Number and distribution of silver-stained nucleolar organizer regions and evolutionary relationships in domestic pigs

Cytogenetic screening of a Canadian swine breeding nucleus using a newly developed karyotyping method named oligo-banding

BACKGROUND

The frequency of chromosomal rearrangements in Canadian breeding boars has been estimated at 0.91 to 1.64%. These abnormalities are widely recognized as a potential cause of subfertility in livestock production. Since artificial insemination is practiced in almost all intensive pig production systems, the use of elite boars carrying cytogenetic defects that have an impact on fertility can lead to major economic losses. To avoid keeping subfertile boars in artificial insemination centres and spreading chromosomal defects within populations, cytogenetic screening of boars is crucial. Different techniques are used for this purpose, but several issues are frequently encountered, i.e. environmental factors can influence the quality of results, the lack of genomic information outputted by these techniques, and the need for prior cytogenetic skills. The aim of this study was to develop a new pig karyotyping method based on fluorescent banding patterns.

RESULTS

The use of 207,847 specific oligonucleotides generated 96 fluorescent bands that are distributed across the 18 autosomes and the sex chromosomes. Tested alongside conventional G-banding, this oligo-banding method allowed us to identify four chromosomal translocations and a rare unbalanced chromosomal rearrangement that was not detected by conventional banding. In addition, this method allowed us to investigate chromosomal imbalance in spermatozoa.

CONCLUSIONS

The use of oligo-banding was found to be appropriate for detecting chromosomal aberrations in a Canadian pig nucleus and its convenient design and use make it an interesting tool for livestock karyotyping and cytogenetic studies.

Nucleolar organizer region banding in crossbred and non-descript pigs of India

Aim:

The objective of this experiment was to study the nucleolar organizer region (NOR)-banding pattern in Large White Yorkshire (LWY) crossbred and non-descript pigs and finding differences in the number of NORs between animals and between genetic groups.

Materials and Methods:

The experiment was carried out on 15 females, and 15 males of LWY crossbred and non-descript pigs to study NOR-banding pattern by employing ammoniacal silver staining technique.

Results:

A total of 63 and 65 number of good metaphases were prepared in LWY crossbred, and non-descript pigs and a total of 168 and 143 number of NORs were detected on the 8th and 10th chromosomes in both genetic groups, respectively. The mean number of NORs per metaphase was 2.67 and 2.20 in LWY crossbred and non-descript pigs, respectively. LWY crossbred pig had high mean number of silver-stained NORs (Ag-NORs) per metaphase compared to non-descript pig. In general, it was observed that the highest frequency of metaphases (%) examined had two number of NORs, while the lowest frequency (%) had four number of NORs. The number of NORs observed per metaphase on secondary constrictions of the 8th and 10th chromosome pair in both genetic groups ranged from 2 to 4. The Chi-square test of significance revealed that the observed frequencies do not differ significantly from the expected frequencies.

Conclusion:

The results confirmed differences across breeds in occurrence and number of NORs on chromosomes in pigs. The mean numbers of NORs present per metaphase vary between the animals indicating the existence of polymorphism for the number of NORs. A higher number of Ag-NORs were observed on chromosome pair 10 in both the genetic groups. It was concluded that NORs were more morphologically distinct and greater on chromosome pair 10 than on pair 8, which suggests a dominant role of chromosome 10 in the global production of ribosomal RNA.

An integrated RH map of porcine chromosome 10

BACKGROUND

Whole genome radiation hybrid (WG-RH) maps serve as "scaffolds" to significantly improve the orientation of small bacterial artificial chromosome (BAC) contigs, order genes within the contigs and assist assembly of a sequence-ready map for virtually any species. Here, we report the construction of a porcine: human comparative map for pig (Sus scrofa) chromosome 10 (SSC10) using the IMNpRH2(12,000-rad) porcine WG-RH panel, integrated with the IMpRH(7000-rad) WG-RH, genetic and BAC fingerprinted contig (FPC) maps.

RESULTS

Map vectors from the IMNpRH2(12,000-rad) and IMpRH(7,000-rad) panels were merged to construct parallel framework (FW) maps, within which FW markers common to both panels have an identical order. This strategy reduced map discrepancies between the two panels and significantly improved map accuracy. A total of 216 markers, including 50 microsatellites (MSs), 97 genes and ESTs, and 69 BAC end sequences (BESs), were ordered within two linkage groups at two point (2 pt) LOD score of 8. One linkage group covers SSC10p with accumulated map distances of 738.2 cR(7,000) and 1814.5 cR(12,000), respectively. The second group covers SSC10q at map distances of 1336.9 cR(7,000) and 3353.6 cR(12,000), yielding an overall average map resolution of 16.4 kb/cR(12,000) or 393.5 kb per marker on SSC10. This represents an approximately 2.5-fold increase in map resolution over the IMpRH(7,000-rad) panel. Based on 127 porcine markers that have homologous sequences in the human genome, a detailed comparative map between SSC10 and human (Homo sapiens) chromosome (HSA) 1, 9 and 10 was built.

CONCLUSION

This initial comparative RH map of SSC10 refines the syntenic regions between SSC10 and HSA1, 9 and 10. It integrates the IMNpRH2(12,000-rad) and IMpRH(7,000-rad), genetic and BAC FPC maps and provides a scaffold to close potential gaps between contigs prior to genome sequencing and assembly. This map is also useful in fine mapping of QTLs on SSC10.

Oxidant-induced decrease of the expression of nucleolar organizer regions in pig lymphocytes can be useful for monitoring the cellular effects of oxidative stress

The technique of silver staining of nucleolar organizer regions (AgNORs) was chosen to estimate the transcriptionally active metaphase and interphase nucleolar organizer regions (NORs) in pig peripheral blood lymphocytes exposed to oxidative agents in vitro. The quantitative analysis of AgNORs was performed by using the counting method and the morphometric method. We found that hydrogen peroxide and 2,2'-azobis-(2-amidinopropane)-dihydrochloride (AAPH) induced a concentration-dependent decrease in NORs activity - in the case of metaphase the NORs activity was exclusively seen on chromosome pairs 8 - which can be considered as another estimate of cellular oxidative stress. Moreover, biomarkers of cyto- and genotoxicity, such as the percentage of apoptotic and necrotic cells, the nuclear division index (NDI) and formation of micronuclei (MN) were used to measure harmful effects provoked by the agents tested.

The mechanisms determining the nucleolar-organizing regions inactivation of domestic horse chromosomes

Cytogenetic investigations of the nucleolar-organizing regions (NORs) show that there is variation in the transcriptional activity of rDNA in many organisms. As a consequence, genetic polymorphism of these regions has been detected. The aim of the present study was to evaluate the hypothetic genetic mechanisms determining the NORs polymorphism of the domestic horse chromosomes. Molecular cytogenetic analyses were carried out on Hucul horses and the following techniques were used: fluorescence in situ hybridization (FISH), telomere primed in situ synthesis (PRINS), in situ nick-translation with HpaII, silver staining (AgNOR) and C-banding technique (CBG). The obtained results suggest that variation in the number and size of silver deposits is related to the number of rDNA copies, DNA methylation and the localization of ribosomal DNA loci in telomeric regions. Moreover, we have found that chromosome pairs 28 and 31 are characterized by higher variation in the NORs number.

Interstitial telomeric sites and NORs in Hartmann's zebra (Equus zebra hartmannae) chromosomes

Interstitial telomeric sites (ITSs) are considered as signatures of chromosomal rearrangements that take place during karyotype evolution. Understanding that equids have undergone rapid karyotype evolution compared with the average in other mammals, a search of these signatures was carried out in the Hartmann's mountain zebra (Equus zebra hartmannae; EZH) chromosomes. Six consistent ITSs were identified on five of the zebra chromosomes (EZH1p, 1q, 2q, 5q, 6q and 11q). The location of these ITSs coincided with fusion points of some of the evolutionarily conserved human-Hartmann's zebra chromosomal segments suggesting that the sequences are remnants of fusion events between ancestral chromosomes. Incidentally, three of the ITSs also matched with the presence of constitutive heterochromatin. Further, ribosomal gene clusters were localized on five zebra chromosomes and the data were compared with those in other equid species. The findings offer preliminary evidence on the likely evolution of some of the Hartmann's zebra chromosomes and add to the current search for clues that lead to the ancestral chromosomal configuration in equids.