thermogenesis at 25 degrees C and 4 degrees C were investigated. The level of resting metabolic rate was closely related to thyroid status: high in the hyperthyroid case and low in the hypothyroid case. However, no increase in resting metabolic rate occurred in either case during further cold acclimation. Hyperthyroidism resulted in an increased nonshivering thermogenesis, which was much enhanced by lower temperature, but hypothyroidism led to a suppressed nonshivering thermogenesis in the cold. The state-4 and state-3 respirations and the activities of cytochrome-c oxidase of liver mitochondria were elevated in hyperthyroid animals but attenuated in hypothyroid ones. However, these levels were scarcely changed after further cold acclimation. Both hyperthyroidism and cold acclimation induced the recruitment of brown adipose tissue, but brown adipose tissue was different biochemically in the two cases: in hyperthyroidism, the total protein was reduced, while fat content increased; in cold acclimation, the total and mitochondrial proteins were increased. However, in hypothyroid voles, the normal adaptive changes in brown adipose tissue were impaired in further cold acclimation. The activity of cytochromec oxidase in brown adipose tissue was increased by hyperthyroidism and enhanced in further cold. In contrast, its activity was inhibited in hypothyroid animals, though activated to some extent in cold. These results demonstrate that normal thyroid function is essential for the cold-induced increase of resting metabolic rate and nonshivering thermogenesis and that there is a synergism between thyroid hormone and cold acclimation in the regulation of nonshivering thermogenesis in Brandt's vole. In addition, the blunted response of brown adipocytes to the cold may be the cytological mechanism for the suppressed nonshivering thermogenesis found with hypothyroidism.

Development of water-conducting cells in the antipodal liverwort Symphyogyna brasiliensis (Metzgeriales).

The thallus of the metzgerialean liverwort Symphyogyna brasiliensis Nets contains a strand of dead thick-walled tells with helicoidally-arranged pits that arc presumably involved in water transport. During the first phase of differentiation these cells undergo a 13-16-fold elongation while remaining thin-walled and almost unchanged in diameter. During subsequent maturation the walls become strongly thickened by deposition of highly electron-opaque material on extraplasmodesmal areas and of transparent material forming collars around plasmodesmata. Whilst the growing wall shows an ordered microribrillar texture and is strongly reactive to PATAg staining for carbohydrates, the material associated with plasmodesmata is amorphous and PATAg-negative. A dense cortical array of microtobules (MTs) overlies the growing wall except in proximity to plasmodesmata, which are closely associated with tubular endoplasmic reticulum (ER). During cellular maturation plasmodesmata undergo extensive secondary elongation by incorporation of cortical ER supposedly continuous with desmotubules. Quantitative analysis of plasmodesmal frequencies in relation to cellular elongation and wall thickness indicates that there is no de novo formation of plasmodesmata. Cortical MTs, wall microfibrils and secondarily-modified plasmodesmata are consistently co-aligned, all forming helices of about 45°. During maturation the Golgi apparatus proliferates and a vast number of vesicles containing PATAg-positive material are produced from a membrane domain interpreted as trans Golgi network, whilst PATAg-negative vesicles are formed along the fenestrated margins of C& and media) dictyosomal cisternae. Exocytosis of PATAg-positive vesicles is confined to extraplasmodesmal areas. In ageing cells abundant fibrillar material, also positive to PATAg-test, accumulates within pleomorphic membrane-bounded tubules. Final cytoplasmic dissolution involves the lysis of all cellular membranes and the liberation of the membrane-bounded fibrillar material, that is subsequently deposited onto the walls. The eventual dissolution of the plugs of amorphous electron-transport material results in the formation of open pits. Similarities in the cytological mechanisms underlying pore development in water-conducting cells of Symphyogyna and in the sieve elements of angiosperms are discussed.

Callose localization in cell walls during meiotic and apomeiotic megasporogenesis in diploid alfalfa (Medicagospp.)

SUMMARYThe localization of callose during meiotic and apomeiotic megasporogenesis in diploid species of the genus Medicago was investigated by fluorescence microscopy. Callose deposition and cytoembryological details were studied by treating flowers with a clearing/staining medium. Ovules were classified into different developmental stages and the distance of growing integuments from the micropyle measured. The relationship between the stage of development and the growth of integuments proved to be an effective method for correlating megasporogenesis events with callose deposition and assessing the cytological mechanism of apomeiosis. Within meiotic ovules callose deposition appeared as thin micropylar caps in the megaspore mother cells and as thick cell plates between dyad, triad and tetrad cells. The degradation of callose started at the beginning of embryo sac differentiation. Ovules with diplosporic cells were characterized by complete absence of callose. When callose was present at the megaspore moth...

The cytological and genetical mechanisms of plant domestication exemplified by four crop models

Plant domestication includes a number of evolutionary mechanisms, such as mutation, hybridization, polyploidization, selection, and genetic drift. These evolutionary mechanisms will be used as entities in a graphical model system to be used as a formal descriptive method to describe the process of plant domestication. The most important crops can be arranged in four different crop domestication models: the bread wheat model, the cotton model, the soybean model, and the chili pepper model. Mixed-model situations occur. The existence of reproductive isolation connects the crop models with the gene pool concept. Cluster and analysis of taxa based on a numerical representation of crossability data allows a precise characterization of the different gene pools. The combined approach of crop domestication models, dendrograms, and gene pool representation creates a clear description of the mechanisms leading to a particular set of reproductive barriers and the extent of these barriers. The four models described here are general outlines. The included entities can be fine tuned to specific crop situations. This will be worked out forAllium cepa (onion) andLesquerella densipila (an oil seed crop). These crop domestication models can also be used in developing breeding strategies by illustrating the rationale of desired approaches such as hybridization improvement techniques or transformation.

Meiosis and some aspects of cytological mechanisms of chromosomal sex determination in nematode species

Meiotic division in Toxascaris leonina, Baylisascaris transfuga, Hexametra sp., Toxocara canis, T. cati, Heterakis gallinarum and Physaloptera clausa of the class Nematoda were studied. Chromosomal sex determination type in different taxa is described. The XX-XO type of sex determination predominates in the species examined. A Y chromosome occurs only in B. transfuga. A correlation between the number and length of sex chromosomes and autosomes in the different taxa was established.

Meiotic studies on a Brassica campestris–alboglabra monosomic addition line and derived B. campestris primary trisomics

Diakinesis chromosomes were studied in pollen mother cells of Brassica campestris (2n = 20, genome AA), B. alboglabra (2n = 18, genome CC), a B. campestris-alboglabra monosomic addition line (AA + 1 chromosome from the C genome), and four derived B. campestris primary trisomics. The nucleolar chromosomes of B. campestris were distinguishable by their morphology at diakinesis. The alien C-genome chromosome in the addition line paired preferentially with the nucleolar chromosome of the A genome. Very rarely, it paired with another pair of the A genome. Thus, it was concluded that the alien C-genome chromosome of the addition line is primarily homoeologous to the nucleolar chromosome and secondarily to another chromosome of the A genome. Three of the four derived B. campestris trisomic plants were identified as B campestris nucleolar trisomics. Trisomy in the fourth plant involved another chromosome. The cytological mechanism underlying the origin of trisomics in the addition line and chromosome homoeology relationships between B. campestris and B. alboglabra are envisaged.

Second meiotic division restitution (SDR) 2n pollen formation in diploid and hexaploid species of Asparagus

Plants of two cultivars of 2x A. officinalis L. and two related ornamental species, 2x A. plumosus Baker and 6x A. densiflorus (Kunth) Jessop cv. Sprengeri were screened for production of pollen of heterogeneous size. Twenty four plants out of thirty-one studied produced this type of pollen, with frequencies of 3% or more large pollen. Some of these plants also produced giant grains. The numerically unreduced pollen arose by the lack of chromosome migration at Anaphase II in either one or both cells of a meiocyte, followed by the absence of cytokinesis in cells with abnormal chromosome behavior, a Second Meiotic Division Restitution (SDR) cytological mechanism. Double unreduced pollen resulted from a similar mechanism acting at both Anaphase I and Anaphase II stages. The high frequency of plants which produced 2n pollen in significant amounts is taken as an indication that the modified meiosis observed is under genetic control.

Cytological mechanisms of 2n egg formation in a diploid genotype of Medicago sativa subsp. falcata

Gametes formed without meiosis in alfalfa would be useful in basic and applied research. Therefore, the cytological analysis of macrosporogenesis of a diploid plant of Medicago sativa subsp. falcata (L.) Arcangeli (named PG-F9), previously selected as a good 2n egg producer, was conducted. A stain-clearing technique was applied which also allowed the analysis of microsporogenesis to be performed. Two mechanisms of 2n egg formation were determined: absence of the second meiotic division (mechanism of the SDR type) and apomeiosis. In particular, it is noteworthy that 2n eggs produced via apomeiosis should retain the same parental genotype, because apomeiotic cells give rise directly to the female gametophyte without undergoing meiosis. The presence of binucleate female gametophytes in the ovules at an early stage of development confirmed the presence of apomeiosis in PG-F9. These abnormalities concerned only macrosporogenesis; in fact, the analysis of microsporogenesis showed the production of normal tetrads of microspores after normal meiosis. Possible application of the discovered mechanisms of 2n egg production in breeding research are discussed.

Skin Color in Apples—Influence of Copigmentation and Plastid Pigments on Shade and Darkness of Red Color in Five Genotypes

The biochemical and cytological mechanisms responsible for the differences in red color quality of apples (Malus domestics Borkh.) were investigated. Copigmentation, the increase in absorbance maxima (λ max) from anthocyanin and flavonoid interactions, is known to be a mechanism for producing variation in shade of red in flowers. In intact apple skin cells, the mean λ max was 550 nm, with no significant difference between genotypes. Furthermore, the ratio of flavonols and proanthocyanidins to anthocyanins was similar for all genotypes. Therefore, copigmentation is not a mechanism producing different shades of red in apples. Darkness of red skin was positively related to the proportion of red cells in the skin and the size of the vacuoles containing anthocyanins. Measurements of plastid pigments, chlorophyll, and carotenoids, compared with L*, a*, b* measurements, indicated that the visual blending of plastid pigments and anthocyanins has an important influence on red coloration of apple skin.

Cytological basis of 2n pollen formation in a wide range of 2x, 4x, and 6x taxa from tuber-bearing Solanum species

Cytological mechanisms of 2n pollen formation were investigated in microsporogenesis and sporad development in 2x, 4x, and 6x taxa of tuber-bearing Solanum species that were originally collected from Mexico, and Central and South America. Parallel spindles and tripoles were seen at anaphase II in the majority of genotypes observed. The occurrence of parallel spindles and tripoles were well correlated with the occurrence of dyads and triads at the sporad stage. Further, the frequency of 2n pollen predicted by frequencies of parallel spindles and tripoles and by frequencies of dyads and triads were also well correlated with observed 2n pollen frequencies. Premature cytokinesis at telophase I to prophase II was seen only in 2x and 4x species from series Conicibaccata. The frequency of premature cytokinesis corresponded well with the frequency of dyads leading to 2n pollen formation. It was concluded that the major mechanisms of 2n pollen formation in tuber-bearing Solanum species is parallel spindles.

Cytology of 2n Pollen Formation in Diploid Crested Wheatgrass, Agropyron cristatum

Hybrids between induced tetraploids of diploid crested wheatgrass, Agropyron cristatum (L.) Gaertn. (2n = 2x = 14), and tetraploid crested wheatgrass, A. desertorum (Fisch. ex Link) Schult. (2n = 4x = 28), have provided improved germplasm with unique agronomic traits. Introgression from diploids to tetraploids would be simplified if diploids that produce 2n gametes (gametes with the sporophytic chromosome number) could be identified. This study was initiated to determine if 2n pollen is produced in diploid crested wheatgrass and to determine the cytological mechanisms of 2n pollen formation. Pollen diameters were investigated by measuring 60 pollen grains from each of 600 diploid and 100 tetraploid genotypes. Pollen diameters averaged 37.9 μm (SE = 0.17) and 41.1 μm (SE = 0.33) for diploid and tetraploid genotypes, respectively, with a range of 35.3 to 41.3 μm among the diploids, and 39.7 to 46.8 μm among the tetraploids. Approximately 16% of the diploid genotypes produced some pollen grains with twice the mean volume of n pollen. Cytological analyses of 17 individuals that produced variable‐sized pollen indicated that 2n pollen was produced via restitution at the second meiotic division. Meiotic abnormalities that could give rise to 2n microspores include i) arrest at metaphase II, ii) lack of sister chromatid movement in anaphase II, iii) movement of all or most sister chromatids to a single pole in anaphase II, and iv) abnormal cytokinesis after telophase II. The formation of 2n pollen in diploid crested wheatgrass should facilitate efficient transfer of germplasm from diploids to tetraploids within the genus Agropyon.

Cytological Mechanism of 2n Microscope Formation in Garden Asparagus

Microsporogenesis was studied in 42 randomly chosen Fz plants of garden asparagus (Asparagus officinalis L. cv. UC 157) (2n = 2x = 20) that had been previously screened for production of pollen of heterogeneous size. At the tetrad stage, the average frequencies of tetrads, triads, and dyads were 58.9%, 15.4%, and 25.9%, respectively. Dyads and triads originated from the lack of chromosome migration toward opposite poles at anaphase II in either one or both cells of a microsporocyte, followed by the absence of cytokinesis in telophase II. The resulting 2n microspores were, therefore, genetically equivalent to second meiotic division restitution products. The observation that all plants examined produced 2n microspores in high frequencies is taken as an indication that the modified meiosis in these plants is under genetic control.