Cloning and expression profiling of the VLDLR gene associated with egg performance in duck (Anas platyrhynchos)

Identification and Characterization of the Very-Low-Density Lipoprotein Receptor Gene from Branchiostoma belcheri: Insights into the Origin and Evolution of the Low-Density Lipoprotein Receptor Gene Family

Low-density lipoprotein receptors (LDLRs) are a class of cell-surface endocytosis receptors that are mainly involved in cholesterol homeostasis and cellular signal transduction. Very-low-density lipoprotein receptors (VLDLRs), which are members of the LDLR family, have been regarded as multi-function receptors that fulfill diverse physiological functions. However, no VLDLR gene has been identified in protochordates to date. As a representative protochordate species, amphioxi are the best available example of vertebrate ancestors. Identifying and characterizing the VLDLR gene in amphioxi has high importance for exploring the evolutionary process of the LDLR family. With this study, a new amphioxus VLDLR gene (designated AmphiVLDLR) was cloned and characterized using RACE-PCR. The 3217 nt transcript of the AmphiVLDLR had a 2577 nt ORF, and the deduced 858 amino acids were highly conserved within vertebrate VLDLRs according to their primary structure and three-dimensional structure, both of which contained five characteristic domains. In contrast to other vertebrate VLDLRs, which had a conserved genomic structure organization with 19 exons and 18 introns, the AmphiVLDLR had 13 exons and 12 introns. The results of a selective pressure analysis showed that the AmphiVLDLR had numerous positive selection sites. Furthermore, the tissue expression of AmphiVLDLR using RT-qPCR showed that AmphiVLDLR RNA expression levels were highest in the gills and muscles, moderate in the hepatic cecum and gonads, and lowest in the intestines. The results of the evolutionary analysis demonstrated that the AmphiVLDLR gene is a new member of the VLDLR family whose family members have experienced duplications and deletions over the evolutionary process. These results imply that the functions of LDLR family members have also undergone differentiation. In summary, we found a new VLDLR gene homolog (AmphiVLDLR) in amphioxi. Our results provide insight into the function and evolution of the LDLR gene family.

The study of candidate genes in the improvement of egg production in ducks – a review

Duck is the second-largest poultry species aside from chicken. The rate of egg production is a major determinant of the economic income of poultry farmers. Among the reproductive organs, the ovary is a major part of the female reproductive system which is highly important for egg production. Based on the importance of this organ, several studies have been carried out to identify candidate genes at the transcriptome level, and also the expression level of these genes at different tissues or egg-laying conditions, and single nucleotide polymorphism (SNPs) of genes associated with egg production in duck. In this review, expression profile and association study analyses at SNPs level of different candidate genes with egg production traits of duck were highlighted. Furthermore, different studies on transcriptome analysis, Quantitative Trait Loci (QTL) mapping, and Genome Wide Association Study (GWAS) approach used to identify potential candidate genes for egg production in ducks were reported. This review would widen our knowledge on molecular markers that are associated or have a positive correlation to improving egg production in ducks, for the increasing world populace.

Genome-Wide Association Study of Egg-Laying Traits and Egg Quality in LingKun Chickens

Egg production is the most important trait of laying hens. To identify molecular markers and candidate genes associated with egg production and quality, such as body weight at first oviposition (BWF), the number of eggs produced in 500 days (EN500), egg weight (EW), egg shell thickness (EST), egg shell strength (ESS), and Haugh unit (HU), a genome-wide analysis was performed in 266 LingKun Chickens. The results showed that thirty-seven single nucleotide polymorphisms (SNPs) were associated with all traits (p < 9.47 × 10⁻⁸, Bonferroni correction). These SNPs were located in close proximity to or within the sequence of the thirteen candidate genes, such as Galanin And GMAP Prepropeptide (GAL), Centromere Protein (CENPF), Glypican 2 (GPC2), Phosphatidylethanolamine N-Methyltransferase (PEMT), Transcription Factor AP-2 Delta (TFAP2D), and Carboxypeptidase Q (CPQ) gene related to egg-laying and Solute Carrier Family 5 Member 7 (SLC5A7), Neurocalcin Delta (NCALD), Proteasome 20S Subunit Beta 2 (PSMB2), Slit Guidance Ligand 3 (SLIT3), and Tubulin Tyrosine Ligase Like 7 (TTLL7) genes related to egg quality. Interestingly, one of the genes involved in bone formation (SLIT3) was identified as a candidate gene for ESS. Our candidate genes and SNPs associated with egg-laying traits were significant for molecular breeding of egg-laying traits and egg quality in LingKun chickens.

Expression and analysis of ESR1, IGF-1, FSH, VLDLR, LRP, LH, PRLR genes in Pekin duck and Black Muscovy duck

In order to explore the influence of egg-laying regulatory genes on egg production in ducks at different laying stages, Pekin duck and Black Muscovy duck were used in this study, including early laying stage (20-30 weeks old), peak laying period (31-48 weeks old) and late laying stage (49-66 weeks old). Relative quantitative RT-PCR was used to detect the mRNA transcription level of selected egg-laying regulatory genes in the ovary tissues of ducks at different laying stages. Study shows: during the laying period of Pekin duck, ESR1, LRP1, IGF-1 and LHR were involved in the regulation of egg-laying, and the high expression of LRP1 in the late stage could inhibit egg production. Still, the expression products of the other three genes showed promoting effect. During the laying period of Black Muscovy duck, FSH, VLDLR, IGF-1, PRLR, LHR and LRP1 participated in the regulation of egg-laying, in which the expression products of the first five genes could promote egg production, while LRP1 showed inhibitory effect. Through our experiments, these data will provide strong theoretical support for the breeding of Pekin duck and Black Muscovy duck.

Comparative transcriptomic analysis of high and low egg-producing duck ovaries

The egg-laying rate is an important indicator of egg production of laying ducks. Egg production directly impacts the economic benefits of the duck industry. In order to obtain better insight into the molecular mechanisms associated with the process of egg production, comparative transcriptomic analysis of the ovaries of Jinding ducks with high and low egg production was performed using the Illumina HiSeq 2500 system. A total of 843 differentially expressed genes (DEGs) was identified, 367 that were down-regulated and 476 that were up-regulated in high egg production (HEP) ovaries, as compared with low egg production (LEP) ovaries. Some genes, such as MC5R, APOD, ORAI1, and DYRK4, were more active in HEP ovaries, indicating that these genes may play important roles in regulation of egg production. Among these 843 DEGs, 685 were assigned to gene ontology (GO) categories. Of these, 25 genes were related to reproduction, and 30 were related to the reproductive process, including some associated with ovarian follicle development, circadian regulation of gene expression, circadian rhythm, and estrogen receptor binding. Furthermore, some important functional pathways were revealed, such as the steroid biosynthesis pathway, the endocrine and other factor-regulated calcium reabsorption pathways, circadian rhythm, the neuroactive ligand-receptor interaction pathway, fatty acid biosynthesis, and the calcium-signaling pathway, which appear to be much more active in the HEP group, as compared to those of the LEP group. The results of this study provide very useful information that may contribute to future functional studies of genes involved in bird reproduction.

Association of VLDLR haplotypes with abdominal fat trait in ducks

Abstract. This study aimed to determine the correlation among VLDLR (very low-density lipoprotein receptor) gene polymorphisms, body weight and abdominal fat deposition of Gaoyou ducks. A total of 267 Gaoyou ducks from one pure line was employed for testing. The polymorphisms of the VLDLR gene were screened by polymerase chain reaction and DNA sequencing. Four novel single nucleotide polymorphisms (SNPs) (g.151G  >  A, g.170C  >  T, g.206A  >  G and g.278–295del) were identified in the 5'-UTR and signal peptide region. Furthermore, eight haplotypes were identified based on the four SNPs. The H8 was the most common haplotype with a frequency of more than 31 %. The four SNPs and their haplotype combinations were shown to be significantly associated with body weight at 6–10 weeks of age (P < 0. 05 or P < 0. 01) and abdominal fat percentage (AFP) (P < 0. 05 or P < 0. 01). Remarkably, the H1H1 diplotype had an effect on increasing body weight and decreasing AFP from the 6th to the 10th weeks of age. However, increasing positive effects of the H5H8 diplotype were observed for both body weight and AFP. This study suggests that the VLDLR gene plays an important role in the regulation of body weight and fat-related traits and may serve as a potential marker for the marker-assisted selection program during duck breeding.

The association of very low-density lipoprotein receptor (VLDLR) haplotypes with egg production indicates VLDLR is a candidate gene for modulating egg production

The very low-density lipoprotein receptor (VLDLR) transports egg yolk precursors into oocytes. However, our knowledge of the distribution patterns of VLDLR variants among breeds and their relationship to egg production is still incomplete. In this study, eight single nucleotide polymorphisms (SNPs) that account for 87% of all VLDLR variants were genotyped in Nick Chick (NC, n=91), Lohmann Brown (LohB, n=50) and Lueyang (LY, n=381) chickens, the latter being an Chinese indigenous breed. Egg production by NC and LY chickens was recorded from 17 to 50 weeks. Only four similar haplotypes were found in NC and LohB, of which two accounted for 100% of all NC haplotypes and 92.5% of LohB haplotypes. In contrast, there was considerable haplotypic diversity in LY. Comparison of egg production in LY showed that hens with NC-like haplotypes had a significantly higher production (p < 0.05) than those without the haplotypes. However, VLDLR expression was not significantly different between the haplotypes. These findings indicate a divergence in the distribution of VLDLR haplotypes between selected and non-selected breeds and suggest that the near fixation of VLDLR variants in NC and LohB is compatible with signature of selection. These data also support VLDLR as a candidate gene for modulating egg production.

Endothelin Receptor B2 (EDNRB2) Gene Is Associated with Spot Plumage Pattern in Domestic Ducks (Anas platyrhynchos)

Endothelin receptor B subtype 2 (EDNRB2) is a seven-transmembrane G-protein coupled receptor. In this study, we investigated EDNRB2 gene as a candidate gene for duck spot plumage pattern according to studies of chicken and Japanese quail. The entire coding region was cloned by the reverse transcription polymerase chain reaction (RT-PCR). Sequence analysis showed that duck EDNRB2 cDNA contained a 1311bp open reading frame and encoded a putative protein of 436 amino acids residues. The transcript shared 89%-90% identity with the counterparts in other avian species. A phylogenetic tree based on amino acid sequences showed that duck EDNRB2 was evolutionary conserved in avian clade. The entire coding region of EDNRB2 were sequenced in 20 spot and 20 non-spot ducks, and 13 SNPs were identified. Two of them (c.940G>A and c.995G>A) were non-synonymous substitutions, and were genotyped in 647 ducks representing non-spot and spot phenotypes. The c.995G>A mutation, which results in the amino acid substitution of Arg332His, was completely associated with the spot phenotype: all 152 spot ducks were carriers of the AA genotype and the other 495 individuals with non-spot phenotype were carriers of GA or GG genotype, respectively. Segregation in 17 GA×GG and 22 GA×GA testing combinations confirmed this association since the segregation ratios and genotypes of the offspring were in agreement with the hypothesis. In order to investigate the underlying mechanism of the spot phenotype, MITF gene was used as cell type marker of melanocyte progenitor cells while TYR and TYRP1 gene were used as cell type markers of mature melanocytes. Transcripts of MITF, TYR and TYRP1 gene with expected size were identified in all pigmented skin tissues while PCR products were not obtained from non-pigmented skin tissues. It was inferred that melanocytes are absent in non-pigmented skin tissues of spot ducks.

Two novel linkage SNPs of VLDLR gene intron 11 are associated with laying traits in two quail populations

Abstract. The very low density lipoprotein receptor (VLDLR) is an important multifunctional receptor and plays a key role in chicken reproduction. This study is designed to investigate the effect of variants in the VLDLR gene on quail laying traits. Two quail populations were studied – yellow feather quail and chestnut feather quail; 384 individuals per breed were used. The laying traits (the weight of the first egg, the age of the first egg, egg weight, the weight of 20-week-old and the egg number of 20-week-old) were measured and recorded. The polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP) method was developed to genotype those individuals. The results showed two novel polymorphisms, i.e. two linkage variations in intron 11 of the VLDLR gene (363T > C and 392C > T). They are associated with the weight of the first egg, the age of the first egg and egg number of 20-week-old in yellow feather and chestnut feather quail. The two variations in intron 11 of VLDLR may be linked with potential major loci or genes affecting some laying traits.

Cooperation of luteinizing hormone signaling pathways in preovulatory avian follicles regulates circadian clock expression in granulosa cell

Ovulation in birds is triggered by a surge of luteinizing hormone (LH), and the ovulatory cycle is affected by the circadian rhythms of clock genes transcription levels in follicles. The influence of LH signaling cascades action on circadian clock genes was investigated using granulosa cells of preovulatory follicles from Roman hens cultured in a serum-free system. The expression of core oscillators (Bmal1, Clock, Cry1, Per2, and Rev-erbβ), clock-controlled gene (Star), Egr-1 and LHr was measured by quantitative real-time PCR. Significant changes in clock genes transcription levels were observed in control groups over 24 h, indicating that cell-autonomous rhythms exist in granulosa cells. Intriguingly, the transcript levels of clock genes increased with LH treatment during 24 h of culture; they peaked 4 h in advance of controls and second but weaker oscillations were also observed. It appeared that LH changed the cell-autonomous rhythm and cycle time of clock genes. To further investigate the LH signaling cascades, inhibitors of cyclic adenosine monophosphate (cAMP), p38 mitogen-activated protein kinases (p38MAPK) and extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathways were used. The transcript levels of clock genes were suppressed by blocking cAMP, but increased with similar expression patterns by blocking the p38MPAK and ERK1/2 pathways over 24 h. Thus, the influence of LH signaling cascades in chicken ovulation is mediated by the cAMP pathway and also involves the p38MAPK and ERK1/2 pathways.