Refinement of Bos taurus sequence assembly based on BAC-FISH experiments

Molecular Cytogenetics in Domestic Bovids: A Review

The discovery of the Robertsonian translocation (rob) involving cattle chromosomes 1 and 29 and the demonstration of its deleterious effects on fertility focused the interest of many scientific groups on using chromosome banding techniques to reveal chromosome abnormalities and verify their effects on fertility in domestic animals. At the same time, comparative banding studies among various species of domestic or wild animals were found useful for delineating chromosome evolution among species. The advent of molecular cytogenetics, particularly the use of fluorescence in situ hybridization (FISH), has allowed a deeper investigation of the chromosomes of domestic animals through: (a) the physical mapping of specific DNA sequences on chromosome regions; (b) the use of specific chromosome markers for the identification of the chromosomes or chromosome regions involved in chromosome abnormalities, especially when poor banding patterns are produced; (c) better anchoring of radiation hybrid and genetic maps to specific chromosome regions; (d) better comparisons of related and unrelated species by comparative FISH mapping and/or Zoo-FISH techniques; (e) the study of meiotic segregation, especially by sperm-FISH, in some chromosome abnormalities; (f) better demonstration of conserved or lost DNA sequences in chromosome abnormalities; (g) the use of informatic and genomic reconstructions, in addition to CGH arrays, to predict conserved or lost chromosome regions in related species; and (h) the study of some chromosome abnormalities and genomic stability using PCR applications. This review summarizes the most important applications of molecular cytogenetics in domestic bovids, with an emphasis on FISH mapping applications.

Anchoring the CerEla1.0 Genome Assembly to Red Deer (Cervus elaphus) and Cattle (Bos taurus) Chromosomes and Specification of Evolutionary Chromosome Rearrangements in Cervidae

Simple Summary

The red deer (Cervus elaphus) de novo genome assembly (CerEla1.0) has provided a great resource for genetic studies in various deer species. In this study, we used gene order comparisons between C. elaphus CerEla1.0 and B. taurus ARS-UCD1.2 genome assemblies and fluorescence in situ hybridization (FISH) with bovine BAC probes to verify the red deer-bovine chromosome relationships and anchor the CerEla1.0 C-scaffolds to karyotypes of both species. We showed the homology between bovine and deer chromosomes and determined the centromere-telomere orientation of the CerEla1.0 C-scaffolds. Using a set of BAC probes, we were able to narrow the positions of evolutionary chromosome breakpoints defining the family Cervidae. In addition, we revealed several errors in the current CerEla1.0 genome assembly. Finally, we expanded our analysis to other Cervidae and confirmed the locations of the cervid evolutionary fissions and orientation of the fused chromosomes in eight cervid species. Our results can serve as a basis for necessary improvements of the red deer genome assembly and provide support to other genetic studies in Cervidae.

Abstract

The family Cervidae groups a range of species with an increasing economic significance. Their karyotypes share 35 evolutionary conserved chromosomal segments with cattle (Bos taurus). Recent publication of the annotated red deer (Cervus elaphus) whole genome assembly (CerEla1.0) has provided a basis for advanced genetic studies. In this study, we compared the red deer CerEla1.0 and bovine ARS-UCD1.2 genome assembly and used fluorescence in situ hybridization with bovine BAC probes to verify the homology between bovine and deer chromosomes, determined the centromere-telomere orientation of the CerEla1.0 C-scaffolds and specified positions of the cervid evolutionary chromosome breakpoints. In addition, we revealed several incongruences between the current deer and bovine genome assemblies that were shown to be caused by errors in the CerEla1.0 assembly. Finally, we verified the centromere-to-centromere orientation of evolutionarily fused chromosomes in seven additional deer species, giving a support to previous studies on their chromosome evolution.

Identification of Some Errors in the Genome Assembly of Bovidae by FISH

From an economic point of view, Bovidae represent the most important family of the Ruminantia suborder. Thus, the mitochondrial and nuclear genomes of Bos taurus were among the first genomes to be sequenced after the sequencing of the human genomes. Over the millennia, the evolution of the genomes of the 3 main species belonging to the Bovidae family - B. taurus (BTA), Ovis aries (OAR), and Capra hircus (CHI) - has led to few chromosome rearrangements. Certainly, the availability and free access to the animal genomes significantly contributed to the improvement of animal genetics; however, some errors may exist due to the high automation in the genomic assembly construction process. In this work, some differences between the genomes of cattle, goat, and sheep highlighted by bioinformatics analysis have been verified by FISH, confirming that some errors persist even in the most recent genome assemblies. This type of approach has allowed us to detect a misassembly of a region belonging to BTA16 and to the homologues OAR12 and CHI16, a misassembly of a short tract in BTA22, OAR19, and CHI22, an incorrect mapping of a region of BTA21 and of CHI27 and OAR26, a discrepancy in the BTA26, OAR22, and CHI26 assemblies, a missed inversion in CHI1 compared to BTA1 and OAR1, and the exact assembly of a region of about 7 Mb in OAR10 and CHI12. Incorrect positioning of genomic tracts can cause unintended consequences in genetic analyses, especially when the data represent a starting point for the construction of genetic tools. In the new genomic assemblies published after the conclusion of our experiments, however, the accuracy in the construction of animal assemblies has been much improved, even if the new assemblies present more extended unmapped portions than the previous versions. The gap could be filled by comparative analyses between similar species or FISH.

ChromaWizard: An open source image analysis software for multicolor fluorescence in situ hybridization analysis

Multicolor image analysis finds its applications in a broad range of biological studies. Specifically, multiplex fluorescence in situ hybridization (M-FISH) for chromosome painting facilitates the analysis of individual chromosomes in complex metaphase spreads and is widely used to detect both numerical and structural aberrations. While this is well established for human and mouse karyotypes, for which species sophisticated software and analysis tools are available, other organisms and species are less well served. Commercially available software is proprietary and not easily adaptable to other karyotypes. Therefore, a publically available open source software that combines flexibility and customizable functionalities is needed. Here we present such a tool called "ChromaWizard" which is based on popular scientific image analysis libraries (OpenCV, scikit-image, and NumPy). We demonstrate its functionality on the example of primary Chinese hamster (Cricetulus griseus) fibroblasts metaphase spreads and on Chinese Hamster Ovary cell lines known for the large number of chromosomal rearrangements. The application can be easily adapted to any kind of available labeling kits and is independent of the used organism and instrumentation. It allows direct inspection of the original hybridization signals and enables either manual or automatic assignment of colors, making it a functional and versatile tool that can be used also for other multicolor applications.

Revealing misassembled segments in the bovine reference genome by high resolution linkage disequilibrium scan

Background

Misassembly signatures, created by shuffling the order of sequences while assembling a genome, can be detected by the unexpected behavior of marker linkage disequilibrium (LD) decay. We developed a heuristic process to identify misassembly signatures, applied it to the bovine reference genome assembly (UMDv3.1) and presented the consequences of misassemblies in two case studies.

Results

We identified 2,906 single nucleotide polymorphism (SNP) markers presenting unexpected LD decay behavior in 626 putative misassembled contigs, which comprised less than 1 % of the whole genome. Although this represents a small fraction of the reference sequence, these poorly assembled segments can lead to severe implications to local genome context. For instance, we showed that one of the misassembled regions mapped to the POLL locus, which affected the annotation of positional candidate genes in a GWAS case study for polledness in Nellore (Bos indicus beef cattle). Additionally, we found that poorly performing markers in imputation mapped to putative misassembled regions, and that correction of marker positions based on LD was capable to recover imputation accuracy.

Conclusions

This heuristic approach can be useful to cross validate reference assemblies and to filter out markers located at low confidence genomic regions before conducting downstream analyses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3049-8) contains supplementary material, which is available to authorized users.

Genetic variants and signatures of selective sweep of Hanwoo population (Korean native cattle)

Although there have been many studies of native Korean cattle, Hanwoo, there have been no selective sweep studies in these animals. This study was performed to characterize genetic variation and identify selective signatures. We sequencedthe genomes of 12 cattle, and identified 15125420 SNPs, 1768114 INDELs, and 3445 CNVs. The SNPs, INDELs, and CNVs were similarly distributed throughout the genome, and highly variable regions were shown to contain the BoLA family and GPR180, which are related to adaptive immunity. We also identified the domestication footprints of the Hanwoo population by searching for selective sweep signatures, which revealed the RCN2 gene related to BPV resistance. The results of this study may contribute to genetic improvement of the Hanwoo population in Korea. [BMB Reports 2013; 46(7):346-351]

Acceleration of X-chromosome gene order evolution in the cattle lineage

The gene order on the X chromosome of eutherians is generally highly conserved, although an increase in the rate of rearrangement has been reported in the rodent lineage. Conservation of the X chromosome is thought to be caused by selection related to maintenance of dosage compensation. However, we herein reveal that the cattle (Btau4.0) lineage has experienced a strong increase in the rate of X-chromosome rearrangement, much stronger than that previously reported for rodents. We also show that this increase is not matched by a similar increase on the autosomes and cannot be explained by assembly errors. Furthermore, we compared the difference in two cattle genome assemblies: Btau4.0 and Btau6.0 (Bos taurus UMD3.1). The results showed a discrepancy between Btau4.0 and Btau6.0 cattle assembly version data, and we believe that Btau6.0 cattle assembly version data are not more reliable than Btau4.0. [BMB Reports 2013; 46(6): 310-315]

Physical mapping of 20 unmapped fragments of the btau_4.0 genome assembly in cattle, sheep and river buffalo

The recent advances in sequencing technology and bioinformatics have revolutionized genomic research, making the decoding of the genome an easier task. Genome sequences are currently available for many species, including cattle, sheep and river buffalo. The available reference genomes are very accurate, and they represent the best possible order of loci at this time. In cattle, despite the great accuracy achieved, a part of the genome has been sequenced but not yet assembled: these genome fragments are called unmapped fragments. In the present study, 20 unmapped fragments belonging to the Btau_4.0 reference genome have been mapped by FISH in cattle (Bos taurus, 2n = 60), sheep (Ovis aries, 2n = 54) and river buffalo (Bubalus bubalis, 2n = 50). Our results confirm the accuracy of the available reference genome, though there are some discrepancies between the expected localization and the observed localization. Moreover, the available data in the literature regarding genomic homologies between cattle, sheep and river buffalo are confirmed. Finally, the results presented here suggest that FISH was, and still is, a useful technology to validate the data produced by genome sequencing programs.

Fine mapping of copy number variations on two cattle genome assemblies using high density SNP array

Background

Btau_4.0 and UMD3.1 are two distinct cattle reference genome assemblies. In our previous study using the low density BovineSNP50 array, we reported a copy number variation (CNV) analysis on Btau_4.0 with 521 animals of 21 cattle breeds, yielding 682 CNV regions with a total length of 139.8 megabases.

Results

In this study using the high density BovineHD SNP array, we performed high resolution CNV analyses on both Btau_4.0 and UMD3.1 with 674 animals of 27 cattle breeds. We first compared CNV results derived from these two different SNP array platforms on Btau_4.0. With two thirds of the animals shared between studies, on Btau_4.0 we identified 3,346 candidate CNV regions representing 142.7 megabases (~4.70%) of the genome. With a similar total length but 5 times more event counts, the average CNVR length of current Btau_4.0 dataset is significantly shorter than the previous one (42.7 kb vs. 205 kb). Although subsets of these two results overlapped, 64% (91.6 megabases) of current dataset was not present in the previous study. We also performed similar analyses on UMD3.1 using these BovineHD SNP array results. Approximately 50% more and 20% longer CNVs were called on UMD3.1 as compared to those on Btau_4.0. However, a comparable result of CNVRs (3,438 regions with a total length 146.9 megabases) was obtained. We suspect that these results are due to the UMD3.1 assembly's efforts of placing unplaced contigs and removing unmerged alleles. Selected CNVs were further experimentally validated, achieving a 73% PCR validation rate, which is considerably higher than the previous validation rate. About 20-45% of CNV regions overlapped with cattle RefSeq genes and Ensembl genes. Panther and IPA analyses indicated that these genes provide a wide spectrum of biological processes involving immune system, lipid metabolism, cell, organism and system development.

Conclusion

We present a comprehensive result of cattle CNVs at a higher resolution and sensitivity. We identified over 3,000 candidate CNV regions on both Btau_4.0 and UMD3.1, further compared current datasets with previous results, and examined the impacts of genome assemblies on CNV calling.

A false single nucleotide polymorphism generated by gene duplication compromises meat traceability

Controlling meat traceability using SNPs is an effective method of ensuring food safety. We have analyzed several SNPs to create a panel for bovine genetic identification and traceability studies. One of these was the transversion g.329C>T (Genbank accession no. AJ496781) on the cytochrome P450 17A1 gene, which has been included in previously published panels. Using minisequencing reactions, we have tested 701 samples belonging to eight Spanish cattle breeds. Surprisingly, an excess of heterozygotes was detected, implying an extreme departure from Hardy-Weinberg equilibrium (P<0.001). By alignment analysis and sequencing, we detected that the g.329C>T SNP is a false positive polymorphism, which allows us to explain the inflated heterozygotic value. We recommend that this ambiguous SNP, as well as other polymorphisms located in this region, should not be used in identification, traceability or disease association studies. Annotation of these false SNPs should improve association studies and avoid misinterpretations.