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International Journal of Genomics
Volume 2017, Article ID 9604762, 14 pages
https://doi.org/10.1155/2017/9604762
Research Article

Preparing and Analyzing Expressed Sequence Tags (ESTs) Library for the Mammary Tissue of Local Turkish Kivircik Sheep

1Faculty of Art and Science, Molecular Biology and Genetics, Yıldız Technical University, Istanbul, Turkey
2Department of Biology, Faculty of Art and Science, Ege University, Izmir, Turkey
3Department of Biochemistry, Faculty of Veterinary Medicine, Istanbul University, Istanbul, Turkey
4Department of Surgery, Faculty of Veterinary Medicine, Istanbul University, Istanbul, Turkey

Correspondence should be addressed to Nehir Ozdemir Ozgenturk; moc.oohay@rimedzorihen

Received 18 October 2016; Accepted 7 December 2016; Published 23 January 2017

Academic Editor: Ferenc Olasz

Copyright © 2017 Nehir Ozdemir Ozgenturk et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Kivircik sheep is an important local Turkish sheep according to its meat quality and milk productivity. The aim of this study was to analyze gene expression profiles of both prenatal and postnatal stages for the Kivircik sheep. Therefore, two different cDNA libraries, which were taken from the same Kivircik sheep mammary gland tissue at prenatal and postnatal stages, were constructed. Total 3072 colonies which were randomly selected from the two libraries were sequenced for developing a sheep ESTs collection. We used Phred/Phrap computer programs for analysis of the raw EST and readable EST sequences were assembled with the CAP3 software. Putative functions of all unique sequences and statistical analysis were determined by Geneious software. Total 422 ESTs have over 80% similarity to known sequences of other organisms in NCBI classified by Panther database for the Gene Ontology (GO) category. By comparing gene expression profiles, we observed some putative genes that may be relative to reproductive performance or play important roles in milk synthesis and secretion. A total of 2414 ESTs have been deposited to the NCBI GenBank database (GW996847–GW999260). EST data in this study have provided a new source of information to functional genome studies of sheep.

1. Introduction

Turkey is an important country in sheep husbandry and there are 33.2 million sheep in the country [1]. Kivircik sheep spread to the Thrace region, Marmara region, and the North Aegean region. They are fed for quality meat, milk, and wool. They are adapted to adverse environmental conditions and resistant parasites. Birth and adult body weight and daily weight gain are 3.7–4 kg, 50–70 kg, and 263 g, respectively. In addition, lactation period is 180 days, lactation milk yield is 83 kg, and wool production yield is 1.5 kg [2].

Colostrum is the first lacteal secretion from the mammary tissue of the mammals after birth [3]. Compared to normal milk, colostrum is clearly richer in contents of immunoglobulin, growth factors, protein, nonprotein nitrogen, fat, ash, minerals, and vitamins [4]. Chemical composition and immunoglobulin level particularly in colostrums exhibits a change within the first 24 h after birth [57]. It is only secreted during the first 72 hours of lactation [8] and the excretion is intense at the first 12–36 hours [7]. Sufficient colostrums intake of newborn lambs in the first days of their lives plays essential role in their healthy growth and reaching to ideal market weight [9]. The raisers are desired to breed very productive animal, so colostrums secretion is a crucially important stage for breeding. Because the compounds of colostrums are important, the genes which are related to colostrums secretion have been the center of interest.

A cDNA library containing the information from the mRNA of a particular tissue or organism is an efficient tool for research on gene structure, function, and manipulation [10]. The production of Expressed Sequence Tags (ESTs) begins with the construction of cDNA libraries. The first description of EST was reported from humans in 1991 [11]. ESTs, which are obtained in the results of sequencing of cDNA clones, are very important data for genomics studies [12].

In our study, we used one of the native breeds known as Kivircik sheep in Turkey. First of all, mammary tissues of Kivircik sheep were collected in two stages, before parturition and after parturition (the secretion of colostrums known to be considerably high), and two different cDNA libraries were constructed from those tissues. By using the bioinformatic tools, the ESTs were analyzed. Finally, obtained ESTs were compared with other genes of distinct organisms found in databases and putative functions of the genes were estimated. We aimed in this study to obtain mammary gland of gene expression profile at prenatal and postnatal stages and to compare the genetic components of colostrums secretion according to these two stages in Kivircik sheep.

2. Materials and Methods

2.1. Tissue Material

Kivircik sheep (Ovis aries) in farm of the Faculty of Veterinary Science at University of Istanbul was used in this study. The mammary tissues from the same sheep were taken by biopsy in 6–8 h during high period of the colostrums secretion after parturition and one week before parturition.

2.2. cDNA Library Construction, Quality Controls, and Sequencing

Total RNA was isolated from 0,377 gr prenatal stage and 1,316 gr postnatal stage mammary tissues with the RNeasy Kit (Qiagen). mRNA was made pure from total RNA using the Oligotex Spin-Column Protocol (Oligotex mRNA Mini Kit, Qiagen, Valencia, CA). Two different cDNA libraries for pre- and postnatal stage tissue were established with 0,23 g and 0,75 g mRNA, respectively. cDNA libraries were constructed with the CloneMiner cDNA Library Construction Kit according to the manufacturer’s instructions (Invitrogen, Carlsbad, CA, USA). Double-stranded cDNA was cloned into pDONR222 vector and transformed into E. coli strain DH5 (Invitrogen, Carlsbad, CA, USA). Each cDNA library was plated onto LB-kanamycin agar medium and individual grown colonies were picked into 384-well plates with SOB medium and inoculated overnight. After the addition of glycerol (10% v/v), the library was stored at −80°C.

Plasmid DNA was isolated from casually selected 142 clones with alkaline lysis method [13]. Isolated DNA was digested with Bgl1701 and analyzed by 1% agarose gel electrophoresis for identifying insert size.

As a template, randomly selected 3072 clones were used for PCR amplification of the cloned cDNA by M13 universal primers. Automated sequencing was performed on an automated high-throughput pipeline using the ABI 3730 capillary sequencer (PE Applied Biosystems, Foster City, CA) at the Genome Sequencing Center, Washington University in St. Louis (WUSTL).

2.3. Sequence Analysis

For analysis of the raw EST data, the low-quality, adapter, and the vector sequences were removed with Phred software [14, 15] (CodonCode Corp., Dedham, MA). The remaining EST sequences were reprocessed by using “cross-match” program which is application of Phrap for the vector sequence trimming [14, 15].

Prenatal and postnatal period EST sequences were assembled separately into contigs with Contig Assembly Program 3 (CAP3) [16, 17]. The default values were used for all the parameters. Results were evaluated by using BEAP program which was developed for editing and representing of alignment [18]. Putative functions of all unique sequences and contigs were designated by gene homology based on BLAST [19]. For Blastn and statistical analysis of all the EST sequences, Geneious software was used [20]. ESTs that showed high similarity in GenBank were classified according to molecular function, biological process, cellular component, protein class, and pathway, respectively, based on Panther classification [21].

3. Results

3.1. Characterization of cDNA Libraries

Two different cDNA libraries were constructed at prenatal and postnatal stages from mammary gland tissue. In cDNA library for the prenatal period, there occurred 3.6 103 clones and its average insert length was 1 kb (min 200 bp to 1500 bp) (Figure 1). On the other hand, cDNA library for postnatal stage, which consisted of 4.3 103 clones, has an average insert length of 0.9 kb ranging from 200 to 1500 kb (Figure 2). After construction of cDNA libraries, 1536 clones were randomly selected and sequenced from prenatal stage library; 1536 clones were randomly selected and sequenced from postnatal stage library. Therefore, 3072 EST sequences were generated.

Figure 1: (a) Prenatal cDNA clones obtaining with alkaline lysis method. (b) Cutting of prenatal cDNA clones with Bsp1407I restriction enzyme.
Figure 2: (a) Postnatal cDNA clones obtaining with alkaline lysis method. (b) Cutting of postnatal cDNA clones with Bsp1407I restriction enzyme.
3.2. BLAST Analysis of ESTs

From obtained 3072 EST sequences, raw EST data were processed and base-called with Phred-Cross_match computer program. The EST sequences were trimmed and vector, adapter, and low-quality bases removed. While 318 low-quality sequences were determined in prenatal cDNA library, 340 low-quality sequences were removed in cDNA library which was constructed at postnatal stage (Table 1). The remaining 1218 high-quality prenatal EST sequences and 1196 high-quality postnatal EST sequences were achieved. Putative functions of all unique sequences were designated by gene homology based on BLAST. Blastn analysis was done using Geneious software. Obtained data was evaluated statistically by this program.

Table 1: The results of analyzing ESTs with Phred software and Geneious software.

According to these results, length of prenatal ESTs changes between 542 bp and 1587 bp; the shortest postnatal EST and the longest postnatal EST were 605 bp and 1535 bp, respectively (Table 1 and Figures 3 and 4).

Figure 3: The length of prenatal ESTs.
Figure 4: The length of postnatal ESTs.

Pursuant to Blastn results of 1218 prenatal ESTs, 154 of them (12.7%) which showed ≥80 similarity or e value ≤ to known sequences of other organisms in NCBI were determined. Also the 1001 ESTs (82.3%) showed significant similarities to putative genes with score of below 80% homology and 63 ESTs had no similarity to situated sequences in databases.

Among the 1196 postnatal ESTs, 268 of them (22.4%) showed 80% and higher similarity; 923 ESTs (77.2%) showed similarities with score of <80 bits. Also 38 ESTs did not match any registrated sequences in NCBI.

All 2414 EST sequences were submitted to GenBank. ESTs were registered to NCBI under the accession numbers of GW996847 to GW999260.

3.3. GO Analysis of ESTs

Total 422 ESTs which show 80% and over 80% similarity to known sequences of other organisms in NCBI were classified according to GO terms such as molecular function, biological process, cellular components, protein class, and pathway with PANTHER database (Figures 5 and 6). 154 and 268 ESTs of these total ESTs were prenatal and postnatal ESTs, respectively. According to the statistical results of PANTHER of high similarity prenatal and postnatal ESTs, 137 and 127 different putative genes were listed and classified, respectively, by GO terms. 39 prenatal ESTs and 37 postnatal EST were uncategorized.

Figure 5: GO terms distribution in the molecular function (a), the biological process (b), the cellular components (c), and the protein class (d) for prenatal ESTs.
Figure 6: GO terms distribution in the molecular function (a), the biological process (b), the cellular components (c), and the protein class (d) for postnatal ESTs.

9 different types of molecular functions were found in prenatal ESTs. The common molecular function GO terms are “binding” which consists of 43 proteins and “catalytic activity” which consists of 27 proteins. Moreover, most of the postnatal ESTs in the molecular function GO term were assigned the same categories, “binding” and “catalytic activity.” In the biological process category that recognizes series of events or molecular functions, 56 and 30 of prenatal ESTs and 55 and 20 of postnatal ESTs were viewed in “metabolic process” and “cellular process,” respectively. The cellular component GO identifies locations at the levels of subcellular structures and macromolecular complexes. In this GO term category, the prenatal ESTs were accumulated more than postnatal ESTs in “cell part,” “macromolecular complex,” and “organelle.” According to protein class GO term classification of Panther database, 23 of prenatal ESTs and 20 of postnatal ESTs most observed “nucleic acid binding” category. On the other hand, Panther database gives information about pathway GO term. When we analyzed prenatal and postnatal ESTs, “B cell activation,” “T cell activation,” and “Huntington disease” categories included some prenatal proteins; moreover, “apoptosis signaling pathway,” “p53 pathway,” and “Parkinson disease” categories contained some postnatal proteins.

3.4. Analyzing of Contigs

1218 high-quality prenatal EST sequences and 1196 high-quality postnatal EST sequences were appointed to achieve contig. Fragment assembly was done with the CAP3 software. According to CAP3 result the prenatal stage EST sequences into 23 contigs and postnatal stage EST sequences into 27 contigs were assembled. Furthermore, in prenatal stage contigs, the longest contig is 1394 bp, it is 2068 in postnatal stage contig. The number of singlets is 1164 and 1059, respectively (Table 2).

Table 2: The analysis of prenatal and postnatal contigs by CAP3.

The results which were obtained by CAP3 software were evaluated with the program BEAP which was developed for editing and representing of alignment (Figure 7).

Figure 7: The result of Contig 20 that was obtained by CAP3 with BEAP.

All of the contigs were designated by Blastn in NCBI. The Blastn results of prenatal and postnatal contigs are shown in Tables 3 and 4, respectively.

Table 3: The Blastn results of prenatal contigs.
Table 4: The Blastn results of postnatal contigs.

Among the Blastn results of prenatal contigs, 7 of them showed high similarities. These are a unique protein associated with intracellular transfer of membrane by coated vesicles “clathrin,” protein that plays role in protein synthesis, “elongation factor-1,” a member of the mitochondrial carrier family, “alpha solute carrier family, member 13,” protein that related with immune system, “immunoglobulin gamma one chain,” a gene that encodes the glycodelin protein, “progestagen-associated endometrial protein (PAEP),” a major component of a specific type of lipoprotein called very low-density lipoproteins (VLDLs), and “apolipoprotein E.” Additionally, 10 of 23 prenatal contigs showed similarities to putative different ribosomal proteins in NCBI with score of <80 (Table 3).

As shown in Table 4, 11 of 27 postnatal contigs have similarities with score of ≥80 bits or e value ≤ 10−10 according to Blastn. 3 postnatal contigs of high similarity postnatal contigs include 3 different ribosomal proteins. In addition, 5 postnatal contigs interacte with calcium and calcium phosphate and self-aggregate to be organized into a supramolecular structure “alpha casein S1 and S2.” The other two contigs show high similarities to putative proteins which are “sperm associated antigen 8 (SPAG8)” and “translation elongation factor 1 alpha.” Important putative genes of the postnatal contigs which showed resemblance below 79% in NCBI are “collagen type III alpha 1,” “nuclear protein, transcriptional regulator,” “beta-2-microglobulin,” “thymosin beta 4,” “beta-casein,” and different ribosomal proteins.

4. Discussion

EST projects are powerful tools for analyzing gene expression patterns in a given tissue and/or at a certain stage and the identification of genes [2224]. These technologies are very important for obtaining genomic sequence information of organisms [25]. In the molecular studies, generation of ESTs is a perfect and unique approach because it allows both expression and estimation and discovery of new genes to be conducted at the same time. Consequently, analysis of expression of a large number of gene profile supplies for scientists to find their functions and facilitates the understanding activity of biological processes in specific tissue or cell of organisms at the certain stage [26].

In this study, we obtained two different EST libraries from Kivircik sheep mammary gland at one week before parturition and 6–8 h after parturition. Because colostrums are only secreted during the first 72 hours after parturition [8], we aimed in this project to achieve putative gene profiling at different time and find the genetic components of colostrums secretion in Kivircik sheep.

The results of the analyzing contig and GO analysis of ESTs and the Blastn analysis of the ESTs showed 80% and higher similarity to putative genes in NCBI. Moreover, we have determined gene expression profile of mammary gland at the different developmental stages, which are prenatal and postnatal.

Table 5 shows Blastn results of some breed-specific expressed genes in all ESTs of prenatal and postnatal ESTs. According to these results, levels of ribosomal protein, transcription factors, and translation factors were found about the same level at both stages. CREB2F, SRF, AKNA, CRTC2, TCF20, and SPDEF transcription factors were observed in prenatal ESTs and also ELF5, NUPR1, BTAF1, TCF12, TFE3, and TCF20 transcription factors were observed in postnatal ESTs. Due to the high level of protein synthesis in the lactation phase, expression of the proteins that participate in ribosomal structure increases at the preparation lactation stage. Also, the genes that have function in transcriptional and translational regulation are expressed highly before parturition [27]. Additionally, we have observed known putative genes related to immune system, growth, and lipid metabolism. These expressed genes which improve quality of milk are “immunoglobulins,” “MHC,” and “beta-2-microglobulin.” Because lactation begins few weeks before parturition [28] and we choose sequenced cDNA clones randomly, these putative genes are found approximately at the same rate in prenatal and postnatal ESTs. Besides, expressions of genes related to milk production are minimal, because the milk is not secreted from mammary gland before the parturition. On the other hand, the milk proteins that affect milk quality are defined as significant milk genes. We have observed their expressions to be high in the lactation stage such as “caseins (alpha-S1, alpha-S2, beta, and kappa casein)” and whey proteins “beta-lactoglobulin” and “alpha-lactalbumin.” Expression of the milk proteins was slightly near the end of gestation, which can be used as a symbol of the maturity of mammary epithelial cells [29].

Table 5: The percentage and number of some expressed genes in prenatal and postnatal ESTs.

On the other hand, in respect of GO analysis, GO terms were identified and observed on five categories, such as molecular function, biological process, cellular components, protein class, and pathways. Prenatal and postnatal high-similarity ESTs were classified with Panther database according to GO terms. According to comparison of these results, prenatal and postnatal ESTs were classified almost in the same category.

After our study, using large-scale EST sequencing as strategy, we have constructed Kivircik sheep mammary gland gene expression profiles in different stages and found prelactation and lactation (especially secreting colostrum) specific genes. These results will help us understand the comparison of mammary gland gene expression profiles between two distinct stages and provide new clues for genomic research. Because our ESTs data just only matched the other organisms in NCBI by approximately 50%, it will also provide the increasing number of ESTs of Ovis aries in NCBI Genbank database and new genes about Ovis aries. This work will improve with the research of different developmental stage of mammary gland.

Competing Interests

The authors declare that there are no competing interests regarding the publication of this paper.

Acknowledgments

This research has been supported by Yıldız Technical University Scientific Research Projects Coordination Department (Project no. 27-01-07-02).

References

  1. TurkStat, Animal Production Statistics, 2016, http://www.tuik.gov.tr/.
  2. General Directorate of Agricultural Research and Policie, Türkiye Evcil Hayvan Genetik Kaynakları Tanıtım Kataloğu, General Directorate of Agricultural Research and Policie, Ankara, Turkey, 2009.
  3. R. Pakkanen and J. Aalto, “Growth factors and antimicrobial factors of bovine colostrum,” International Dairy Journal, vol. 7, no. 5, pp. 285–297, 1997. View at Publisher · View at Google Scholar · View at Scopus
  4. F. O. Uruakpa, M. A. H. Ismond, and E. N. T. Akobundu, “Colostrum and its benefits: a review,” Nutrition Research, vol. 22, no. 6, pp. 755–767, 2002. View at Publisher · View at Google Scholar · View at Scopus
  5. S. E. Pattinson and E. W. Thomas, “The effect of sire breed on colostrum production of crossbred ewes,” Livestock Production Science, vol. 86, no. 1–3, pp. 47–53, 2004. View at Publisher · View at Google Scholar · View at Scopus
  6. A. Argüello, N. Castro, M. J. Zamorano, A. Castroalonso, and J. Capote, “Passive transfer of immunity in kid goats fed refrigerated and frozen goat colostrum and commercial sheep colostrum,” Small Ruminant Research, vol. 54, no. 3, pp. 237–241, 2004. View at Publisher · View at Google Scholar · View at Scopus
  7. J. C. Chen, C. J. Chang, H. C. Peh, and S. Y. Chen, “Serum protein levels and neonatal growth rate of Nubian goat kids in Taiwan area,” Small Ruminant Research, vol. 32, no. 2, pp. 153–160, 1999. View at Publisher · View at Google Scholar · View at Scopus
  8. S. Kráčmar, J. Kuchtík, M. Baran et al., “Dynamics of changes in contents of organic and inorganic substances in sheep colostrum within the first 72 h after parturition,” Small Ruminant Research, vol. 56, no. 1-3, pp. 183–188, 2005. View at Publisher · View at Google Scholar · View at Scopus
  9. H. S. Umberger, “Profitable artificial rearing of lambs,” Animal and Poultry Sciences Publication No. 410-023, Virginia Cooperative Extension, 1997. View at Google Scholar
  10. S.-Y. Ying, “Complementary DNA libraries: an overview,” Applied Biochemistry and Biotechnology—Part B Molecular Biotechnology, vol. 27, no. 3, pp. 245–252, 2004. View at Publisher · View at Google Scholar · View at Scopus
  11. M. D. Adams, J. M. Kelley, J. D. Gocayne et al., “Complementary DNA sequencing: expressed sequence tags and human genome project,” Science, vol. 252, no. 5013, pp. 1651–1656, 1991. View at Publisher · View at Google Scholar · View at Scopus
  12. L.-X. Du, S.-F. Liu, J. Zhu et al., “Construction of SMART cDNA library of sheep ovary and identification of candidate gene by homologous cloning,” Agricultural Sciences in China, vol. 6, no. 11, pp. 1390–1395, 2007. View at Publisher · View at Google Scholar · View at Scopus
  13. J. Sambrook, E. F. Fritsch, and T. Maniatis, Moleculer Clonning: A Laboratory Manuel, vol. 2nd, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, USA, 1989.
  14. B. Ewing, L. Hillier, M. C. Wendl, and P. Green, “Base-calling of automated sequencer traces using phred. I. accuracy assessment,” Genome Research, vol. 8, no. 3, pp. 175–185, 1998. View at Publisher · View at Google Scholar · View at Scopus
  15. B. Ewing and P. Green, “Base-calling of automated sequencer traces using phred. II. Error probabilities,” Genome Research, vol. 8, no. 3, pp. 186–194, 1998. View at Publisher · View at Google Scholar · View at Scopus
  16. X. Huang and A. Madan, “CAP3: a DNA sequence assembly program,” Genome Research, vol. 9, pp. 868–877, 1999. View at Publisher · View at Google Scholar · View at Scopus
  17. X. Huang, “An improved sequence assembly program,” Genomics, vol. 33, no. 1, pp. 21–31, 1996. View at Publisher · View at Google Scholar · View at Scopus
  18. J. E. Koltes, Z.-L. Hu, E. Fritz, and J. M. Reecy, “BEAP: the BLAST extension and alignment program—a tool for contig construction and analysis of preliminary genome sequence,” BMC Research Notes, vol. 2, article 11, 2009. View at Publisher · View at Google Scholar · View at Scopus
  19. S. F. Altschul, W. Gish, W. Miller, E. W. Meyers, and D. J. Lipman, “Basic local alignment search tool,” Journal of Molecular Biology, vol. 215, no. 3, pp. 403–410, 1990. View at Publisher · View at Google Scholar · View at Scopus
  20. M. Kearse, R. Moir, A. Wilson et al., “Geneious basic: an integrated and extendable desktop software platform for the organization and analysis of sequence data,” Bioinformatics, vol. 28, no. 12, pp. 1647–1649, 2012. View at Publisher · View at Google Scholar · View at Scopus
  21. P. D. Thomas, M. J. Campbell, A. Kejariwal et al., “PANTHER: a library of protein families and subfamilies indexed by function,” Genome Research, vol. 13, no. 9, pp. 2129–2141, 2003. View at Publisher · View at Google Scholar · View at Scopus
  22. S. Chen, H. Miao, F. Chen, B. Jiang, J. Lu, and W. Fang, “Analysis of expressed sequence tags (ESTs) collected from the inflorescence of chrysanthemum,” Plant Molecular Biology Reporter, vol. 27, no. 4, pp. 503–510, 2009. View at Publisher · View at Google Scholar · View at Scopus
  23. W.-K. Wang, C.-C. Liu, T.-Y. Chiang, M.-T. Chen, C.-H. Chou, and C.-H. Yeh, “Characterization of expressed sequence tags from flower buds of alpine Lilium formosanum using a subtractive cDNA library,” Plant Molecular Biology Reporter, vol. 29, no. 1, pp. 88–97, 2011. View at Publisher · View at Google Scholar · View at Scopus
  24. D. Yang, Z. H. Tang, L. P. Zhang, C. P. Zhao, and Y. L. Zheng, “Construction, characterization, and expressed sequence tag (EST) analysis of normalized cDNA library of thermo-photoperiod-sensitive genic male sterile (TPGMS) wheat from spike developmental stages,” Plant Molecular Biology Reporter, vol. 27, no. 2, pp. 117–125, 2009. View at Publisher · View at Google Scholar · View at Scopus
  25. Y. Gueguen, J.-P. Cadoret, D. Flament et al., “Immune gene discovery by expressed sequence tags generated from hemocytes of the bacteria-challenged oyster, Crassostreagigas,” Gene, vol. 303, pp. 139–145, 2003. View at Publisher · View at Google Scholar · View at Scopus
  26. T. Thanh, V. T. Q. Chi, M. P. Abdullah et al., “Construction of cDNA library and preliminary analysis of expressed sequence tags from green microalga Ankistrodesmus convolutus Corda,” Molecular Biology Reports, vol. 38, no. 1, pp. 177–182, 2011. View at Publisher · View at Google Scholar · View at Scopus
  27. Z. Su, X. Dong, B. Zhang et al., “Gene expression profiling in porcine mammary gland during lactation and identification of breed-and developmental-stage-specific genes,” Science in China Series C, vol. 49, no. 1, pp. 26–36, 2006. View at Publisher · View at Google Scholar · View at Scopus
  28. Y. Gao, X. Lin, K. Shi, Z. Yan, and Z. Wang, “Bovine mammary gene expression profiling during the onset of lactation,” PLoS ONE, vol. 8, no. 8, Article ID e70393, 2013. View at Publisher · View at Google Scholar · View at Scopus
  29. G. W. Robinson, R. A. McKnight, G. H. Smith, and L. Hennighausen, “Mammary epithelial cells undergo secretory differentiation in cycling virgins but require pregnancy for the establishment of terminal differentiation,” Development, vol. 121, no. 7, pp. 2079–2090, 1995. View at Google Scholar · View at Scopus