Chromosomal Heteromorphism with Reference to Cancer and Reproduction in Human: An Appraisal

Background and Objectives: Human cell nucleus has, the genome consisting of euchromatin and heterochromatin. The euchromatin has gene-rich and actively functional. The heterochromatin has two components namely constitutive and facultative, where the former is highly polymorphic. It is related to numerous diseases like cancer and infertility which is now well known, though it was earlier thought to be inactive; hence the implication of these polymorphic variants of chromosomes is reviewed with respect to acrocentric and non- acrocentric types. Methodologies: The polymorphic variants can be detected by C, G, Q and R banding techniques. We usually follow G band preparation of karyotypes following World Health Organisation (WHO) manuals and their role in cancer and reproduction is reviewed. Review and Conclusion: It is emphasized that most of the p and q arms of 1, 9, 16, D and G groups and X, Y chromosomes exhibited polymorphism which are related to cancerous and infertile conditions in both sexes. Data on few non-acrocentric chromosomes like 2, 4, 8, 10, 12, 18, 19 and 20 are not available. Our review however indicated that the evaluation of specific heteromorphic variants needs to be detected using specific probes for confirmation of anomaly to assist affected cases, though earlier data indicated ambiguous information with few cases analyzed regarding assisted reproductive technologies and malignancy condition. This appraisal thus would play a key role in human chromosomal heteromorphic abnormalities and recommend genetic tests and counseling ultimately made available to the affected cases.

Evolution of recombination rates between sex chromosomes

In species with genetic sex-determination, the chromosomes carrying the sex-determining genes have often evolved non-recombining regions and subsequently evolved the full set of characteristics denoted by the term ‘sex chromosomes’. These include size differences, creating chromosomal heteromorphism, and loss of gene functions from one member of the chromosome pair. Such characteristics and changes have been widely reviewed, and underlie molecular genetic approaches that can detect sex chromosome regions. This review deals mainly with the evolution of new non-recombining regions, focusing on how certain evolutionary situations select for suppressed recombination (rather than the proximate mechanisms causing suppressed recombination between sex chromosomes). Particularly important is the likely involvement of sexually antagonistic polymorphisms in genome regions closely linked to sex-determining loci. These may be responsible for the evolutionary strata of sex chromosomes that have repeatedly formed by recombination suppression evolving across large genome regions. More studies of recently evolved non-recombining sex-determining regions should help to test this hypothesis empirically, and may provide evidence about whether other situations can sometimes lead to sex-linked regions evolving. Similarities with other non-recombining genome regions are discussed briefly, to illustrate common features of the different cases, though no general properties apply to all of them. This article is part of the themed issue ‘Evolutionary causes and consequences of recombination rate variation in sexual organisms’.

Chromosomal variability in the wild ornamental species of Symphysodon (Perciformes: Cichlidae) from Amazon

Cytogenetic studies were conducted on three discus species which inhabit the Amazon in Brazil: Symphysodon haraldi from Manacapuru, S. aequifasciatus from Tefé and S. discus from Barcelos. All individuals showed 2n=60 chromosomes, most of them biarmed. No sexual chromosomal heteromorphism was verified. However, different karyotypic formulae, owing to the presence of subtelocentric chromosomes, were verified for S. aequifasciatus and S. discus. One of the karyotypic formulae from S. aequifasciatus (cytotype 2) differs from the others, due to one of the homologues in the first chromosome pair being significantly larger than the other. A large variability was observed toward the nucleolar organizer regions (NOR) of S. haraldi and S. aequifasciatus. Although the number of silver-stained blocks varied from 2 to 5, confirming different NOR patterns, at least seven homologue pairs were involved with NORs. In S. discus only two marks were observed, however two chromosome pairs were involved, characterizing a multiple NOR system for the three species. The heterochromatic blocks were mainly located in the pericentromeric region of all chromosomes but, in some of them, they are also located in the proximal regions, both in the short and long arms. Moreover, in the cytotype 2 from S. aequifasciatus, an interstitial heterochromatic block was observed on the long arm of the largest homologue of the first pair. A direct comparison of karyotypes from more related genera (Heros, Uaru, Mesonauta and Pterophyllum), makes it clear that a succession of chromosomal rearrangements, mainly pericentric inversions, translocations and fissions/fusions occurred resulting in the present diploid number and intraspecific karyological variability found in Symphysodon.

An Approach for Quantitative Assessment of Fluorescence In Situ Hybridization (FISH) Signals for Applied Human Molecular Cytogenetics

A number of applied molecular cytogenetic studies require the quantitative assessment of fluorescence in situ hybridization (FISH) signals (for example, interphase FISH analysis of aneuploidy by chromosome enumeration DNA probes; analysis of somatic pairing of homologous chromosomes in interphase nuclei; identification of chromosomal heteromorphism after FISH with satellite DNA probes for differentiation of parental origin of homologous chromosome, etc.). We have performed a pilot study to develop a simple technique for quantitative assessment of FISH signals by means of the digital capturing of microscopic images and the intensity measuring of hybridization signals using Scion Image software, commonly used for quantification of electrophoresis gels. We have tested this approach by quantitative analysis of FISH signals after application of chromosome-specific DNA probes for aneuploidy scoring in interphase nuclei in cells of different human tissues. This approach allowed us to exclude or confirm a low-level mosaic form of aneuploidy by quantification of FISH signals (for example, discrimination of pseudo-monosomy and artifact signals due to over-position of hybridization signals). Quantification of FISH signals was also used for analysis of somatic pairing of homologous chromosomes in nuclei of postmortem brain tissues after FISH with “classical” satellite DNA probes for chromosomes 1, 9, and 16. This approach has shown a relatively high efficiency for the quantitative registration of chromosomal heteromorphism due to variations of centromeric alphoid DNA in homologous parental chromosomes. We propose this approach to be efficient and to be considered as a useful tool in addition to visual FISH signal analysis for applied molecular cytogenetic studies.