Mutations in Both the Surface and Transmembrane Envelope Glycoproteins of the RAV-2 Subgroup B Avian Sarcoma and Leukosis Virus Are Required to Escape the Antiviral Effect of a Secreted Form of the TvbS3 Receptor †.

The subgroup A through E avian sarcoma and leukosis viruses ASLV(A) through ASLV(E) are a group of highly related alpharetroviruses that have evolved to use very different host protein families as receptors. We have exploited genetic selection strategies to force the replication-competent ASLVs to naturally evolve and acquire mutations to escape the pressure on virus entry and yield a functional replicating virus. In this study, evolutionary pressure was exerted on ASLV(B) virus entry and replication using a secreted for of its Tvb receptor. As expected, mutations in the ASLV(B) surface glycoprotein hypervariable regions were selected that knocked out the ability for the mutant glycoprotein to bind the sTvbS3-IgG inhibitor. However, the subgroup B Rous associated virus 2 (RAV-2) also required additional mutations in the C-terminal end of the SU glycoprotein and multiple regions of TM highlighting the importance of the entire viral envelope glycoprotein trimer structure to mediate the entry process efficiently. These mutations altered the normal two-step ASLV membrane fusion process to enable infection.

A New Chicken Genome Assembly Provides Insight into Avian Genome Structure.

The importance of the Gallus gallus (chicken) as a model organism and agricultural animal merits a continuation of sequence assembly improvement efforts. We present a new version of the chicken genome assembly (Gallus_gallus-5.0; GCA_000002315.3), built from combined long single molecule sequencing technology, finished BACs, and improved physical maps. In overall assembled bases, we see a gain of 183 Mb, including 16.4 Mb in placed chromosomes with a corresponding gain in the percentage of intact repeat elements characterized. Of the 1.21 Gb genome, we include three previously missing autosomes, GGA30, 31, and 33, and improve sequence contig length 10-fold over the previous Gallus_gallus-4.0. Despite the significant base representation improvements made, 138 Mb of sequence is not yet located to chromosomes. When annotated for gene content, Gallus_gallus-5.0 shows an increase of 4679 annotated genes (2768 noncoding and 1911 protein-coding) over those in Gallus_gallus-4.0. We also revisited the question of what genes are missing in the avian lineage, as assessed by the highest quality avian genome assembly to date, and found that a large fraction of the original set of missing genes are still absent in sequenced bird species. Finally, our new data support a detailed map of MHC-B, encompassing two segments: one with a highly stable gene copy number and another in which the gene copy number is highly variable. The chicken model has been a critical resource for many other fields of study, and this new reference assembly will substantially further these efforts.

Validation of Alternative Transcript Splicing in Chicken Lines that Differ in Genetic Resistance to Marek's Disease.

Utilizing RNA-seq data, 1,574 candidate genes with alternative splicing were previously identified between two chicken lines that differ in Marek's disease (MD) genetic resistance under control and Marek's disease virus infection conditions. After filtering out 1,530 genes with splice variants in the first or last exon, 44 genes were screened for possible exon loss or gain using PCR and gel electrophoresis. Consequently, 7 genes exhibited visually detectable differential expression of splice variants between lines 6 (MD resistant) and 7 (MD susceptible), and the resultant PCR products verified by DNA sequencing. Birds from inbred line 6 have transcripts that preferentially retain an exon compared to line 7 chickens for ITGB2, SGPL1, and COMMD5. Birds from inbred line 7 have alleles that preferentially retain an exon compared to line 6 for MOCS2. CCBL2 exon 1a is absent and ATAD1 exon 2 is truncated by 87 nucleotides in transcripts expressed by line 7 compared to those from line 6. For CHTF18, line 6 transcripts have an indel mutation with 7 additional nucleotides in exon 21 compared to line 7. The current study validates 7 genes with alternatively spliced isomers between the two chicken lines, which helps provide potential underlying mechanisms for the phenotypic differences.

A comparative physical map reveals the pattern of chromosomal evolution between the turkey (Meleagris gallopavo) and chicken (Gallus gallus) genomes.

BACKGROUND: A robust bacterial artificial chromosome (BAC)-based physical map is essential for many aspects of genomics research, including an understanding of chromosome evolution, high-resolution genome mapping, marker-assisted breeding, positional cloning of genes, and quantitative trait analysis. To facilitate turkey genetics research and better understand avian genome evolution, a BAC-based integrated physical, genetic, and comparative map was developed for this important agricultural species. RESULTS: The turkey genome physical map was constructed based on 74,013 BAC fingerprints (11.9 × coverage) from two independent libraries, and it was integrated with the turkey genetic map and chicken genome sequence using over 41,400 BAC assignments identified by 3,499 overgo hybridization probes along with > 43,000 BAC end sequences. The physical-comparative map consists of 74 BAC contigs, with an average contig size of 13.6 Mb. All but four of the turkey chromosomes were spanned on this map by three or fewer contigs, with 14 chromosomes spanned by a single contig and nine chromosomes spanned by two contigs. This map predicts 20 to 27 major rearrangements distinguishing turkey and chicken chromosomes, despite up to 40 million years of separate evolution between the two species. These data elucidate the chromosomal evolutionary pattern within the Phasianidae that led to the modern turkey and chicken karyotypes. The predominant rearrangement mode involves intra-chromosomal inversions, and there is a clear bias for these to result in centromere locations at or near telomeres in turkey chromosomes, in comparison to interstitial centromeres in the orthologous chicken chromosomes. CONCLUSION: The BAC-based turkey-chicken comparative map provides novel insights into the evolution of avian genomes, a framework for assembly of turkey whole genome shotgun sequencing data, and tools for enhanced genetic improvement of these important agricultural and model species.

Comparative BAC-based mapping in the white-throated sparrow, a novel behavioral genomics model, using interspecies overgo hybridization.

BACKGROUND: The genomics era has produced an arsenal of resources from sequenced organisms allowing researchers to target species that do not have comparable mapping and sequence information. These new "non-model" organisms offer unique opportunities to examine environmental effects on genomic patterns and processes. Here we use comparative mapping as a first step in characterizing the genome organization of a novel animal model, the white-throated sparrow (Zonotrichia albicollis), which occurs as white or tan morphs that exhibit alternative behaviors and physiology. Morph is determined by the presence or absence of a complex chromosomal rearrangement. This species is an ideal model for behavioral genomics because the association between genotype and phenotype is absolute, making it possible to identify the genomic bases of phenotypic variation. FINDINGS: We initiated a genomic study in this species by characterizing the white-throated sparrow BAC library via filter hybridization with overgo probes designed for the chicken, turkey, and zebra finch. Cross-species hybridization resulted in 640 positive sparrow BACs assigned to 77 chicken loci across almost all macro-and microchromosomes, with a focus on the chromosomes associated with morph. Out of 216 overgos, 36% of the probes hybridized successfully, with an average number of 3.0 positive sparrow BACs per overgo. CONCLUSIONS: These data will be utilized for determining chromosomal architecture and for fine-scale mapping of candidate genes associated with phenotypic differences. Our research confirms the utility of interspecies hybridization for developing comparative maps in other non-model organisms.

Multi-platform next-generation sequencing of the domestic turkey (Meleagris gallopavo): genome assembly and analysis.

A synergistic combination of two next-generation sequencing platforms with a detailed comparative BAC physical contig map provided a cost-effective assembly of the genome sequence of the domestic turkey (Meleagris gallopavo). Heterozygosity of the sequenced source genome allowed discovery of more than 600,000 high quality single nucleotide variants. Despite this heterozygosity, the current genome assembly (∼1.1 Gb) includes 917 Mb of sequence assigned to specific turkey chromosomes. Annotation identified nearly 16,000 genes, with 15,093 recognized as protein coding and 611 as non-coding RNA genes. Comparative analysis of the turkey, chicken, and zebra finch genomes, and comparing avian to mammalian species, supports the characteristic stability of avian genomes and identifies genes unique to the avian lineage. Clear differences are seen in number and variety of genes of the avian immune system where expansions and novel genes are less frequent than examples of gene loss. The turkey genome sequence provides resources to further understand the evolution of vertebrate genomes and genetic variation underlying economically important quantitative traits in poultry. This integrated approach may be a model for providing both gene and chromosome level assemblies of other species with agricultural, ecological, and evolutionary interest.

Inhibition of Marek's disease virus replication by retroviral vector-based RNA interference.

RNA interference (RNAi) is a promising antiviral methodology. We recently demonstrated that retroviral vectors expressing short-hairpin RNAs (shRNA-mirs) in the context of a modified endogenous micro-RNA (miRNA) can be effective in reducing replication of other retroviruses in chicken cells. In this study, similar RNAi vectors are shown to inhibit replication of the avian herpesvirus, Marek's disease virus (MDV, also known as gallid herpesvirus type 2), and its close relative, herpesvirus of turkeys (HVT). Cells expressing shRNA-mirs targeting the MDV or HVT gB glycoprotein gene or the ICP4 transcriptional regulatory gene show significant inhibition of viral replication. Not only are viral titers reduced, but observed plaque sizes are significantly smaller when the virus is grown on cells in which RNAi is effective. We also describe a modified retroviral delivery vector that expresses a shRNA-mir containing up to three RNAi target sequences and employ this vector with multiple targets within the MDV gB gene or within both the gB and ICP4 genes. The use of targets within multiple genes potentially can provide a larger antiviral effect and/or make it more difficult for viral escape mutations to evolve.

Inhibition of avian leukosis virus replication by vector-based RNA interference.

RNA interference (RNAi) has recently emerged as a promising antiviral technique in vertebrates. Although most studies have used exogenous short interfering RNAs (siRNAs) to inhibit viral replication, vectors expressing short hairpin RNAs (shRNA-mirs) in the context of a modified endogenous micro-RNA (miRNA) are more efficient and are practical for in vivo delivery. In this study, replication competent retroviral vectors were designed to deliver shRNA-mirs targeting subgroup B avian leukosis virus (ALV), the most effective of which reduced expression of protein targets by as much as 90% in cultured avian cells. Cells expressing shRNA-mirs targeting the tvb receptor sequence or the viral env(B) sequence significantly inhibited ALV(B) replication. This study demonstrates efficient antiviral RNAi in avian cells using shRNA-mirs expressed from pol II promoters, including an inducible promoter, allowing for the regulation of the antiviral effect by doxycycline.

Marek's disease virus up-regulates major histocompatibility complex class II cell surface expression in infected cells.

Many herpesviruses modulate major histocompatibility complex (MHC) expression on the cell surface as an immune evasion mechanism. We report here that Marek's disease virus (MDV), a lymphotrophic avian alphaherpesvirus, up-regulates MHC class II cell surface expression in infected cells, contrary to all other herpesviruses examined to date. This MDV-induced class II up-regulation was detected both in vitro and in vivo. This effect was not solely an indirect effect of interferon, which is a highly potent natural inducer of MHC class II expression, since MHC class II up-regulation in cultured primary fibroblast cells was confined to the infected cells only. MHC class II up-regulation was also observed in infected cells of the bursa of Fabricius during the lytic phase of MDV infection in birds and upon reactivation of MDV from latency in an MDV-transformed cell line. As MDV is a strictly cell-associated virus and requires activated T cells for its life cycle, this up-regulation of MHC class II in infected cells may contribute to virus spread within the infected host by increasing the chance of contact between productively infected cells and susceptible activated T cells.

Integration of the cytogenetic and physical maps of chicken chromosome 17.

The chicken genome, like those of most avian species, contains numerous microchromosomes that cannot be distinguished by size alone. Unique properties attributed to the microchromosomes include high GC content and gene density, and an enhanced recombination rate. Previously, microchromosome GGA 17 was shown to align with the consensus genetic linkage group E41W17, and bacterial artificial chromosome (BAC) clones containing E41W17 markers were isolated and assigned on the physical BAC map as well as the recently assembled draft chicken genome sequence. For this study, these same BACS were utilized as probes for fluorescence in-situ hybridization (FISH) to develop the GGA 17 cytogenetic map. Here we detail the chromosome order of ten BAC DNAs, thereby deriving a cytogenetic map of GGA 17 that is simultaneously integrated with both the linkage map and genome sequence. The location of the FISH probes together with the morphological appearance of the chromosome suggested that GGA 17 is an acrocentric chromosome whose cytogenetic map orientation is reversed from that currently indicated by the linkage map and draft genome sequence. The reversed orientation and the centromere location of GGA 17 were confirmed experimentally by dual-colour FISH hybridization using terminal BACs and the centromere-specific CNM oligonucleotide as probes. An advantage of this cyto-genomic approach is the improved alignment of the sequence and linkage maps with cytogenetic features such as the centromere, telomeres, p and q arms, and staining patterns indicating GC versus AT content.

Telomerase activity and differential expression of telomerase genes and c-myc in chicken cells in vitro.

This study examined telomerase activity and gene expression profiles for three genes in Gallus gallus domesticus: telomerase reverse transcriptase (chTERT), telomerase RNA (chTR), and c-myc. Expression of these genes was studied in chicken embryonic stem (chES) cells, chicken embryo fibroblasts (CEFs), and DT40 cells using quantitative real-time polymerase chain reaction. Our results establish that, relative to transcription levels in telomerase-negative CEFs, chTERT and chTR are up-regulated in telomerase-positive chES cells. Transcription levels of chTERT, chTR, and c-myc are dramatically up-regulated in telomerase-positive DT40 cells, relative to CEFs and chES cells. These results are consistent with a model in which telomerase activity is up-regulated in proliferating embryonic stem cells requiring stable telomeres to endure multiple rounds of cell division; down-regulated in differentiated, lifespan-limited cells; and dramatically up-regulated in immortalized, transformed cells for which uncontrolled proliferation is correlated with c-myc dysregulation and telomerase activity.

Use of chicken models for the analysis of human disease.

The chicken has been a valuable model organism for genetic analysis since early in the last century. This unit provides an overview of the primary strategic approaches through which the chicken is used as a model for human genetics and disease. These approaches are reviewed with key advantages and disadvantages noted for each. In addition, the chicken offers a model system for several human inherited diseases, both single gene-based and complex traits. This unit briefly describes two such models as examples: autoimmune pathology causing Hashimoto's thyroiditis and sex-linked dwarfism.

A BAC-based physical map of the chicken genome.

A genome-wide physical map constructed with bacterial artificial chromosomes (BACs) is an essential component in linking phenotypic traits to the responsible genetic variation in the genomes of plants and animals. We have constructed a physical map of the chicken genome from 57,091 BACs (7.9-fold haploid genome coverage) by restriction fingerprint analysis using high-resolution polyacrylamide gel electrophoresis. The physical map consists of 2331 overlapping BAC contigs and is estimated to span 1510 Mb in physical length. BAC contigs were verified manually and by screening the BACs with 367 DNA markers. A total of 361 of the contigs have been anchored to the existing chicken genetic map. This map represents the first genome-wide, BAC-based physical map of the chicken genome. It provides a powerful platform for many areas of chicken genomics, including targeted marker development, fine mapping of genes and QTL alleles, positional cloning, analysis of avian genome organization and evolution, chicken-mammalian comparative genomics, and large-scale genome sequencing.