Maize (Zea mays L.) is an important cereal crop with 2n = 20 chromosomes. For assessing irradiation effects, dry seeds of maize were treated with nine different irradiation doses viz., 5, 10, 15, 20, 25, 30, 35, 40 and 50kR at BARC, Mumbai. The gamma irradiated seed progenies displayed a wide spectrum of meiotic anomalies, the most being lagging chromosomes, chromosome stickiness, multivalents, unoriented bivalents, bridges and micronuclei. Other aberrations identified include chromosome scattering, univalents, disturbed polarity and precocious chromosome movement. The spectrum of these meiotic aberrations was found higher at metaphase than anaphase/telophase. Also, the frequency of aberrations showed positive correlation with the increasing gamma irradiation dose. In addition, gamma irradiation doses decreased germination and survival percentage as well. The irradiation doses ranging from 20-30kR were found beneficial in producing rapid cytogenetic variations and average lethality which could be beneficial in future cytological and mutation breeding programmes.
Till date, several classical and modern methodological tools had been employed to investigate the phenomenon of cytomixis for developing a clear understand-ing about its occurrence, causes, mechanism, and role in plant evolution. These attempts have succeeded to some extent in modifying general perception about it ever since its discovery. Based on the published literature as well as own work, the authors opined that cytomixis has no correlation with ploidy level/genomic status. Rather, it is the genetic makeup and environmental conditions which are responsible for cytomixis. We also assume that recipient meiocytes adjust to extra chromatin either through exclusion or condensation of chromatin. Other-wise, meiocytes depicted spindle abnormalities leading to gametes with the variable genetic constitution. Such gametes led to the origin of aneuploids/polyploids. High frequency of cytomixis during early stages has a direct correlation with the easier passage of genetic material. In some cases, whole nucleus migration resulted into syncytes which yielded unreduced pollen, leading to the origin of polyploids. Another evolutionary aspect of cytomixis is that it induces high pollen sterility in plants of cold deserts acquiring perennial habits and vegetative modes of propagation. We are of the opinion that like other cytological processes, cytomixis is a natural meiotic aberration of potential evolutionary significance.
Chromosome analysis of tea plant (Camellia sinensis), an important commercial crop used to prepare beverages throughout the world, was conducted using fluorescence in situ hybridization (FISH) and chromosome image analyzing system IV (CHIAS IV) software. Chromosomes of C. japonica, a popular woody plant used as an ornamental tree and an important breeding resource for tea plants, were used for comparison with C. sinensis. Both C. sinensis and C. japonica comprised 30 chromosomes. The 5S rDNA, a fundamental repeat sequence and a landmark for FISH, was used for Camellia karyotyping. We observed one 5S rDNA locus on two chromosomes in both the species and confirmed the presence of bivalents in the meiotic cells of interspecific hybrids between C. japonica and C. sinensis. These results suggest that the two species have homoeologous chromosomes. C. sinensis chromosomes were analyzed using CHIAS IV, which performs quantitative karyotyping. This is the first report on the karyotyping of the Japanese tea plant C. sinensis by FISH and quantitative image analysis using CHIAS IV. This report will accelerate the use of cytological and genetic linkage map analysis in tea breeding.
Despite the efforts of numerous researchers over the years, inner structure of a chromosome is still controversial, although several models have been proposed to date. It is now well known that there are two important structural components to the chromosome, the chromosome scaffold and chromatin fibers. The chromosome scaffold, which is mainly composed of four different proteins, is a protein axis extending longitudinally in both chromatids. The chromatin fiber, which is composed of a DNA strand with histone proteins, is also packed in each chromatid. We used focused ion beam/scanning electron microscope (FIB/SEM) to elucidate these two structural components using human chromosomes. FIB/SEM effectively cuts the human chromosomes by its focused Ga ion beam and the cross-sections were visualized by the resolution of scanning electron microscope. As a result, the chromosome scaffold has been confirmed to be located in the central region of each chromatid. The distribution of chromatin fiber in the chromosome’s inner space was also detected. It seems to be distributed more or less randomly within a chromosome. These results strongly indicated that the nanotechnology afforded by FIB/SEM is an effective method to reveal the chromosome’s inner structure in detail.
