Differential expression of fatty acid transporters and fatty acid synthesis-related genes in crop tissues of male and female pigeons (Columba livia domestica) during incubation and chick rearing

Characteristics of glucolipid metabolism and oxidative stress in breeding pigeons (Columba livia) during lactation

Breeding pigeons is a fundamental source of profit in various enterprises but little is known on the metabolic laws governing their lactation. In this study, we analysed the metabolic profile of different sex of breeding pigeons (Columba livia, European pigeons, Mimas) during lactation. We found that male pigeons exhibited catabolism during lactation. Extension of lactation resulted in increased weight loss, then slow recovery of body weight. Conversely, the weight loss in female pigeons peaked on the seventh day of lactation. They then gradually recovered their body weight. Male pigeons showed more duration of combing, while female pigeons showed more duration of resting. In male pigeons, except for triglyceride (TG), which increased, blood lipid indexes barely changed during lactation. Conversely, in females, both TG and total cholesterol increased in middle and late lactation. The level of oxidative stress in female pigeons during lactation was higher than in males, lipid peroxide malondialdehyde, hydrogen peroxide (H2 O2 ), plasma calcium (Ca) and phosphorus (P) levels increased in late lactation. Levels of estradiol and progesterone in female pigeons increased during lactation, whereas those of luteotropic hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL) and testosterone gradually decreased. As per LC-MS spectra analysis, the differential metabolites in the plasma on the day of hatching and before laying in female pigeons in lactation were enriched in retrograde endocannabinoid signalling, α-linolenic acid, arachidonic acid, choline, glycerophospholipid metabolisms, and valine, leucine, and isoleucine degradations. Levels of fatty acids, amino acids, sphingomyelin and phosphatidylinositol related to the secretion of pigeon milk had reduced, whereas the levels of phosphatidylcholine, phosphatidylethanolamine, and TG, which are all related to egg production, had increased. In conclusion, our study systematically revealed the different metabolic characteristics of male and female breeding pigeons during lactation. This is useful for precision feeding of pigeons and applicable in nutritional interventions for improved production.

Comprehensive Transcriptomic and Metabolomic Analysis Revealed the Functional Differences in Pigeon Lactation between Male and Female during the Reproductive Cycle

Lactation is a unique reproductive behavior in pigeons, with the crop serving as the organ responsible for secreting pigeon milk. Both male and female pigeons can produce crop milk and rear their offspring through a division of labor. Since the time of the secretion of pigeon crop milk is different in the process of feeding the young, whether the metabolism and formation of pigeon milk use the same mechanism is a very interesting scientific question. However, the metabolic dynamics and underlying genetic mechanisms involved in the formation of pigeon crop milk remain unclear, particularly during the incubation-feeding reproductive cycle. In this study, we integrated lactation-associated metabolism and transcriptome data from the crop tissues of both male and female pigeons during the brooding and feeding stages. We mapped the changes in metabolites related to milk formation in the crop tissues during these stages. Through metabolome profiling, we identified 1413 metabolites among 18 crop tissues. During the breeding cycles, the concentrations of estrone, L-ergothioneine, and L-histidine exhibited the most dynamic changes in females. In contrast, estrone, L-anserine, 1-methylhistidine, homovanillate, oxidized glutathione, and reducing glutathione showed the most dynamic changes in males. Gender-specific differences were observed in the metabolome, with several metabolites significantly differing between males and females, many of which were correlated with cytokine binding, immunity, and cytochrome P450 activity. Using this dataset, we constructed complex regulatory networks, enabling us to identify important metabolites and key genes involved in regulating the formation of pigeon milk in male and female pigeons, respectively. Additionally, we investigated gender-associated differences in the crop metabolites of pigeons. Our study revealed differences in the modulation of pigeon crop milk metabolism between males and females and shed light on the potential functions of male and female pigeon milk in the growth, development, and immunity of young pigeons, an area that has not been previously explored. In conclusion, our results provide new insights into the metabolic regulation of pigeon crop milk formation during the brooding and breeding stages. Furthermore, our findings lay the foundation for the accurate development of artificial pigeon milk.

Pigeon during the Breeding Cycle: Behaviors, Composition and Formation of Crop Milk, and Physiological Adaptation

Pigeon is an important economic poultry species in many countries. As an altricial bird, its growth and development are largely reliant on pigeon milk produced by the crop tissue in the first week. During the breeding cycle, pigeons undergo a series of behavioral changes. Pigeon milk is generally characterized by having high concentrations of proteins and lipids, and a complicated regulatory network is involved in the milk formation. Hormones, especially prolactin, could promote the proliferation of crop epidermal cells and nutrient accumulation. The expression of target genes associated with these important biological processes in the crop epidermis is affected by non-coding RNAs. Meanwhile, signaling pathways, such as target of rapamycin (TOR), Janus kinase/signal transducer and activator of transcription proteins (JAK/STAT), protein kinase B (Akt), etc., influence the production of crop milk by either enhancing protein synthesis in crop cells or inducing apoptosis of crop epidermal cells. In order to adapt to the different breeding periods, pigeons are physiologically changed in their intestinal morphology and function and liver metabolism. This paper reviews the behaviors and physiological adaptations of pigeon during the breeding cycle, the composition of pigeon crop milk, and the mechanism of its formation, which is important for a better understanding of the physiology of altricial birds and the development of artificial crop milk.

Spatio-temporal transcriptome dynamics coordinate rapid transition of core crop functions in 'lactating' pigeon

Pigeons (Columba livia) are among a select few avian species that have developed a specialized reproductive mode wherein the parents produce a 'milk' in their crop to feed newborn squabs. Nonetheless, the transcriptomic dynamics and role in the rapid transition of core crop functions during 'lactation' remain largely unexplored. Here, we generated a de novo pigeon genome assembly to construct a high resolution spatio-temporal transcriptomic landscape of the crop epithelium across the entire breeding stage. This multi-omics analysis identified a set of 'lactation'-related genes involved in lipid and protein metabolism, which contribute to the rapid functional transitions in the crop. Analysis of in situ high-throughput chromatin conformation capture (Hi-C) sequencing revealed extensive reorganization of promoter-enhancer interactions linked to the dynamic expression of these 'lactation'-related genes between stages. Moreover, their expression is spatially localized in specific epithelial layers, and can be correlated with phenotypic changes in the crop. These results illustrate the preferential de novo synthesis of 'milk' lipids and proteins in the crop, and provides candidate enhancer loci for further investigation of the regulatory elements controlling pigeon 'lactation'.

Metabolomics analysis of the effects of different litter size on reproductive metabolism and oxidative stress in breeding pigeon (Columba livia)

The pigeon breeding industry employs a high-rearing pattern to achieve economic benefits. However, too many squabs consume more energy of the breeding pigeons causing adverse effects on their breeding performance. To determine the optimal rearing patterns and the effects of different numbers of squabs on reproductive performance, oxidative stress, and glucolipid metabolism of lactating breeding pigeons in winter, three rearing patterns consisting of "2 + 2″, "2 + 3″ and "2 + 4" (a pair of breeding pigeons feeding two, three and four squabs, respectively) were adopted using European Mimas white pigeons breed. The feed intake, bodyweight loss, feed-to-meat ratio, and squab mortality were linearly increased with the number of squabs during lactation, while the bodyweight recovery rate and squab growth performance were significantly slowed down after lactation. Similarly, the laying rate was linearly decreased on days 16, 17, and 18 of lactation, with a similar pattern on the re-laying rate on days 11, 12, and 13 after first laying. In addition, the number of non-laying pigeons in the second batch was significantly increased, implying that the number of squabs significantly affected the reproductive performance of female pigeons. The eggshell weight and thickness in the "2 + 3″ group were significantly increased. However, the number of squabs in the "2 + 3″ group had no significant effect on plasma calcium (Ca) and phosphorus (P) levels. Analysis of the glucolipid metabolism index and oxidative stress level of pigeons further revealed that the contents of triglyceride (TG), total cholesterol (T-CHO), and low-density lipoprotein cholesterol (LDL-C) in the plasma of male pigeons were significantly decreased with the increase in the number of squabs, but there was no obvious oxidative stress. On the contrary, glucose (GLU), TG, malondialdehyde (MDA) in the plasma of female pigeons were significantly increased, total antioxidant capacity (T-AOC) were significantly decreased, implying that the female pigeons suffered more oxidative stress and more dramatic changes in glucolipid metabolism. Metabolomics revealed that the differential metabolites in the plasma of female pigeons in "2 + 2″, "2 + 3″, and "2 + 4″ groups were significantly enriched in the fatty acid, phospholipid, sphingolipid metabolism, and the Krebs cycle pathways, especially under "2 + 4″ rearing pattern. Overall, in female pigeons, the available lipids were reduced; hence, their body turned to sugar dysplasia and protein utilization mode, increasing the oxidative stress level and decreasing their reproductive performance. Therefore, an increased number of squabs significantly affects the body condition and reproductive performance of breeding pigeons. The "2 + 3″ rearing pattern is the most suitable for winter breeding pigeon production under the current nutrition level.

Differential expression of glucose metabolism-related genes and AMP-activated protein kinases in crop tissue of male and female pigeons (Columba livia domestica) during the incubation and chick-rearing periods

The objective of this study was to explore the carbohydrate contents of crop milk, insulin and glucose concentrations in serum and the expression patterns of AMP-activated protein kinases (AMPKs) and genes related to glucose metabolism in pigeon crops during the breeding period. Crop milk was collected from squabs of rearing Day 1 (R1) to R6. Contents of total sugar and reducing sugar increased to the maximum levels at R6 (p < 0.05). Forty-two pairs of adult pigeons were allotted to seven groups by different breeding stages, and their crops and serum were sampled. No significant differences were found in either insulin or glucose levels in serum. The glucose transporter 2 gene level was the greatest at R15 in females, whereas it was at R7 in males. However, sodium-dependent glucose transporters 1 expression in both sexes decreased from incubation Day 17 (I17) to R7. In females, glucokinase expression peaked at R1, and at R1 and R7 in males. Pyruvate kinase mRNA levels peaked at R7 in females and at R15 males. The mRNA abundance of fructose-1,6-bisphosphatase 1 in both sexes and glucose-6-phosphatase in females decreased after I10. While phosphoenolpyruvate carboxykinase 1 expression decreased after I17 (p < 0.05). Protein levels of AMPKα in crops were minimized at R1 (p < 0.05). In females, expression of AMPKα1 and AMPKα2 was inhibited at I17 and R1 (p < 0.05). In males, AMPKα1 expression was decreased at R7 (p < 0.05) and AMPKα2 was reduced at I10 and R1. pAMPK expression was the lowest at I17 in females, and it was at R7 and R25 in males. Conclusively, glycolysis in pigeon crops was enhanced during chick-rearing, while gluconeogenesis was significantly inhibited. The stability of the insulin level suggests that it was probably not involved in the regulation of glucose metabolism in crop tissues.

Dynamic changes in protein concentrations of keratins in crop milk and related gene expression in pigeon crops during different incubation and chick rearing stages

1. The objective of this study was to examine the keratin composition of crop milk, the variation of epithelial thickness and keratin (K) gene expression in samples from young pigeon during incubation and chick rearing.2. Crop milk was collected from 1-, 3- and 5-day-old squab crops for keratin content analysis. Results showed that K4 accounted for the highest proportion of all detected keratins.3. In total, 42 pairs of adult pigeons were allocated to seven groups according to different stages to collect crop samples. Gene expression studies showed that the K3 gene expression was maximised at rearing Day 15 (15) and R1 in males and females, respectively. K6a gene level was the greatest at R15 in females, whereas it peaked at incubation Day 4 (I4) in males. The K12, K13, K23 and K80 gene levels were inhibited at the peak period of crop milk formation in comparison with I4. In females, K cochleal expression peaked at I10, whereas it was the greatest at R25 in males. K4 and K14 gene expression was the highest at I10 in females, while K4 and K14 were minimised at I17 and R7 in males, respectively. Gene expressions of K5, K8, K19 and K20 in males and K19 in females were maximised at R1. The K5, K20 and K75 gene levels in females peaked at R7. K75 and K8 expressions in males and females reached a maximum value at R25 and I17, respectively.4. The epithelial thickness of male and female crops reached their greatest levels at R1 and had the highest correlation with K19.5. These results emphasised the importance of keratinisation in crop milk formation, and different keratins probably play various roles during this period. The K19 was probably a marker for pigeon crop epithelium development. The sex of the parent pigeon affected keratin gene expression profiles.

Prolactin promotes crop epithelial proliferation of domestic pigeons (Columba livia) through the Hippo signaling pathway

The purpose of this study was to investigate whether prolactin (PRL) regulates the proliferation of pigeon crop epithelium through the Hippo signaling pathway during the breeding cycle. Twenty-four pairs of adult pigeons were allotted to four groups by different breeding stages, and their crops and serum were sampled. Eighteen pairs of young pigeons were selected and divided into three groups for the injection experiments. The results showed that the serum PRL content and crop epithelial thickness of pigeons increased significantly at day 17 of incubation (I17) and day 1 of chick-rearing (R1). In males, the mRNA levels of yes-associated transcriptional regulator (YAP) and snail family transcriptional repressor 2 (SNAI2) were peaked at I17, and the gene levels of large tumor suppressor kinase 1 (LATS1), serine/threonine kinase 3 (STK3), TEA domain transcription factor 3 (TEAD3), connective tissue growth factor (CTGF), MYC proto-oncogene (c-Myc) and SRY-box transcription factor 2 (SOX2) reached the maximum value at R1. In females, the gene expression of YAP, STK3, TEAD3, and SOX2 reached the greatest level at I17, the expression profile of SAV1, CTGF, and c-Myc were maximized at R1. In males, the protein levels of LATS1 and YAP were maximized at R1 and the CTGF expression was upregulated at I17. In females, LATS1, YAP, and CTGF reached a maximum value at I17, and the expression level of phosphorylated YAP was minimized at I17 in males and females. Subcutaneous injection of prolactin (injected for 6 d, 10 μg per kg body weight every day) on the left crop of pigeons can promote the proliferation of crop epithelium by increasing the CTGF level and reducing the phosphorylation level of YAP. YAP-TEAD inhibitor verteporfin (injection for 6 d, 2.5 mg per kg body weight every day) can inhibit the proliferation of crop epithelium induced by prolactin by inhibiting YAP and CTGF expression. In conclusion, PRL can participate in crop cell proliferation of pigeons by promoting the expression of YAP and CTGF in Hippo pathway.

miR-193-5p negatively regulates PIK3CD to promote crop fibrocyte proliferation in pigeon (Columba livia)

The crop of pigeon has specific characteristics as producing crop milk in the lactating period. However, the exact mechanisms underlying the regulation of crop lactation remain unclear. miRNAs, the essential regulators of gene expression, are implicated in various physiological and biological activities. In this study, we discovered a new miRNA that regulated phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit delta (PIK3CD) and crop fibrocyte proliferation. Results of the luciferase reporter assay suggested that miR-193-5p suppressed PIK3CD expression by targeting a conserved binding site in the 3'-untranslated region (UTR) of PIK3CD mRNA. MiR-193-5p promoted crop fibrocyte proliferation and migration, whereas PIK3CD inhibited these effects. These findings suggested an important regulatory role of miR-193-5p in crop fibrocyte proliferation, suggesting that miR-193-5p and PIK3CD might be important regulators of crop milk production.

Lipid accumulation and oxidative stress in the crop tissues of male and female pigeons during incubation and chick-rearing periods

The current study aimed to evaluate the changes in lipid accumulation and oxidative status in pigeon crops during different breeding stages. Forty-two pairs of adult pigeons were randomly assigned to 7 groups. Lipid droplet accumulation in pigeon crops was visualized by using oil red O staining from d 17 of incubation (I17) to d 7 of chick rearing (R7). Transmission electron microscopy analysis showed swollen mitochondria with disintegration of cristae and typical characteristics of endoplasmic reticulum stress in crop tissues at R1 compared with those at I4. During the peak of pigeon milk formation, the concentrations of reactive oxygen species, and oxidative damage markers (advanced oxidation protein products, 8-hydroxy-2 deoxyguanosine, and malondialdehyde) and the enzyme activities of total superoxide dismutase and glutathione peroxidase were all elevated significantly (P < 0.05). The protein concentration of B-cell lymphoma-2 associated X in crop tissues was significantly higher at R1, while the level of B-cell lymphoma-2 protein in males was the highest at I4 (P < 0.05). The ratio of B-cell lymphoma-2 associated X protein (Bax)/B-cell lymphoma-2 (Bcl-2) in both male and female crops peaked at R1 (P < 0.05). Gene expression of the key enzymes involved in mitochondrial and peroxisomal fatty acid β-oxidation was investigated in crops. In males, the gene expression of carnitine palmitoyltransferase 1a peaked at R15, and that of carnitine palmitoyltransferase 2 increased significantly from R1 to R15 (P < 0.05). The mRNA abundance of long chain 3-hydroxyacyl-CoA dehydrogenase increased to the maximum value at R1 and I17 in males and females, respectively. From I17 to R7, the mRNA levels of acyl-CoA oxidase 1 and acyl-CoA oxidase 2 were decreased in pigeon crops (P < 0.05). Conclusively, lipid droplet accumulation was found in male and female pigeon crops from the end of incubation to the early stage of chick rearing. Although antioxidant defence and mitochondrial fatty acid β-oxidation were both mobilized, oxidative stress in crop tissues still occurred during the peak of milk formation.

Sex-Dependent Regulation of Placental Oleic Acid and Palmitic Acid Metabolism by Maternal Glycemia and Associations with Birthweight

Pregnancy complications such as maternal hyperglycemia increase perinatal mortality and morbidity, but risks are higher in males than in females. We hypothesized that fetal sex-dependent differences in placental palmitic-acid (PA) and oleic-acid (OA) metabolism influence such risks. Placental explants (n = 22) were incubated with isotope-labeled fatty acids (13C-PA or 13C-OA) for 24 or 48 h and the production of forty-seven 13C-PA lipids and thirty-seven 13C-OA lipids quantified by LCMS. Linear regression was used to investigate associations between maternal glycemia, BMI and fetal sex with 13C lipids, and between 13C lipids and birthweight centile. Placental explants from females showed greater incorporation of 13C-OA and 13C-PA into almost all lipids compared to males. Fetal sex also influenced relationships with maternal glycemia, with many 13C-OA and 13C-PA acylcarnitines, 13C-PA-diacylglycerols and 13C-PA phospholipids positively associated with glycemia in females but not in males. In contrast, several 13C-OA triacylglycerols and 13C-OA phospholipids were negatively associated with glycemia in males but not in females. Birthweight centile in females was positively associated with six 13C-PA and three 13C-OA lipids (mainly acylcarnitines) and was negatively associated with eight 13C-OA lipids, while males showed few associations. Fetal sex thus influences placental lipid metabolism and could be a key modulator of the impact of maternal metabolic health on perinatal outcomes, potentially contributing toward sex-specific adaptions in which females prioritize survival.

Influence of dietary phosphorus concentrations on the performance of rearing pigeons (Columba livia), and bone properties of squabs

The objective of this study was to investigate the effects of dietary P levels on the performance of rearing pigeons, and bone characteristics of squabs from 7 to 21 d of age. A total of 192 pairs of adult Silver King pigeons (40 wk of age) were used. The pigeons were randomly allocated to one of 4 treatment groups, each consisting of eight replicates of 6 pigeon pairs per replicate. Dietary treatments included the basal diet (containing 0.3% of P), the basal diet supplemented with 0.2, 0.4, or 0.8% inorganic P. And the dietary Ca content was kept at 1.40% across all treatments. The experimental diets were fed to parent pigeons as corn-soybean complete pellet feed, and squabs fed with crop milk secreted by parent pigeons. Pigeons in the group of 0.4% supplemental non-phytate phosphorus (NPP) had shorter (P = 0.045) oviposition interval than those in the control group and group of 0.8% NPP. When the diet was supplemented with 0.8% of NPP, the least average egg weight was observed (P = 0.006). Female breeding birds had much higher (P < 0.01) Ca, P, and ALP in serum than male ones. At 7-d of age, dietary P supplementation influenced P and Ca content in tibia ash of squabs (P < 0.05). The tibia ash Ca content in the group of 0.2% NPP was the highest among the treatments (P = 0.007). At d 21 of age, both the birds in the group of 0.4 and 0.8% NPP had higher tibia breaking strength (P < 0.01) and tibia ash contents (P < 0.001) compared to the ones in the control group. In conclusion, the P deficiency in the diet of parent pigeons could cause poor bone mineralization of squabs, especially impaired the bone-breaking strength and bone ash content. The 0.8% of NPP supplementation in the diet has a positive influence on mineralization of squabs although production depression was observed. Both P and Ca metabolism of female breeding birds were more active than male ones at earlier time points of rearing period. The desirable supplemental NPP level in diet for breeding pigeon was 0.4% according to the performance data in the present trial. The recommended Ca: P ratio for pigeons, which was different from the optimum value for broilers, needs to be studied in the future.

Exploration of Proteomics Analysis of Crop Milk in Pigeons (Columba livia) during the Lactation Period

Pigeon milk is a curdlike substance separated from the mature crop epithelium of breeders, associated with the rapid growth and development of squabs. The aim of this study was to investigate in detail the variations in the content of several important ingredients in crop milk. In this study, we utilized proteomic techniques to investigate the composition and changing pattern of crop milk protein of squabs on days 1 (D1), 3 (D3), and 7 (D7). Our results indicated that the crude protein contents in crop milk decreased with age, and they were up to 50% during the first 3 days. The proteomic data showed that a total of 2558 proteins were identified in all samples from three stages, and the top 15% crop milk proteins were ribosomal protein, keratin, peroxiredoxin, annexin, heat shock protein, and eukaryotic translation protein based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis and normalized spectral abundance factors (NSAFs) calculation. Furthermore, the compositions of crop milk protein between D1 and D3 were quite similar [51 differentially expressed proteins (DEPs)], while great proteomic differences were observed between D1/D3 and D7 (more than 240 DEPs). Additionally, gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that up-regulated DEPs mainly participate in immune response, while down-regulated DEPs were involved in cell differentiation and development as well as tRNA aminoacylation biosynthesis. In conclusion, DEPs were mainly related to protein synthesis, immunity, and antioxidation, which provided effective information for the development of artificial squab milk products in the future.

Effect of Akt activation on apoptosis-related gene expression in the crop tissues of male and female pigeons (Columba livia)

The current study investigated whether the expression of apoptosis genes in the pigeon crops was affected by the Akt signaling pathway during crop milk formation. First, 78 pairs of adult White King pigeons were randomly assigned to 7 groups, and the expression of apoptosis-related genes and Akt signaling pathway-related proteins in the crop tissues during different breeding stages were examined. The results showed that the mRNA levels of Bak, caspase-3, caspase-6, and caspase-9 in female crops all increased and reached their highest levels at d 17 of incubation (I17). In male crops, the levels of caspase-3 and caspase-9 gene expression peaked at d 1 of chick rearing (R1). The lowest level of Bcl-2 gene expression in females was observed at I17. The expression ratios of p-Akt (Ser473)/Akt and p-Akt (Thr308)/Akt in male crops decreased to their minimum at R1, while it was observed at d 7 of chick rearing (R7) in females. Second, 36 pairs of adult pigeons were divided into 3 groups and were subjected to SC79 injections with dosages of 0, 0.02, or 0.04 mg/kg bodyweight. The SC79 injections resulted in a considerable decrease in growth performance of pigeon squabs. In male crops, the expression ratios of p-Akt (Ser473)/Akt and p-Akt (Thr308)/Akt were significantly elevated in the 0.02 mg/kg SC79 group, while in female crops, they were higher in the 0.04 mg/kg SC79 group (P < 0.05). The SC79 injection inhibited the gene expression of Bak in female crops, but enhanced the gene expression of Bcl-2 in both male and female crops. In the 0.04 mg/kg SC79 group, a 50.7 to 75.7% decrease was observed in the expression of caspase-3, caspase-6, and caspase-9 in male and female pigeon crops. Expression of the caspase-8 gene and total Akt protein in pigeon crops was not changed in different breeding stages or after SC79 injection. In conclusion, the expression of genes related to mitochondria-dependent apoptosis can be regulated by the Akt signaling pathway, which may play a potential role in pigeon milk formation.

Identification of microRNA-Associated-ceRNA Networks Regulating Crop Milk Production in Pigeon (Columba livia)

Pigeon belongs to altrices. Squab cannot forage independently. Nutrition can only be obtained from crop milk secreted by male and female pigeon. miRNA could regulate many biological events. However, the roles of miRNA and ceRNA in regulating crop milk production are still unknown. In this study, we investigated the miRNAs expression profile of female pigeon crop, explored the potential key genes, and found the regulatory mechanisms of crop milk production. A total of 71 miRNAs were identified differentially expressed significantly. Meanwhile, miR-20b-5p, miR-146b-5p, miR-21-5p, and miR-26b-5p were found to be the key miRNAs regulating lactation. Target genes of these miRNAs participated mainly in cell development; protein and lipid synthesis; and ion signaling processes, such as cell-cell adhesion, epithelial cell morphogenesis, calcium signaling pathway, protein digestion, and absorption. In the ceRNA network, miR-193-5p was located in the central position, and miR-193-5p/CREBRF/LOC110355588, miR-460b-5p/GRHL2/MSTRG.132954, and miR-193-5p/PIK3CD/LOC110355588 regulatory axes were believed to affect lactation. Collectively, our findings enriched the miRNA expression profile of pigeon and provided novel insights into the microRNA-associated-ceRNA networks regulating crop milk production in pigeon.

Effects of incubation and chick rearing on intestinal morphology, digestive enzyme activities, and mRNA expression of nutrient transporter genes in the pigeon (Columba livia) under artificial farming conditions

The present study investigated the changes in morphology, enzyme activities in the pancreas and mucosa, and nutrient transporter gene expression in the duodenum and jejunum in male and female pigeons during the incubation and chick-rearing periods. Forty-two pairs of White King pigeons with 2 fertile eggs per pair were randomly divided into 7 groups by different breeding stages. The crypt depth of the duodenum and jejunum reached the peak at day 1 (R1) and day 7 (R7) of chick rearing, respectively. The jejunum surface area increased to a maximum value at R1. Amylase activity in the pancreas decreased to the lowest value at R1, whereas trypsin and lipase activities peaked at 17 D of incubation (I17) and R7, respectively. In male pigeons, mucosal Na⁺-K⁺-ATPase activity in the duodenum and jejunum was the highest at R15 and it was at I17 in female pigeons. Jejunum sucrose activity in female pigeons was higher at I4 than that at I17 (P < 0.05). The gene expression of FAT/CD36 and I-FABP in the duodenum gradually increased and then declined in the late chick-rearing period. SGLT1 in the jejunum decreased to a lower level at I17 and R25 in male pigeons (P < 0.05). GLUT2 expression in female duodenum and male jejunum decreased to a lower value at I17 compared with that at R15 (P < 0.05). In the late of incubation (from I10 to I17), expression of duodenum CAT1, B⁰AT1, and PepT1 and jejunum CAT1, ASCT1, and PepT1 in female pigeons was significantly reduced (P < 0.05), whereas opposite results were found in male jejunum CAT1 and duodenum ASCT1. In conclusion, variations of intestinal morphology, activities of pancreatic and mucosal enzymes, and gene expression of nutrient transporters during incubation and chick-rearing periods, underlying potential changes of digestive and absorptive function and intestinal adaptation with sexual effects, may represent a complicated response to stimuli of different breeding stages.

The profiling of amino acids in crop milk and plasma and mRNA abundance of amino acid transporters and enzymes related to amino acid synthesis in the crop tissue of male and female pigeons during incubation and chick-rearing periods

The present study was carried out to investigate the changes in amino acid (AA) contents of crop milk and plasma and mRNA abundance of AA transporters and AA synthesis-related enzymes in the crop tissue of male and female pigeons during incubation and chick-rearing periods. Forty-two pairs of adult White King pigeons with 2 fertile eggs per pair were randomly divided into 7 groups by different breeding stages. The AA content of crop milk decreased from day 1 (R1) to day 25 (R25) of chick rearing (P < 0.05). In both male and female adult pigeons, the contents of Thr, Leu, Val, His, Asp, and Pro in plasma increased to maximum levels on R25. Parental sex effect and interaction between stage and sex were observed in the AA contents of pigeon plasma (P < 0.05). For AA transporters, the mRNA abundances of SNAT2, ASCT1, LAT1, and y+LAT2 in the male crops reached the highest value on day 17 of incubation (I17), and the peak mRNA levels of PAT-1, xCT, b0,+AT, and CAT1 were found on R7 (P < 0.05). In females, the abundances of ASCT1, B0AT1, asc-1, and CAT1 mRNA peaked on R1, whereas the maximum levels of LAT1, PAT-1, b0,+AT, and y+LAT2 were observed on R7. For enzymes involved in AA synthesis, the highest gene expressions of glutamate dehydrogenase 1, acetolactate synthase in both parent pigeons, and L-threonine 3-dehydrogenase in female pigeon crops were attained on I17. The expressions of ornithine-δ-aminotransferase, glutamic-oxal(o)acetic transaminase 1, glutamic-oxal(o)acetic transaminase 2, asparagine synthetase, serine hydroxymethyltransferase 2, and glutamic-pyruvic transaminase 2 in both sexes and argininosuccinate lyase and L-threonine 3-dehydrogenase in males were the highest on R1. In conclusion, AA used for pigeon crop milk formation may originate from plasma and intracellular synthesis. The genes involved in AA transport and synthesis varied significantly with sexual effects, indicating that other factors should be considered in future explorations of the mechanism of protein formation in crop milk.

Prolactin induces lipid synthesis of organ-cultured pigeon crops

The objective of this research was to examine the effects of prolactin (PRL) on the lipid synthesis of organ-cultured pigeon crops in vitro. In experiment 1, the histology, activities of enzymes, and expression of genes involved in metabolism and apoptosis of organ-cultured pigeon crops were analyzed over a 7-d culture period. The results showed that cultured crops maintained their structural integrity for up to 3 d in vitro. Beyond 3 d, caspase-3 activity and Bak1 gene expression increased with day of culture, whereas the activities of succinate dehydrogenase, Na+-K+-ATPase, Ca2+-Mg2+-ATPase, total ATPase, and gene expression of Bcl-2 and CK-19 diminished (P < 0.05). In experiment 2, the crops were cultured for 24, 36, and 48 h in medium containing 0, 25, or 50 ng/mL PRL, respectively, and the accumulation of lipid droplets, lipid content, and expression of fatty acid transportation- and lipogenesis-related genes were analyzed. The results showed that the crops with PRL supplements showed higher amounts of lipid droplets than those of the controls, and the droplets were mainly located in the basal nutritive layer in response to PRL. The efficacy of inducing lipid accumulation increased as the concentration of PRL increased. Crops with 50 ng/mL PRL incubated for 36 h displayed the maximal lipid content. Increasing the concentration of PRL from 0 to 50 ng/mL resulted in a dose-dependent increase in the expression of acetyl-CoA carboxylase, fatty acid synthase, fatty acid translocase, fatty acid binding protein 5, acyl-CoA binding protein, and peroxisome proliferator-activated receptor γ genes after incubation for 36 h (P < 0.05). Therefore, our results indicated that the organ-cultured pigeon crops maintained good viability for up to 3 d in vitro. Furthermore, PRL induced the lipid synthesis of organ-cultured pigeon crops in a dose- and time-dependent manner, which was related to the increased expression of genes involved in fatty acid transportation and lipogenesis.

Changes in hormone profiles, growth factors, and mRNA expression of the related receptors in crop tissue, relative organ weight, and serum biochemical parameters in the domestic pigeon (Columba livia) during incubation and chick-rearing periods under artificial farming conditions

The present study was conducted to determine the changes in concentrations of hormones and growth factors and their related receptor gene expressions in crop tissue, relative organ weight, and serum biochemical parameters in male and female pigeons during incubation and chick-rearing periods under artificial farming conditions. Seventy-eight pairs of 60-week-old White King pigeons with 2 fertile eggs per pair were randomly divided into 13 groups by different breeding stages. Serum prolactin and insulin-like growth factor-1 (IGF-1) concentrations in crop tissue homogenates were the highest in both male and female pigeons at 1 d of chick-rearing (R1), while epidermal growth factor (EGF) in female pigeons peaked at d 17 of incubation (I17) (P < 0.05). mRNA expression of the prolactin and EGF receptors in the crop tissue increased at the end of incubation and the early chick-rearing stage in both sexes. However, estrogen, progesterone, and growth hormone receptor expression each decreased during the early chick-rearing stage (P < 0.05). In male pigeons, IGF-1 receptor gene expression reached its peak at R7, while in female pigeons, it increased at the end of incubation. The relative weight of breast and abdominal fat in both sexes and thighs in the males was lowest at R7, and then gradually increased to the incubation period level. Serum total protein, albumin, and globulin concentrations increased to the highest levels at I17 (P < 0.05). Total cholesterol, triglyceride, and low-density lipoprotein reached their highest values at I17 in male pigeons and R25 in female pigeons (P < 0.05). In conclusion, hormones, growth factors, and their receptors potentially underlie pigeon crop tissue development. Changes in organs and serum biochemical profiles suggested their different breeding-cycle patterns with sexual effects.