Sex-determining systems are highly divergent between different lineages of reptiles. Birds and snakes have genotypic sex determination (GSD), whereas all crocodilians and tuataras exhibit temperature-dependent sex determination (TSD). Sex-determining mechanisms in turtles and squamates include GSD with male heterogamety, GSD with female heterogamety, and TSD. Comparing different sex determination systems with molecular cytogenetics and comparative genomics thus provides unparalleled opportunities to capture the evolution of sex chromosomes and determination systems in action. The direct or indirect comparison of linkage homologies of sex chromosomes among various GSD species highlighted that the evolutionary process of sex determination systems in reptiles is highly complex and includes transitions between sex determination systems in closely related species and cryptic transitions (ZW system to new ZW system, XY system to new XY system). This approach also revealed the convergent evolution of sex chromosomes, in which ancestral autosomes suitable for the role of sex determination evolved into new sex chromosomes multiple times during reptile evolution. We discuss the mechanisms and rules underlying the evolution of the sex chromosomes and sex determination systems, such as the association of simple short repeat amplification with the heterochromatinization of sex-specific chromosomes and temperature-induced transition from GSD to TSD.
Chromosome condensation is essential for the faithful transmission of genetic information to daughter cells during cell division. However, the chromosome condensation process is still poorly understood. This review discusses the functions of two major factors involved in chromosome condensation, chromosomal proteins and divalent cations. Proteome analysis of the human metaphase chromosome revealed its protein components and identified proteins were functionally calcified. The chromosome scaffold, an axial structure of the chromosome that is composed of several proteins, is important for constructing the X-shaped chromosome morphology. Through conventional fluorescence microscopy, the scaffold structure in a single chromatid has been observed as a single thick axis. However, using super-resolution microscopy and electron microscopy, the scaffold structure was found to be formed of two thin axes. It will generate both stiffness and elasticity to chromosome. Although chromosome structure is severely altered without the scaffold structure, chromosome condensation can also be achieved by divalent cations. Depletion of one of the divalent cations, Ca2+, causes defects in mitotic progression and chromosome condensation after the breakdown of the nuclear envelope. Fluorescence lifetime imaging microscopy-Förster resonance energy transfer and electron microscopy experiments demonstrated that chromosome condensation is influenced by Ca2+. Ca2+ is also required for the stabilization of kinetochore microtubules by loading CENP-F into the kinetochore.
Till date, several classical and modern methodological tools had been employed to investigate the phenomenon of cytomixis for developing a clear understanding 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. Otherwise, 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.
Cucumis melo, one of the most important horticultural crops in Cucumis genus, has great variation in fruit sizes and shapes. Although comparative karyotype in Cucumis species has been conducted by several researchers, detailed information on molecular cytogenetic variation in relation to centromeres and ribosomal DNA (rDNA) loci within C. melo groups and other Cucumis species are still limited. Here, we conducted physical mapping of centromeric DNA (Cmcent), and two rDNAs (5S and 45S) on C. metuliferus, C. hystrix, and nineteen accessions of C. melo that were classified into subspecies melo and agrestis. Fluorescence in situ hybridization (FISH) using 5S and 45S rDNA probes revealed cytogenetic variation among C. melo accessions. C. melo subsp. melo and agrestis showed high conservation in terms of the number of 5S and 45S rDNA loci. However, the 45S rDNA signal intensities and the 5S rDNA locations are variable among C. melo species tested. Most of the accessions in agrestis group had two pairs of 45S rDNA with similar FISH signal intensities, except P90. Great variations were observed in melo group where some of the accessions showed similar patterns to subsp. agrestis while others showed two pairs of 45S rDNA loci with variable signal intensities. Our findings provide useful information for distinguishing C. melo accessions using centromeric DNA and rDNA probes, which would be important for breeding programs in melon.
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.
Genomic study is very important for understanding function of chromosomes, delving into evolution, and providing useful information for the strategies of genome manipulation. Karyotyping using Fluorescence in situ Hybridization (FISH) technique is one of the ways to achieve these goals. Although many publications describing cucumber chromosome analyses have been appeared, information on Indonesian cucumber is still rare. In the present study, karyotypes of four Indonesian pureline cultivars, five Indonesian hybrid cultivars and two landrace varieties of Japanese cucumber were observed and compared. Repetitive sequences of 45S and 5S rDNAs and one type of tandem repetitive DNA sequence (type I) were hybridized as FISH probes to mitotic chromosomes of cucumber. The results showed variation in the numbers of 45S rDNA signals, especially in the Indonesian hybrid cultivars which had the most various numbers of signals (eight, nine, and ten signals) among the accessions studied. Two Indonesian hybrid cultivars and one Japanese landrace variety had unpaired signals of 45S rDNA. On the contrary, all accessions had same number of 5S rDNA signals (a pair signals) on chromosome 5. The type I signals distributed on all of the cucumber chromosomes either at one end of the chromosome arm or at both ends of the arms in almost of all cucumber accessions. However, three accessions of Indonesian cucumber did not have type I signals on one of their chromosomes (i.e., chromosome 7). The data showed that Indonesian cucumbers investigated here had different karyotypes from the previous reports, implying potential genetic variation in Indonesian varieties.
The genus Apios (Leguminosae) comprises 6 species, which are distributed in East Asia and North America. The Japanese Apios comprises only one species: A. fortunei Maxim., distributed from Hokkaido to Kyushu. The Apios fortunei we examined had 2n=22 chromosomes. We are the first to report this chromosome number and karyotype for this species. Our present report is the first to show the karyotype of genus Apios, because no studies have ever tried to examine the karyotype on this genus.