Interphase nuclear structure was studied in five Phaseolus plant species. All the species showed chromocentric nuclear organization in both meristematic and differentiated cells. The number of chromocenters appeared to be a species-specific character. Percentage heterochromatin values, as determined by using three different staining techniques, were higher in meristematic cells than those in differentiated cells. There was a close correspondence between percentage heterochromatin and amount of highly repetitive DNA implying a possible involvement of the latter in chromatin condensation.
Interphase nuclear structure was studied in 15 leguminous species. Eleven species showed chromocentric interphase nuclei while the remaining 4 had reticulate nuclei. The number of chromocenters appeared to be dependent on the number of chromosomes (2n). The total proportion of condensed chromatin as determined by planimetry was found to vary from 11-24% in chromocentric nuclei and 29-62% in reticulate nuclei. The condensed chromatin amount showed a direct correlation with the nuclear DNA content (2C). Though the interphase nuclear structure remained same in differentiated cells, the amount of condensed chromatin was considerably less than that in the meristematic cells, indicating underreplication of heterochromatin during differentiation. HCl-Giemsa method seems to be the simplest method for detection of underreplication in plants.
Chromosome banding techniques, used extensively for studying the linear differentiation of plant and animal chromosomes, have yielded useful information with respect to chromo some orientation and somatic association in interphase nuclei, genome analysis in polyploids and chromosomal changes during evolution (Lavania and Sharma 1983). Compared to the large number of reports of chromosome banding in mitotic metaphase chromosomes, relatively few reports are available on meiotic chromosome banding (Natarajan and Natarajan 1972, Marks 1974, Kranz 1976, de Jong and Oud 1979, Dietrich et al. 1981, Ferrer et al. 1984, Fomi naya and Jouve 1985, Jouve et al. 1985. Meiotic banding has also been used mainly in studies on hybrid cytogenetics of triticale and wheat (Jouve et al. 1980, 1982a, b, Naranjo and Lacadena 1982. Apart from this approach, only a few attempts have been made so far to study and compare the behaviour of heterochromatin in meiosis and mitosis (Natarajan and Natarajan 1972, Marchi et al. 1982). Furthermore the observations in a few plants indicate that the euchromatin undergoes more condensation than heterochromatin during cell division (Stack 1984). Also, as the meiotic metaphase chromosomes are condensed than the corresponding mitotic chromosomes, it is essential to ascertain whether this differential condensation affects the stainability and behaviour of heterochromatin during cell division. Such studies are ex pected to furnish useful information about the differential behaviour of heterochromatin if any, and the role of heterochromatin during cell division.In the present communication, we report visualization of heterochromatin of Allium cepa and Rhoeo discolor during meiosis and give a comparative account of its behaviour during mitosis and meiosis. These two plants were selected specifically because A. cepa has telomeric heterochromatin while R. discolor shows the presence of centromeric heterochromatin.Materials and methods a) Fixation: For mitotic analysis, root tips of A. cepa and R. discolor were fixed in cold ethanol: glacial acetic acid (3:1) for about 12h and then stored in 70% ethanol till further use. For meiotic preparations, flower buds were fixed and stored as above. b) Preparation of air-dried slides: For banding treatments, air-dried preparations were used. Root tips were macerated for 10min in 1N HCl at room temperature. Meriste matic portion was then squashed in a drop of 45% acetic acid. For meiotic preparations, anthers were squashed in 45% acetic acid. Sporogenous tissue was squeezed from an thers and anther walls were removed. Cover-glasses were then detached using liquid nitrogen and the slides were air-dried after immersion in 90% ethanol. c) C-banding: C-banding was performed by the standard BSG (Barium hydroxide-saline -Giemsa) technique of Sumner (1972) with a slight modification. Air-dried preparations 1 NCL Communication No. 3701.
Digestion of nuclear DNAs of five plants, namely Cucurbita maxima (red gourd), Trichosanthes anguina (snake gourd), Cucumis sativus (cucumber), Cajanus cajan (pigeon pea) and Phaseolus vulgaris (french bean) with the restriction endonuclease MboI yielded discrete size classes with molecular weights in the range of 0.5 to 5 kbp. The MboI digestion pattern of Cot 0.1 DNA in french bean is comparable with that of total DNA, indicating that these bands represented highly repeated DNA sequences. Cleavage of the DNAs with varying amounts of MboI indicated the dispersed nature of the repeat families. Southern hybridization studies using french bean highly repetitive DNA as a probe indicated more homology with repeats of pigeon pea and less homology with red gourd, snake gourd and cucumber repeats.
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