Increase of IMF content improves the quality of chicken meat

As expected, the results of the analysis of the 520 female JXY chickens from the selected line (n = 256) at generation 16 and control line (n = 264) showed that the IMF content was more prominently increased in the selected line than that in the control line (P < 0.01) (Fig. 1A). In addition, evaluation of the impact of the increased IMF content on the quality of breast meat revealed that the shear force of the breast meat of the selected line was significantly lower than that of the control line (Fig. 1B). Similarly, the contents of two important volatile substances, namely hexanal and heptanal, in the breast meat of the selected line were significantly higher (P < 0.01, P < 0.05, respectively) than those of the control line (Fig. 1C). However, the meat color of the breast meat was not significantly different between the two lines (Fig. 1D).

Fig. 1
figure 1

Comparison of IMF content and meat quality between the selected line and the control line. A IMF content; (B) Shear force; (C) Contents of hexanal and heptanal; (D) Meat color. Data are expressed as the mean ± standard error of mean (SEM); Selected line: n = 256; Control line: n = 264

TG controls the IMF content in chicken meat

The results on the contents of the main IMF components (TG, PLIP and TCHO) showed that PLIP content was the highest in IMF, and the contents of PLIP and TG were dominant relative to cholesterol that of TCHO (Fig. 2A). Another perspective, the contents of the main IMF components (TG, PLIP and TCHO) between the selected line and the control line revealed, as shown in Fig. 2B, that the contents of PLIP and TG were significantly higher in the breast muscle tissue of the selected line those in the control line (P < 0.01, P < 0.05, respectively), but the increase of the TG content was higher than that of PLIP. However, the TCHO content was not significantly different between the two lines (P > 0.05). Furthermore, Spearman’s correlation analysis showed that the IMF content had a higher positive correlation with the TG content (r = 0.45, P < 0.01), and a relatively weak positive correlation with the PLIP content (r = 0.11, P < 0.05), but not with the TCHO content (Fig. 2C and Additional file 1: Table S1). In addition, the determination of the contents of TG, PLIP and TCHO in myocytes and adipocytes from the pectoral muscle tissue revealed that in adipocytes the TG content was higher (P < 0.01, P < 0.01) than those of PLIP and TCHO, while in myocytes the PLIP content was higher (P < 0.01, P < 0.01) than the TG and TCHO contents (Fig. 2D).

Fig. 2
figure 2

Identification of the decisive ingredients of IMF in chicken meat. A Contents of TG, PLIP and TCHO in the selected line and the control line, respectively. The tissue sample (B). Comparison of TG, PLIP and TCHO contents between the selected line and the control line. C Correlation analysis between IMF, TG, PLIP and TCHO. IMF content had a higher positive correlation with the TG content (P < 0.01), and a relatively weak positive correlation with the PLIP content (P < 0.05). D Contents of TG (mmol/g), PLIP (mmol/L) and TCHO (mmol/gprot) in myocytes or adipocytes derived from the pectoral muscle tissue with the same cell number. Data are expressed as the mean ± standard error of mean (SEM); n = 520 individual (in vivo) or 3 (in vitro)

Change of FA composition determines IMF deposition

A total of 23 common FAs in breast muscle tissue were commonly shared by both the selected line and control line, and the C16:0, C18:0, C18:1n9c, C18:2n6c and C20:4n6 are considered as the main structural components with a proportion exceeding 10% (Table 1). Additionally, the results of the PCA showed that C16:0, C16:1, C18:0, C18:1n9c, C18:2n6c and C20:4n6 were represented in the first principal component, as shown in Fig. 3, suggesting that these long-chain FAs are dominant in the process of IMF deposition. Among 23 common FAs, the proportions of C14:0, C16:1, C18:1n9c and C18:3n3 were significantly increased (P < 0.05 or P < 0.01), but those of C10:0, C18:0, C20:0, C20:3n6, C20:4n6 and C22:6n3 were significantly decreased (P < 0.05 or P < 0.01) in the selected line compared to those in the control line (Table 1). In addition, the correlation analysis found positive correlations between TG and IMF and important long-chain FAs (LCFAs, mainly including C14:0, C14:1, C16:0, C16:1, C18:1n9c and C18:3n3), and negative correlations between TG/IMF and other FAs (mainly including C18:0, C20:0, C21:0, C22:0, C20:3n6, C20:4n6, C20:5n3, C22:6n3, C24:0 and C24:1) (Fig. 4 and Additional file 2: Table S2).

Table 1 Comparison of fatty acid composition in muscle tissue of the selected line and control line of JXY chicken
Fig. 3
figure 3

Principal components analysis (PCA) of fatty acid (FA) in chicken meat between the selected line and the control line. PCA based on data of FA composition of meat from all 520 chickens. Data are expressed as the mean ± standard error of mean (SEM); Selected line: n = 256; Control line: n = 264

Fig. 4
figure 4

Relationships between IMF, TG, PLIP and fatty acids (FAs). The Spearman correlation analysis was conducted in the R statistics software (version 3.6.1). n = 520

FAs in TG determine the total FAs composition of IMF

The breast muscle tissue of each 8 individuals in the groups with high- or low-IMF content were also used to separately extract TG and PLIP to determine the FA composition of IMF. As shown in Table 2, the proportions of C14:0, C16:0, C16:1, C18:1n9c, C18:3n3, and C20:1 were significantly higher, while the proportions of C18:0, C20:0, C20:4n6, C20:4n6, C20:5n3, C22:0, C21:1n9, and C24:0 were significantly (P < 0.05 or P < 0.01) lower in TGs of the high-IMF group than those of the low-IMF group, which was consistent with the changes of C14:0, C16:0, C16:1, C18:1n9c, C18:3n3, and C20:1 proportions in breast muscle tissue between the high-IMF and low-IMF group. However, only the proportions of C12:0 and C15:0 were significantly different (P < 0.05), and the proportions of the remaining FAs did not show significant changes of PLIP in the high-IMF group compared to those in the low-IMF group (P > 0.05).

Table 2 Fatty acid composition in separately extract IMF, TG and PLIP from muscle tissue of JXY chicken with high- or low-IMF content

FA synthesis and extracellular intake are jointly involved in IMF deposition

As shown in Fig. 4 and Additional file 2: Table S2, the results of the correlation analysis revealed the higher positive correlation between C14:0 (the important intermediates in de novo synthesis of FA) and C14:1 (P < 0.01), C16:0 (P < 0.01), C16:1 (P < 0.01), C18:1n9c (P < 0.01) and C18:3n3 (P < 0.01). Also, using our previous RNA-seq data of breast muscle tissue from each 8 individuals in the groups with high- or low-IMF content, some DEGs related to FA synthesis were screened by the following criteria: |log2 FC| ≥ 0.58, with Padj < 0.05. These DEGs are mainly involved in multiple processes of FA metabolism, including the processes of DNL (FASN, SRFBP1), release (LPL), desaturation (SCD5), elongation (ELOVL5, ELOVL7), transport (FABP4, FABP5, FABP9, CD36) and activation (ACSL5) of FAs. Further, the expression levels of all these genes were significantly (P < 0.01) up-regulated in the high-IMF content group compared to those in the low-IMF content group (Fig. 5), and the expression levels of these genes had the significant correlation with the content of IM, TG or FA (Additional file 3: Table S3).

Fig. 5
figure 5

Expression levels of genes related to fatty acid (FA) metabolism in pectoralis of chickens with high- or low-IMF content. Each 8 individuals in the groups with high- or low-IMF content from 520 JXY chickens were used for RNA-sequencing. Genes involved in various processes of FA metabolism, including de novo synthesis (FASN, SRFBP1), release (LPL), desaturation (SCD5), elongation (ELOVL5, ELOVL7), transport (FABP4, FABP5, FABP9, CD36) and activation (ACSL5). Data are expressed as the mean ± standard error of mean (SEM); n = 8

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