A chromosome-level genome assembly for the Silkie chicken resolves complete sequences for key chicken metabolic, reproductive, and immunity genes

Frontiers and emerging topics in a century of Silkie chicken research: insights, challenges, and opportunities

Silkie chickens are a unique breed renowned for their pigmentation, food and medicine homology properties, and distinctive appearance, making them highly valuable in exhibitions, as pets, in medicinal cuisine, and as a model for melanin research. Despite their vast potential, the growing volume of publications and patents related to Silkie chicken highlights the critical need for systematic organization, summarization, and analysis of this wealth of information. For the first time, this study employs bibliometric tools to summarize and analyze 114 years of research on Silkie chicken. Our study demonstrates that academic studies primarily focus on their nutritional value, melanin production, and genetic mechanisms, while patents emphasize food formulations, breeding methods, and purebred identification. Although there has been significant growth in publications and citations since 2001, international collaboration remains limited. This study presents the need for integrated and multidisciplinary research to unlock the full potential of Silkie chicken and provides a foundational framework for future studies and applications.

Cloning, phylogenetic analysis, tissue expression profiling, and functional roles of NPC1L1 in chickens, quails, and ducks

The Niemann-Pick C1-Like 1 (NPC1L1) protein, primarily expressed in the epithelial cells of the small intestine, is essential for cholesterol absorption from both dietary intake and biliary secretion. Despite this conserved function across mammals, the full-length coding sequence of NPC1L1 remains uncharacterized in key avian models including chicken (Gallus gallus), quail (Coturnix japonica), and duck (Anas platyrhynchos). In this study, we successfully cloned the full NPC1L1 mRNA sequence in chicken, quail, and duck, including the entire 5' and 3' untranslated regions, utilizing rapid amplification of cDNA ends methods. Phylogenetic analysis across 12 species, comprising four avian and eight representative mammalian species, revealed that the NPC1L1 sequences in the main poultry species exhibit a high degree of similarity. Despite the phylogenetic divergence of poultry NPC1L1 sequences from their mammalian counterparts, protein sequence alignment revealed that the cholesterol-sensing peptides of NPC1L1 are conserved across all species examined in this study. These findings imply that the NPC1L1 in poultry may also play a role in cholesterol transport. Analysis of tissue gene expression profiles in chickens, quails, and ducks indicated that NPC1L1 is predominantly expressed in the duodenum, jejunum, and liver. Additionally, experiments on medium-to-cell cholesterol transit in primary intestinal epithelial cells confirmed that chicken NPC1L1 is capable of efficiently transporting cholesterol into cells. Further experiments are required to elucidate the biological function of poultry NPC1L1. In summary, this study successfully cloned the full-length sequence of NPC1L1 from chickens, quails, and ducks, and conducted a comprehensive analysis of their evolutionary history and expression patterns. This research establishes a foundation for future investigations into the role of poultry NPC1L1 in cholesterol transport.

Population genomic analysis identifies the complex structural variation at the fibromelanosis (FM) locus in chicken

Phenotypic diversity and its genetic basis are central questions in biology, with domesticated animals offering valuable insights due to their rapid evolution the last 10,000 years. In chickens, fibromelanosis (FM) is a striking pigmentation phenotype characterized by hyperpigmentation. A previous study identified a complex structural variant involving both two large duplications (127.4 and 170.5 kb in size) and inversions associated with upregulated expression of the Endothelin 3 (EDN3) gene. However, the detailed organization of the structural arrangements have remained unclear. In this study, we conducted a comprehensive genomic survey of 517 FM chickens representing 44 different populations. Our results elucidate the complex arrangement of the duplications and inversions at the FM locus based on the large-scale genomic survey, population level genotyping, and linkage disequilibrium analysis, providing conclusive support for one specific configuration of the two large duplications, resolving a controversy that has been unresolved for more than a decade. Our results show that the birth of this complex structural variant must have involved an interchromosomal rearrangement creating fixed heterozygosity due to sequence differences between the two copies of the 127.4 kb duplication. This study shows how population genomics can be used to understand complex structural variations that underlie phenotypic variation.

Common Ancestry of the Id Locus: Chromosomal Rearrangement and Polygenic Possibilities

The diversity in dermal pigmentation and plumage color among domestic chickens is striking, with Black Bone Chickens (BBC) particularly notable for their intense melanin hyperpigmentation. This unique trait is driven by a complex chromosomal rearrangement on chromosome 20 at the Fm locus, resulting in the overexpression of the EDN3 (a gene central to melanocyte regulation). In contrast, the inhibition of dermal pigmentation is regulated by the Id locus. Although prior studies using genetic crosses, GWAS, and gene expression analysis have investigated the genetic underpinnings of the Id locus, its precise location and functional details remain elusive. Our study aims to precisely locate the Id locus, identify associated chromosomal rearrangements and candidate genes influencing dermal pigmentation, and examine the ancestral status of the Id locus in BBC breeds. Using public genomic data from BBC and non-BBC breeds, we refined the Id locus to a ~1.6 Mb region that co-localizes with Z amplicon repeat units at the distal end of the q-arm of chromosome Z within a 10.36 Mb inversion in Silkie BBC. Phylogenetic and population structure analyses reveal that the Id locus shares a common ancestry across all BBC breeds, much like the Fm locus. Selection signatures and highly differentiated BBC-specific SNPs within the MTAP gene position it as the prime candidate for the Id locus with CCDC112 and additional genes, suggesting a possible polygenic nature. Our results suggest that the Id locus is shared among BBC breeds and may function as a supergene cluster in shank and dermal pigmentation variation.

Near telomere-to-telomere genome assemblies of Silkie Gallus gallus and Mallard Anas platyrhynchos restored the structure of chromosomes and "missing" genes in birds

BACKGROUND

Chickens and ducks are vital sources of animal protein for humans. Recent pangenome studies suggest that a single genome is insufficient to represent the genetic information of a species, highlighting the need for more comprehensive genomes. The bird genome has more than tens of microchromosomes, but comparative genomics, annotations, and the discovery of variations are hindered by inadequate telomere-to-telomere level assemblies. We aim to complete the chicken and duck genomes, recover missing genes, and reveal common and unique chromosomal features between birds.

RESULTS

The near telomere-to-telomere genomes of Silkie Gallus gallus and Mallard Anas platyrhynchos were successfully assembled via multiple high-coverage complementary technologies, with quality values of 36.65 and 44.17 for Silkie and Mallard, respectively; and BUSCO scores of 96.55% and 96.97% for Silkie and Mallard, respectively; the mapping rates reached over 99.52% for both assembled genomes, these evaluation results ensured high completeness and accuracy. We successfully annotated 20,253 and 19,621 protein-coding genes for Silkie and Mallard, respectively, and assembled gap-free sex chromosomes in Mallard for the first time. Comparative analysis revealed that microchromosomes differ from macrochromosomes in terms of GC content, repetitive sequence abundance, gene density, and levels of 5mC methylation. Different types of arrangements of centromeric repeat sequence centromeres exist in both Silkie and the Mallard genomes, with Mallard centromeres being invaded by CR1. The highly heterochromatic W chromosome, which serves as a refuge for ERVs, contains disproportionately long ERVs. Both Silkie and the Mallard genomes presented relatively high 5mC methylation levels on sex chromosomes and microchromosomes, and the telomeres and centromeres presented significantly higher 5mC methylation levels than the whole genome. Finally, we recovered 325 missing genes via our new genomes and annotated TNFA in Mallard for the first time, revealing conserved protein structures and tissue-specific expression.

CONCLUSIONS

The near telomere-to-telomere assemblies in Mallard and Silkie, with the first gap-free sex chromosomes in ducks, significantly enhanced our understanding of genetic structures in birds, specifically highlighting the distinctive chromosome features between the chicken and duck genomes. This foundational work also provides a series of newly identified missing genes for further investigation.

Transposable elements shape the landscape of heterozygous structural variation in a bird genome

Avian genomes exhibit compact organization and remarkable chromosomal stability. However, the extent and mechanisms by which structural variation in avian genomes differ from those in other vertebrate lineages are poorly explored. This study generated a diploid genome assembly for the golden pheasant ( Chrysolophus pictus), a species distinguished by the vibrant plumage of males. Each haploid genome assembly included complete chromosomal models, incorporating all microchromosomes. Analysis revealed extensive tandem amplification of immune-related genes across the smallest microchromosomes (dot chromosomes), with an average copy number of 54. Structural variation between the haploid genomes was primarily shaped by large insertions and deletions (indels), with minimal contributions from inversions or duplications. Approximately 28% of these large indels were associated with recent insertions of transposable elements, despite their typically low activity in bird genomes. Evidence for significant effects of transposable elements on gene expression was minimal. Evolutionary strata on the sex chromosomes were identified, along with a drastic rearrangement of the W chromosome. These analyses of the high-quality diploid genome of the golden pheasant provide valuable insights into the evolutionary patterns of structural variation in avian genomes.

Guinea fowl eggshell structural analysis at different scales reveals how organic matrix induces microstructural shifts that enhance its mechanical properties

Guinea fowl eggshells have an unusual structural arrangement that is different from that of most birds, consisting of two distinct layers with different microstructures. This bilayered organization, and distinct microstructural characteristics, provides it with exceptional mechanical properties. The inner layer, constituting about one third of the eggshell thickness, contains columnar calcite crystal units arranged vertically as in most bird shells. However, the thicker outer layer has a more complex microstructural arrangement formed by a switch to smaller calcite domains with diffuse/interlocking boundaries, partly resembling the interfaces seen in mollusk shell nacre. The switching process that leads to this remarkable second-layer microstructure is unknown. Our results indicate that the microstructural switching is triggered by changes in the inter- and intracrystalline organic matrix. During production of the outer microcrystalline layer in the later stages of eggshell formation, the interactions of organic matter with mineral induce an accumulation of defects that increase crystal mosaicity, instill anisotropic lattice distortions in the calcite structure, interrupt epitaxial growth, reduce crystallite size, and induce nucleation events which increase crystal misorientation. These structural changes, together with the transition between the layers and each layer having different microstructures, enhance the overall mechanical strength of the Guinea fowl eggshell. Additionally, our findings provide new insights into how biogenic calcite growth may be regulated to impart unique functional properties. STATEMENT OF SIGNIFICANCE: Avian eggshells are mineralized to protect the embryo and to provide calcium for embryonic chick skeletal development. Their thickness, structure and mechanical properties have evolved to resist external forces throughout brooding, yet ultimately allow them to crack open during chick hatching. One particular eggshell, that of the Guinea fowl, has structural features very different from other galliform birds - it is bilayered, with an inner columnar mineral structure (like in most birds), but it also has an outer layer with a complex microstructure which contributes to its superior mechanical properties. This work provides novel and new fundamental information about the processes and mechanisms that control and change crystal growth during the switch to microcrystalline domains when the second outer layer forms.

High quality assemblies of four indigenous chicken genomes and related functional data resources

Many lines of evidence indicate that red jungle fowl (RJF) is the primary ancestor of domestic chickens. Although multiple versions of RJF (galgal2-galgal5 and GRCg6a) and commercial chickens (GRCg7b/w and Huxu) genomes have been assembled since 2004, no high-quality indigenous chicken genomes have been assembled, hampering the understanding of chicken domestication and evolution. To fill the gap, we sequenced the genomes of four indigenous chickens with distinct morphological traits in southwest China, using a combination of short, long and Hi-C reads. We assembled each genome (~1.0 Gb) into 42 chromosomes with chromosome N50 90.5-90.9 Mb, amongst the highest quality of chicken genome assemblies. To provide resources for gene annotation and functional analysis, we also sequenced transcriptomes of 10 tissues for each of the four chickens. Moreover, we corrected many mis-assemblies and assembled missing micro-chromosomes 29 and 34-39 for GRCg6a. Our assemblies, sequencing data and the correction of GRCg6a can be valuable resources for studying chicken domestication and evolution.

When Livestock Genomes Meet Third-Generation Sequencing Technology: From Opportunities to Applications

Third-generation sequencing technology has found widespread application in the genomic, transcriptomic, and epigenetic research of both human and livestock genetics. This technology offers significant advantages in the sequencing of complex genomic regions, the identification of intricate structural variations, and the production of high-quality genomes. Its attributes, including long sequencing reads, obviation of PCR amplification, and direct determination of DNA/RNA, contribute to its efficacy. This review presents a comprehensive overview of third-generation sequencing technologies, exemplified by single-molecule real-time sequencing (SMRT) and Oxford Nanopore Technology (ONT). Emphasizing the research advancements in livestock genomics, the review delves into genome assembly, structural variation detection, transcriptome sequencing, and epigenetic investigations enabled by third-generation sequencing. A comprehensive analysis is conducted on the application and potential challenges of third-generation sequencing technology for genome detection in livestock. Beyond providing valuable insights into genome structure analysis and the identification of rare genes in livestock, the review ventures into an exploration of the genetic mechanisms underpinning exemplary traits. This review not only contributes to our understanding of the genomic landscape in livestock but also provides fresh perspectives for the advancement of research in this domain.