Fragile sites are known to represent single stranded gaps resulting from despiralization of DNA due to failure of compact folding in metaphase chromosomes. During routine cytogenetic investigation we have observed a “puff”-like appearance and varying degrees of undercondensation in chromosomes 4 and 12 in a case of hairy cell leukemia and chromosome 17 in a case of non-specific mental retardation. The regions involved include previously reported fragile sites. In our view, these unique features represent additional aspects of folate sensitive fragile regions on chromosomes. The possible mechanism(s) that might have led to the origin of undercondensed segments has been discussed in view of the recent understanding of the role of histone and non-histone proteins, divalent cations and the DNA structure in chromatin packaging.
A mutant causing partial desynapsis (dissociation of paired chromosomes) and consequently a high frequency of univalents at metaphase I was found among plants of a wild population of Rhoeo collected in Merida, Yucatan, MEXICO. The seedlings were found to be diploids with 2n=2x=12. The mutant formed from 6 to 12 univalents/cell and had a karyotype 8M+4SM differing from that of the other desynaptic referred to in other work by the same author which has 7M+5SM. The mutant had a high pollen fertility (65.7%). The produced microspores had chromosome numbers from n=5 to n=14. Unreduced pollen grains were formed at TII via second division restitution and comprised 34.14% of the pollen grains.
Identification of the potato chromosomes 1 (the longest one), 2 (the carrier of the nucleolar organizer, NOR) and 12 (the shortest one) is possible at mitosis using a combined Feulgen-haematoxylin staining or a Giemsa C-banding technique. In two aneuploids containing 27 chromosomes, three specimens of chromosome 1 were found through conventional staining. Variation of the length of the NORs and the size of the satellite of homologues or homoeologues of chromosome 2 was observed in various cytotypes. Pachytene analysis identified trisomics of chromosome 2 of interspecific origin, involving S. tuberosum L. ssp. tuberosum Hawkes and S. phureja Juz. et Buk., but only two specimens of chromosome 2 could be identified using the Giemsa C-banding technique. However, in six different interspecific chromosome 2-trisomics, three chromosomes per cell were found to show a hybridization signal by non-radioactive in situ hybridization with heterologous rDNA from pea. The high polymorphism of chromosomes observed after Giemsa C-banding made it impossible to bring the identity of the mitotic chromosomes in accordance with the results from pachytene identification in cells containing one triplicate chromosome.
Treatment of different concentrations of Dithane M-45, Aldrex-30 and Metacid-50 showed positive chromotoxic effects in Allium cepa root meristem. Effects included high lethality for cell division, clumping, bridges, fragments, cytomixis, disturbed polarity etc. Such genotoxic effects warrants frequent use of these chemicals.
In the stage of diakinesis the ring bivalents form 3 types of configurations-7 closed, 6 closed+1 open, 5 closed+2 open ring bivalents. In the majority of investigated cells the open bivalents are attached to the nucleolus. The disorders in the following meiotic stages are insignificant thus resulting in high vitality pollen formation. These results have shown that the studied varieties of winter barley are suitable for use in hybridization programmes with other cereals.
Seed germination in Miconia cabucu was studied under two different conditions in the laboratory: light and dark. The seed is light sensitive. The root hairs are originated on the entire root surface. The length and breadth of root hair was determined. Some variations in the root hair morphology were seen like branching and septate condition. The root hairs last for 30 to 35 days. These variations in the root hair and the positive photoblastic dormancy of seeds are discussed with reference to previous literature.
An interspecific cross was made between S. melongena var. American Wonder and S. incanum, a non-tuberous wild species showing considerable resistance to various diseases and insect pests. Chromosome association and pollen sterility were analysed for the parent and hybrids. The hybrids (n=12) were obtained only when S. incanum was used as a female parent. The hybrids flowered, fruited and seeded normally. Meiosis was mostly normal. Cytomorphology of F1 hybrids has been discussed.
The mitotic chromosome and chromosome configuration in meiosis were studied in the genus Afgekia: A. sericea and A. mahidolae. Both species are diploid which share the same chromosome number, 2n=16, and 8 bivalents at metaphase I.
Until recently, the importance of mitotic crossover between homologous chromosomes and its role in the etiology and evolution of malignancy had not been realized. This was in part secondary to the fact that the only direct evidence for chiasma formation in normal human tissue came from the “pre-banding era.” An actual physical exchange between the chromatids of a homologous pair of chromosomes had not been documented. To date, whatever direct cytogenetic evidence that mitotic crossover exists comes from cancer cells with altered genomes. We report for the first time, demonstration of band to band chromosomal pairing, chiasma formation and chromatid exchange physically visible in normal human blood and amniotic cell culture. With this phenomenon now documented, its role in the malignancy and origin of partial uniparental disomy and consequently in the origin of other hereditary diseases can be relevantly hypothesized.
The duration of the complete mitotic cycle and its component phases were compared with the amounts of 2C nuclear DNA, heterochromatin DNA and euchromatin DNA in four Lathyrus species. The total cycle time as a whole was not correlated with the variation in 2C nuclear DNA amount which was two fold between the 4 related species. However, the duration of synthesis phase was positively correlated with the amounts of total DNA and heterochromatin DNA. The results have further shown that a disproportionate increase in the amount of heterochromatin in the genome can increase the total duration of the mitotic cycle as well as the duration of its component phases.
In this work, intranuclear rod-shaped structures that appeared in NC-4 spores matured without stress conditions were examined with a differential interference microscope (Nomarski-type). It was found that more than 70% of the spores had intranuclear rods, and these spores maintained the structures for at least 30 days after the fruiting bodies were formed. Thus, we suppose that the intranuclear actin rods participate in the stabilization of the dormant stage matured NC-4 spores.
The sequence of oocyte maturation in the fetal ovary of the house shrew, Suncus murinus is analyzed by means of light and electron microscopy of surface-spread silver-stained preparations. The progression of cells is relatively synchronous. Meiotic prophase starts at day 20 of gestation. The peak of pachytene cells occurs on day 23. On day 24 diplotene cells are abundant. They are characterized by premature desynapsis and separation of axial elements of some bivalents. The meotic prophase is completed on day 26, when the majority of oocytes reach the dictyotene stage.
We investigated inner morphology and development of the shoot apical meristem of a day-neutral species Pectis papposa Gray before and after floral initiation from day 2 to day 8 after sowing. Initiation of reproductive phase, which was represented morphologically by bract formation, coincided with histological observations showing disturbance of the tunica-corpus boundary.
The karyotypes of five species of Sesbania were studied. All species had 2n=12 and there was agreement with x=6 for the genus. The majority of chromosomes were metacentric and their length varied from 2.12 to 6.99μm. Karyotype information is presented for the first time for three species. There are correlations between karyotypes and taxonomic arrangement at subgenus level. Some phylogenetic relationships were discussed. Comparisons of TF% index agreed with the proposals presented in literature.
Microsporogenesis was studied in the plants from a selfed clone of Hemerocallis ‘Sleeping Beauty’ showing regular meiosis under normal cultural conditions and desynapsis when subjected to stress of low water combined with high temperature. The progeny, grown under normal field conditions, had non-desynaptic and desynaptic plants in the ratio of 3:1 indicating that the parent clone was heterozygous and that the desynapsis was controlled by a single recessive gene. The studies revealed a gradation among different genotypes in their response to environmental conditions. The stress situation under which the heterozygote showed desynap-tic meiosis did not affect microsporogenesis in other cultivars. However, within the range of variation in environmental conditions during normal outdoor cultivation, the desynapsis heterozygote showed uniform bivalent pairing and non-significant differences in chiasma frequency. Desynapsis homozygotes, on the other hand, showed desynapsis even under normal field conditions and were affected by day to day fluctuations in ambient conditions showing prominent differences in bivalent and chiasma frequencies. The desynaptic plants, besides having low bivalent pairing, were characterized by delayed entry of MMCs into meiosis, high rate of suppression of meiosis, and blockage or highly irregular division following Al. Variation in the number of cells with different bivalents followed a binomial distribution in three desynaptic plants. In one it deviated significantly from the expected which was possibly due to intercellular differences.