Comparison of the Genome-Wide DNA Methylation Profiles between Fast-Growing and Slow-Growing Broilers

Introduction Growth traits are important in poultry production, however, little is known for its regulatory mechanism at epigenetic level. Therefore, in this study, we aim to compare DNA methylation profiles between fast- and slow-growing broilers in order to identify candidate genes for chicken growth. Methylated DNA immunoprecipitation-sequencing (MeDIP-seq) was used to investigate the genome-wide DNA methylation pattern in high and low tails of Recessive White Rock (WRRh; WRRl) and that of Xinhua Chickens (XHh; XHl) at 7 weeks of age. The results showed that the average methylation density was the lowest in CGIs followed by promoters. Within the gene body, the methylation density of introns was higher than that of UTRs and exons. Moreover, different methylation levels were observed in different repeat types with the highest in LINE/CR1. Methylated CGIs were prominently distributed in the intergenic regions and were enriched in the size ranging 200–300 bp. In total 13,294 methylated genes were found in four samples, including 4,085 differentially methylated genes of WRRh Vs. WRRl, 5,599 of XHh Vs. XHl, 4,204 of WRRh Vs. XHh, as well as 7,301 of WRRl Vs. XHl. Moreover, 132 differentially methylated genes related to growth and metabolism were observed in both inner contrasts (WRRh Vs. WRRl and XHh Vs. XHl), whereas 129 differentially methylated genes related to growth and metabolism were found in both across-breed contrasts (WRRh Vs. XHh and WRRl Vs. XHl). Further analysis showed that overall 75 genes exhibited altered DNA methylation in all four contrasts, which included some well-known growth factors of IGF1R, FGF12, FGF14, FGF18, FGFR2, and FGFR3. In addition, we validate the MeDIP-seq results by bisulfite sequencing in some regions. Conclusions This study revealed the global DNA methylation pattern of chicken muscle, and identified candidate genes that potentially regulate muscle development at 7 weeks of age at methylation level.


Introduction
Chicken growth is important economic traits in poultry production. It was determined by the interactions among genetic, nutritional, and environmental factors [1]. Until now, there have been extensive genome-wide association studies, which have identified some genetic factors affecting chicken growth [2,3]. And many candidate genes were reported to have important effects on growth [4][5][6]. Moreover, a large number of quantitative trait loci (QTLs) for chicken growth have been identified [7][8][9][10][11]. However, the genetic mechanisms in chicken growth system are still unknown and, polymorphism or QTL alone can not provide adequate explanations for them. Recently, epigenetic factors especially DNA methylation have received considerable attention because of its potential influence on complex traits and diseases [12]. Nevertheless, so far the epigenetic mechanisms responsible for chicken growth remain poorly understood.
DNA methylation is a stably inherited epigenetic modification in eukaryotes. Previous work has demonstrated the importance of DNA methylation in many biological processes like gene expression regulation, genomic imprinting, X chromosome inactivation, and disease development [13][14][15][16][17][18][19]. Recently, the research on genomic methylation has been extensively conducted in plants and mammals [20][21][22]. In birds, the genome-wide DNA methylation was firstly profiled in the muscle and liver tissues from two breeds including the red jungle fowl and avian broiler using Methylated DNA immunoprecipitation-sequencing (MeDIP-seq) [23].
The objective of the present study was to assay the genome-wide DNA methylation pattern in the muscle and to identify methylated genes that were involved in the chicken growth. Here, we collected breast muscle tissues of the two-tail samples from two chicken breeds exhibiting different growth performance at 7 weeks of age: Recessive White Rock (WRR) and Xinhua Chickens (XH), and compared the DNA methylation differences between these two breeds and within each breed by MeDIP-seq. Our analysis showed the landscape of DNA methylome distribution in the genome, revealed a large number of differentially methylated genes in different comparisons between or within two breeds, and identified genes related to the regulation of chicken growth at 7 weeks of age.

Ethics Statement
All animal experiments were handled in compliance with and approved by the Animal Care Committee of South China Agricultural University (Guangzhou, People's Republic of China) with approval number SCAU#0011. All efforts were made to minimize suffering.

Animals
Two chicken breeds, WRR and XH, were used for DNA methylation investigation in the present study. WRR, a breed with fast growth rate, were obtained from Guangdong Wens Foodstuff Company Ltd, Guangdong, China. XH, a Chinese native breed with slow growth rate, were obtained from Zhicheng Avian Breeding Company Ltd, Guangdong, China. All broilers were reared in cages with a 24-h photoperiod for the first 2 d of age and then changed to a 16-h photoperiod. They were fed with free access to water and fed ad libitum with 16.5% CP and 2, 800 kcal of ME/kg. At 7 weeks of age, according to the body weight records, 3 female birds from each of the two-tail samples of WRR and XH were selected and then four groups including WRR h , WRR l , XH h , and XH l were generated. The BW values were 1,064.0611.1, 695.0624.4, 305.8623.3, and 207.6611.1 g in the WRR h , WRR l , XH h , and XH l group, respectively. Breast muscle tissues of the 12 individuals were collected and stored at 280uC until DNA extraction.

DNA Extraction and Preparation for MeDIP-seq
Genomic DNA was isolated using TaKaRa Universal Genomic DNA Extraction Kit Ver. 3.0 (DV811A) (TaKaRa, Osaka, Japan) according to the manufacturer's protocol and then DNA quality was evaluated by agarose gel electrophoresis and spectrophotometer. DNA from 3 birds within each group was mixed in equal amounts to generate a pooled sample using Quant-iT dsDNA HS Assay Kit (Invitrogen, Carlsbad, CA, USA). Subsequently, these four pooled samples were sonicated to produce DNA fragments ranging from 100-500 bp. After end repairing, phosphorylating and A-tailing with Paired-End DNA Sample Prep kit (Illumina, San Diego, CA, USA), DNA was ligated to an Illumina sequencing primer adaptor. Then the fragments were used for MeDIP enrichment using Magnetic Methylated DNA Immunoprecipitation kit (Diagenod, Liège, Belgium) following the manufacturer's recommendation and the qualifying DNA was used for PCR

Bisulfite Sequencing
Five pairs of primers (Table 1) were designed with Methyl Primer Express Software v1.0, including one pair (P1) for the validation of relatively low methylated regions and four pairs (P2-P5) for high methylated regions. Two micrograms of pooled DNA from each group was firstly treated with the EpiTect Bisulfite kit (Qiagen, Valencia, CA, USA) and used as the template for the following semi-nested PCR amplification. PCR for PM1 and PM2 was performed in 50-mL reaction mixtures containing 50 ng of DNA, 1 mM of each primer and 25 mL Premix EX Taq TM Hot Start Version (TaKaRa, Osaka, Japan) with the conditions as: 94uC for 1 min; 35 cycles of 98uC for 10 s, 62uC for 30 s and 72uC for 30 s; and 72uC for 5 min. Reactions for PM3 to PM5 were carried out in a total volume of 50 mL including 50 ng of DNA, 1 mM of each primer and 2.5 U LA Taq HS (TaKaRa, Osaka, Japan). Both of the first and the second reaction rounds were performed under the following conditions: 94uC for 3 min; 35 cycles of 94uC for 30 s, 62uC for 30 s and 72uC for 30 s; and 72uC for 5 min. The PCR products were purified with a Gel Extraction Kit (Tiangen, Beijing, China) according to the manufacturer's instructions and then cloned into the pMD18-T vector (Takara, Osaka, Japan). For each primer, 10 clones were sequenced by BGI (Shenzhen, Guangdong, China) with commercial service and the resulting data were analyzed using ClustalW.

Bioinformatic Analysis
Raw data obtained from Illumina sequencing were first processed to filter out reads containing adapters, unknown or low quality bases and then were mapped to the chicken reference genome (ftp://ftp.ensembl.org/pub/release-63/fasta/ gallus_gallus/dna/) by SOAPaligner v 2.21 (http://soap. genomics.org.cn/) with no more than 2 bp mismatches [24]. The uniquely mapped data were retained for reads distribution analysis including the distribution in chicken chromosomes and the distribution in different components of the genome. Gene information was downloaded from the public FTP site of Ensembl (ftp://ftp.ensembl.org/pub/release-63/gtf/gallus_gallus/) and the region from transcript starting site to transcript ending site was defined as gene body region. The CpG islands (CGIs) were scanned by CpGPlot (https://gcg.gwdg.de/emboss/cpgplot.html) with the criteria as: length exceeding 200 bp, GC content greater than 50%, and observed-to-expected CpG ratio greater than 0.6. Repeat annotations were obtained from the UCSC database (http://hgdownload.cse.ucsc.edu/goldenPath/rn4/bigZips/ chromOut.tar.gz) and the analysis of reads distribution on repeats was carried out by RepeatMasker (http://www.repeatmasker.org/ Figure 1. Genomic distribution of the uniquely mapped reads. All uniquely mapped reads were classified into four types: reads uniquely mapped to CpG islands (dark blue), genes bodies (green), repeats (red), others (light blue). The percentage for each class was given at the top of each graph. WRR h , WRR l , XH h , and XH l indicated the group of Recessive White Rock with high body weight, Recessive White Rock with low body weight, Xinhua Chickens with high body weight, and Xinhua Chickens with low body weight, respectively. doi:10.1371/journal.pone.0056411.g001 ). Then genome-wide methylation peak scanning was conducted using the MACS V 1.4.2 (http://liulab.dfci.harvard.edu/MACS/) [25]. The number of peaks in different components of the chicken genome (such as promoters, 59 UTR, 39 UTR, exon, intron, intergenic regions, CGIs, and repeats) was analyzed in our study. Moreover, the number of methylated peaks in the whole genome, called total peak number, was also analyzed in each sample and here a peak overlapping among the different components was just counted for one time. The methylation densities in different components of the genome were compared by calculating the ratio of methylated peaks in a particular component to the total area of that region. Statistical analyses of methylation level differences in different components of the genome and CGIs density differences in different size classes were processed with least square method by JMP 8.0 software (http://www.jmp.com/; SAS Institute Inc., Cary, NC, USA). All genes with peaks were used for the subsequent gene ontology (GO) analysis and pathway analysis. GO term information was obtained from the UniProtKB-GOA database (http://www.ebi.ac.uk/GOA/). Genes exhibiting more than 2-fold methylation level changes in different samples were analyzed for GO and KEGG pathway enrichments using the DAVID Functional Annotation Tool (http://david.abcc.ncifcrf. gov/) [26], with P,0.005 and Benjiamini adjusted p,0.05.

Online Data Deposition
The MeDIP-Seq data from this study have been deposited in NCBI Sequence Read Archive with accession number GSE42751 (http://www.ncbi.nlm.nih.gov/geo/query/acc. cgi?acc = GSE42751).

Assemble and Blast Analysis of MeDIP-seq Reads
In the present study, three breast muscle tissues were used to generate one pooled DNA sample for each group of WRR h , WRR l , XH h , and XH l . A range of 36,734,694 to 33,399,566 raw reads were generated for the four groups, respectively. In each group, about 65% of the reads were mapped and about 36% of the reads were uniquely mapped to the chicken genome ( Table 2). The uniquely mapping reads of WRR h , WRR l , XH h , and XH l covered 21.05%, 18.10%, 21.26%, and 20.03% of the chicken genome, respectively.
MeDIP-seq reads were detected in most chromosomal regions (GGA1-28, chromosome Z, chromosome W, and chromosome MT) in each group except for some gaps ( Figure S1, S2). However, no uniquely mapped but just multi-mapped reads could be found in a long region of GGA17 (from 3,180,001 to 11,182,526 bp).  The analysis of read distribution in different components of the genome showed that the uniquely mapped reads were mainly present in repeat elements. A range of 17.42% to 19.84% of them belonged to the gene body regions. The proportion of reads uniquely mapped to CGIs in WRR h , WRR l , XH h , and XH l was only 1.00%, 0.87%, 0.97%, and 1.02%, respectively ( Figure 1).

MeDIP-seq Data Validation
In this study, one region with relatively low methylation and two regions with high methylation were selected randomly to carry out bisulfite sequencing for the validation of MeDIP-seq data. We found that the bisulfite sequencing results were almost in accordance with the MeDIP-seq results ( Figure 2, Figure S3 and S4).

DNA Methylation Profiles of the Chicken
In order to decipher the genome-wide DNA methylation profiles of the chicken, we used the uniquely mapped reads to detect the methylated peak and further analyzed the peak distribution in different components of the genome through the comparison of their methylation densities. Here, the genomic regions 2 Kb upstream and downstream of the TSS were regarded as the proximal promoter. We obtained 44,945, 44,832, 42,747, and 53,821 methylated peaks in WRR h , WRR l , XH h , and XH l , respectively (Table 3). A major portion of them were present in the intergenic regions followed by introns and exons. The average methylation density comparison showed that there were significantly differential methylation levels in different components of the genome (P,0.01) (Figure 3). Among all the classes, the average methylation density of promoters was the lowest followed by CGIs. The exon and intron regions exhibited significantly higher  methylation levels than the intergenic regions (P,0.01). Within the gene body, the methylation density of introns was significantly higher than UTRs and exons (P,0.01). Repeats showed a relatively high methylation level. Moreover, we observed different methylation levels in different repeat types with high methylation in LINE/CR1 (44.5%), LTR/ERVL (20.6%), and simple repeat (9.3%) ( Table 4).

Distribution of DNA Methylation in CGIs
CGIs were associated with the majority of the annotated gene promoters and were reported to be lowly methylated in the vertebrate genome [27,28]. In this study, CGIs were classified into two types based on their methylation status. CGIs containing methylated peaks were regarded as methylated CGIs and the rest were termed as unmethylated. In the chicken genome, there were a total of 33,915 CGIs. Of these CGIs, about 13.0% (n = 4,406) were methylated in WRR h , 11.9% (n = 4,020) in WRR l , 13.0% (n = 4,412) in XH h , and 15.0% (n = 5,084) in XH l ( Table 5). Most of the methylated CGIs were present in the intergenic regions. Within the gene body, exons showed more methylated CGIs than UTRs and introns. Moreover, when classified methylated CGIs of each class according to their sizes, we found that the CGI number significantly decreased (P,0.05) with increase in the size of islands except for that in the 39UTR region and more than 20% of methylated CGIs were in the size range of 200-300 bp ( Figure 4).
The number of unmethylated CGIs was significantly more (P,0.01) than that of methylated CGIs in each size. The densities of unmethylated CGIs in different size classes were significantly different (P,0.05) for each region. Furthermore, we found that unmethylated CGIs were enriched in promoters compared to other classes (25%).

Differentially Methylated Genes Among the Four Samples
Comparison of gene methylation showed that there were 4,085 differentially methylated genes (coverage changes was more than two folds; p value ,0.01) between WRR h and WRR l (WRR h Vs. WRR l ), 5,599 between XH h and XH l (XH h Vs. XH l ), 4,204 between WRR h and XH h (WRR h Vs. XH h ), as well as 7,301 between WRR l and XH l (WRR l Vs. XH l ) (Figure 7, Dataset S2). Moreover, 2,259 differentially methylated genes were found in both WRR h Vs. WRR l and XH h Vs. XH l , while 2,758 were identified in both WRR h Vs. XH h and WRR l Vs. XH l . Of these, 1,400 genes were differently methylated in all of the four comparisons. We subsequently analyzed the direction and degree of methylation difference for the four contrasts in different gene regions. The results showed that there were more down-  (Table S2).

KEGG Pathway Analysis
In order to investigate the pathway categories of differentially methylated genes, we performed a DAVID functional annotation analysis. The results showed that the common differentially methylated genes of the WRR h Vs. WRR l and XH h Vs. XH l contrasts were significantly enriched (Benjiamini adjusted p,0.05) in 9 predicted pathways, including several growth and metabolic related pathways such as Wnt signaling pathway, MAPK signaling pathway, ErbB signaling pathway, focal adhesion, and adherens junction (Table 7). A total of 132 differentially methylated genes involved in these 5 pathways were observed in the contrasts within the two breeds (WRR and XH) ( Table S3)   pathways, including some related to growth and metabolic such as MAPK signaling pathway, adherens junction, focal adhesion, and tight junction (Table 8). There were 129 differentially methylated genes in these 4 pathways, including some affecting growth such as IGF1R, MYH11, MYH15, MYH7B, MYLK2, FGF12, FGF14, FGF18, FGFR2, FGFR3, TGFBR1, and TGFBR2 (Table S4). Further analysis of differentially methylated genes in pathways we concerned showed that 75 genes exhibited altered DNA methylation in all of the four contrasts including WRR h Vs. WRR l , XH h Vs. XH l , WRR h Vs. XH h , and WRR l Vs. XH l (Table 9). Moreover, IGF1R and several genes belonging to the FGF family and receptors (FGF12, FGF14, FGF18, FGFR2, and FGFR3) were contained among them.

DNA Methylation Profiles
Although global DNA methylation surveys have been performed on liver and muscle tissues [23], this study is the first to systematically compare the genome-wide muscle methylation profiles of fast-and slow-growing broilers using two-tail samples of two breeds with different growth performance. The objective was to identify methylated genes affecting chicken growth. In the present study, the MeDIP-seq method was applied and 4 lines were employed in all, each line using pooled DNA samples from 3 birds. Such a pooling strategy can reduce the cost. To confirm results from MeDIP-seq, methylation tests of three regions were done with bisulfite sequencing in each pooled samples. And the methylation levels between the two methods were generally in accord with each other. Reads distribution analysis of our study found that uniquely mapped reads were enriched in the repeats and the gene body regions. It was consistent with previous findings [23].
The scan of methylation enriched regions (called peak) in MeDIP-seq was important to survey the global methylation pattern. In this study, peak distribution analysis demonstrated that promoter and CGIs were hypomethylated, whereas the methylation levels in gene body regions and repeats were relatively high.   These results were in accordance with findings in other species [22,29]. It has been well documented that most of the promoter regions were lowly methylated and promoter DNA methylation had repressive effects on gene expression [30]. DNA methylation in the gene body regions might alter chromatin structure and transcription elongation efficiency [31]. However, in contrast to previous research in animals [22,29,32], we did not observed a higher methylation level in exons than in introns in chickens. Further analysis of the methylation levels in the gene body regions showed that there was no significant difference (P.0.05) among the methylation densities of the first exon (1.0660.14), mid exon (1.4360.14), last exon (1.2360.14), and exons (1.3460.14). Also no significant difference (P.0.05) was found among the methylation levels of the first intron (2.1160.14), mid intron (2.3260.14), last intron (2.5560.14) and the intron region (2.3960.14). On the other hand, it has been demonstrated that most of the CGIs were unmethylated and CGIs could influence local chromatin structure [33,34]. Like the findings in the present study, the majority of methylated CGIs were observed in intragenic and intergenic regions [35,36]. Intragenic or intergenic CGIs were proved to have the characteristics of functional promoters and the methylation of intragenic CGIs played a crucial role in regulating alternative promoters [34,36,37]. In chicken genome, the LINE/ CR1 was the predominant interspersed repeat element and it accounted for over 80% of all interspersed repeats [38]. Our study here found that LINE/CR1 was the predominant repeats of DNA methylation, which was consistent with findings in previous study of chicken [23].

Potential Pathways Involved in Chicken Growth at 7 Weeks of Age
Growth is under complex genetic control [39]. In the current study, in order to uncover its regulation mechanisms, the regulatory network underlying growth was examined. For those differentially methylated genes common for the contrasts compared within breeds or between breeds, enriched growth and metabolic related pathways were explored. As expected, several important pathways were found, including MAPK signaling pathway, Wnt signaling pathway, and ErbB signaling pathway. The MAPK signaling pathway is a well-known signal transduction pathway that can transduce a variety of external signals and subsequently lead to a wide range of cellular responses including growth, differentiation, inflammation and apoptosis. Currently, three major MAPK pathways, the extracellular-signal regulated kinases (ERK1/ERK2), the c-jun N-terminal kinases (JNK), and p38 kinase, have been identified [40]. Previous research showed that the MAPK (RAF/MEK/ERK) signaling pathway played a key role in skeletal muscle and its activation was indispensable for muscle cell proliferation [41]. And the p38 MAPK signaling pathway was proved to be a major regulator of skeletal muscle development [42]. On the other hand, the MAPK pathway is a common target downstream of all ErbB receptors, which are well-known mediators of cell proliferation, differentiation, apoptosis, and cell motility [43]. Thus, the ErbB signaling pathway was also selected as a possible pathway affecting growth in the present study. The Wnt signaling pathway was crucial for embryogenesis in vertebrates. In chicken, the Wnt signaling pathway was found to be strongly associated with some carcass traits [44]. In addition, our analyses also found some pathways related to cell junctions (tight junction, focal adhesion, adherens junction) enriched. Focal adhesion was the signaling center of numerous intracellular pathways that regulated cell growth, survival, and gene expression [45]. Moreover, recent studies suggested that the tight junction was involved in the regulation of cell growth and differentiation, while the adherens junction could limit cell growth [46][47][48]. Therefore, those three pathways were regarded as pathways potentially related to chicken growth at 7 weeks of age in this study.

Function of Potential Methylated Genes Affecting Chicken Growth at 7 Weeks of Age
WRR and XH were two chicken breeds with divergent growth rate. In this study, the body weight of WRR was more than three times of the XH at seven weeks of age. Further, for the two-tail samples within each breed, the body weight of fast-growing samples was about 1.5 times more than slow-growing samples. Therefore, the identified differentially methylated genes within or between the two breeds in breast muscle tissues were potentially involved in chicken growth at 7 weeks of age. Eventually, we found that a total of 75 differentially methylated genes shared by all the four contrasts (WRR h Vs. WRR l , XH h Vs. XH l, WRR h Vs. XH h, and WRR l Vs. XH l ) might contribute to the regulation of chicken growth at 7 weeks of age. Among them, IGF1R and several genes belonging to the FGF family and receptors (FGF12, FGF14, FGF18, FGFR2, and FGFR3) were contained. IGF1R has been well demonstrated to play an important role in the skeletal muscle development [49,50]. In chicken, several polymorphisms of the IGF1R gene were identified to be associated with early growth traits and carcass traits [4]. FGFs were originally isolated as growth factors for fibroblasts, and now they were recognized as growth factors with diverse biological activities [51]. For instance, previous studies in rodents and chicken demonstrated that FGF18 was a pleiotropic growth factor involved in the development of various organs [52,53]. Studies using FGF knockout mice also indicated that FGF18 played a crucial role in development [51]. FGFRs were also demonstrated to have crucial effects on cell proliferation [51]. The results from this study indicated that these genes might affect chicken growth at 7 weeks of age via the change of DNA methylation.
In addition, many other differentially methylated genes related to muscle development were found in both inner contrasts (WRR h Vs. WRR l and XH h Vs. XH l ), including the key modulator of skeletal muscle differentiation, IGF1 and well-known genes related to the biosynthesis of myosin (MYL9 and MYLK) [54]. The methylation of these genes might partially contribute to the chicken growth difference within breeds at 7 weeks of age. On the other hand, some well-known genes related to the biosynthesis of myosin (MYH11, MYH15, MYH7B, and MYLK2) and two genes   [55,56]. We believed that the methylation of these genes might partially contribute to the chicken growth difference between WRR and XH at 7 weeks of age. However, the epigenetic effects of these genes on chicken growth still require further study in the future. In summary, this study provided a comprehensive analysis of DNA methylation profiles of chicken breast muscle and revealed 75 differentially methylated genes between fast-and slow-growing birds at 7 weeks of age. Several genes (IGF1R, FGF12, FGF14, FGF18, FGFR2, and FGFR3) may play key roles in affecting chicken growth at 7 weeks of age. Our observations provide new clues for deciphering the epigenetic mechanisms of chicken growth and will contribute to the improvement of poultry production. Figure S1 Chromosome distribution of reads in WRR h and WRR l . The distribution of reads in the chromosome 1-28, Z, W, and chromosome MT of the chicken genome was shown with red color for each sample. MeDIP-seq reads were plotted in 10 kb windows along chromosome. WRR h and WRR l indicated the group of Recessive White Rock with high body weight and Recessive White Rock with low body weight, respectively. (JPG) Figure S2 Chromosome distribution of reads in XH h and XH l . The distribution of reads in the chromosome 1-28, Z, W, and chromosome MT of the chicken genome was shown with red color for each sample. MeDIP-seq reads were plotted in 10 kb windows along chromosome. XH h and XH l indicated the group of Xinhua Chickens with high body weight and Xinhua Chickens with low body weight, respectively. (JPG) Figure S3 Bisulfite sequencing validation of MeDIP-seq data in one region with relatively low methylation. WRR h , WRR l , XH h , and XH l indicated the group of Recessive White Rock with high body weight, Recessive White Rock with low body weight, Xinhua Chickens with high body weight, and Xinhua Chickens with low body weight, respectively. (JPG) Figure S4 Bisulfite sequencing validation of MeDIP-seq data in one region with relatively low methylation. WRR h , WRR l , XH h , and XH l indicated the group of Recessive White Rock with high body weight, Recessive White Rock with low body weight, Xinhua Chickens with high body weight, and Xinhua Chickens with low body weight, respectively. (JPG)