Unraveling the Genetic Foundations of Broiler Meat Quality: Advancements in Research and Their Impact

As societal progress elevates living standards, the focus on meat consumption has shifted from quantity to quality. In broiler production, optimizing meat quality has become paramount, prompting efforts to refine various meat attributes. Recent advancements in sequencing technologies have revealed the genome’s complexity, surpassing previous conceptions. Through experimentation, numerous genetic elements have been linked to crucial meat quality traits in broiler chickens. This review synthesizes the current understanding of genetic determinants associated with meat quality attributes in broilers. Researchers have unveiled the pivotal insights detailed herein by employing diverse genomic methodologies such as QTL-based investigations, candidate gene studies, single-nucleotide polymorphism screening, genome-wide association studies, and RNA sequencing. These studies have identified numerous genes involved in broiler meat quality traits, including meat lightness (COL1A2 and ACAA2), meat yellowness (BCMO1 and GDPD5), fiber diameter (myostatin and LncIRS1), meat pH (PRDX4), tenderness (CAPN1), and intramuscular fat content (miR-24-3p and ANXA6). Consequently, a comprehensive exploration of these genetic elements is imperative to devise novel molecular markers and potential targets, promising to revolutionize strategies for enhancing broiler meat quality.


Introduction
Chicken muscle is one of the foremost sources of meat globally.It is characterized by its high protein content, nutritional richness, and versatile applications, making it a staple in many diets.Broiler meat quality refers to the characteristics obtained from broiler chickens specifically bred and raised for meat production.Chicken meat's quality is a pivotal determinant of its palatability and sensory attributes.As consumer preferences increasingly veer towards high-quality food products, investigating chicken meat (broiler meat) quality traits garners escalating attention [1][2][3][4][5].
Critical chicken meat quality traits include meat color, water-holding capacity, pH levels, tenderness, and nutrient composition.These attributes influence the meat's appearance, taste, shelf life, and nutritional profile.Consequently, a comprehensive understanding and exploration of chicken meat qualities are imperative to elevating poultry standards, meeting consumer expectations, and propelling advancements in the poultry industry.
In recent years, significant strides have been made in studying chicken meat quality traits [6][7][8][9].Researchers have embraced various methodologies to assess, compare, and analyze chicken characteristics encompassing color, texture, pH, fat content, and nutritional composition.These endeavors have unveiled the underlying factors impacting these traits and elucidated strategies and methodologies for enhancing chicken meat quality [10,11].
However, knowledge gaps and challenges persist despite the progress in studying chicken meat quality traits.Notably, the full extent of different chicken hybrids and strains' influence on meat quality traits remains elusive.At the same time, the effects of feed composition adjustments and breeding management strategies warrant further exploration.Therefore, this paper reviews and synthesizes the latest research advancements in chicken quality traits.Specifically, we will delve into meat color, water retention, pH levels, tenderness, and intramuscular fat.We will scrutinize the factors influencing these traits and elucidate policies and methodologies to enhance chicken meat quality.By providing comprehensive references and scientific insights, we aim to underpin the continual evolution of the chicken industry, offering guidance and inspiration for future research endeavors.

Genetic Factors Influencing Meat Quality Traits in Chicken
Genetic determinants play a pivotal role in shaping the qualitative attributes of chicken meat, encompassing characteristics such as flesh color, water retention, pH value, and tenderness.These genetic factors collectively contribute to chicken meat's overall quality and sensory experience (Table 1).

Meat Color
Meat color is a vital visual indicator of quality for consumers, with brighter hues often associated with superior nutritional value and desirability [34].Recent research has delved into the genetic underpinnings of meat color formation, unveiling candidate genes that influence flesh coloration.Studies have identified genes associated with glycolysis, fatty acid metabolism, protein metabolism, and heme concentration as potential regulators of meat color [7,12,14].Notably, genes such as ACAA2, ACSS3, APOH, ATP5L, CAV3, COL1A2, COX7C, GDPD5, MMP27, RBP4A, SLC2A6, TBXAS1, and UQCR10 have emerged as key players in modulating chicken meat color [6][7][8]12,13,15].Understanding the genetic basis of meat color variation enables targeted breeding strategies and management practices to enhance meat quality and meet consumer preferences.

Water-Holding Capacity
Water-holding capacity (WHC) is a critical attribute affecting meat quality, as excessive water loss results in dry and unpalatable meat.Genetic studies have identified candidate genes in muscle cell membrane permeability, aquaporin protein regulation, and hydroregulation pathways influencing WHC [35][36][37].Notably, genes such as ABCA1, COL6A1, GSTT1L, and the IIb sodium phosphate cotransporter have regulated WHC and intramuscular fat deposition [35,36].The IIb sodium phosphate cotransporter reduces the WHC of thigh meat [35].
The promoters of ABCA1, COL6A1, and GSTT1L were found to be hypermethylated, leading to transcriptional downregulation in hens during the later laying period.This downregulation may account for the significant difference in meat quality observed between juvenile and later-laying-period hens [36].Woody breast muscle disease has also been associated with genetic mutations affecting WHC and overall meat quality [37].Strategies aimed at modulating gene expression and optimizing muscle hydration levels can enhance WHC and improve the sensory attributes of chicken meat.

Tenderness
Meat tenderness is a crucial factor influencing consumer perception and satisfaction.Genetic studies have identified candidate genes, including CAPN1 and CAST, which regulate muscle protein degradation and influence tenderness [18,19].Specific genetic variations within these genes, such as the SNP CAPN1 G3535A and G37868A variant of the CAST gene, have been linked to variations in shear force values and intramuscular fat content [18,19].Understanding the genetic mechanisms underlying meat tenderness enables targeted breeding programs and tenderization strategies to improve overall meat quality.

Muscle Fiber Diameter
Muscle fiber diameter is a crucial metric in assessing muscle structure and development, directly impacting the chewiness and texture of chicken meat [29,38].Birds that grow rapidly have more muscle fibers than those that grow more slowly.Fast-growing chickens display larger muscle fiber diameters and a more significant proportion of glycolytic fibers (type II or quick-twitch, glycolytic fibers) than slow-growing chickens [39][40][41].The fiber type distribution in different muscles influences their dominant metabolic capabilities, subsequently affecting the meat quality of slaughtered animals [42,43].
Understanding the genetic factors influencing muscle fiber diameter is paramount, prompting extensive research into associated genes.Identified candidate genes such as myostatin, IGF-1, and PDK4 are implicated in various stages of muscle growth, fiber formation, and composition [28].During chicken embryogenesis, myostatin downregulates myf5, Pax3, and MyoD, leading to a deficiency in limb muscle size [31].Zhu et al. [28] employed qRT-PCR to measure the expression of these genes and utilized scanning electron microscopy for fiber diameter analysis.Their findings suggest that myostatin, IGF-1, and PDK4 may significantly contribute to observed variations in chicken muscle characteristics.
Further investigations by Li et al. [27] highlighted the regulatory role of the hub gene DYNLL2 and its target miR-148-3p in chicken myogenesis.MyoD and MyoG, members of the myogenic regulator family, activate transcription for myogenic agents by binding to a conserved E-box.They are crucial in muscle cell differentiation and development [30].Interestingly, the long noncoding RNA LncIRS1 has increased muscle fiber diameter and muscle mass in vivo in chickens [29].Additionally, genetic strategies to modulate muscle hydration levels in broiler meat quality could include targeting myomaker, as elucidated by Luo et al. [30], which is essential for chicken myoblast fusion and subsequently impacts muscle development and possibly muscle hydration.
Moreover, the IGFBP3 gene, integral to mammalian embryonic and postnatal development, inhibits chicken primary myoblast proliferation while promoting differentiation, thereby influencing muscle fiber diameter [8].Boonlaos et al. [26] found that Toll-like receptor gene TLR2 and TLR4 mRNA expression correlate with the muscle fiber diameter of white striping chicken.Furthermore, the IGF1 gene encoding Insulin-like Growth Factor 1 is crucial in muscle development [29,44].Studies have revealed a correlation between IGF1 gene polymorphism and chicken muscle fiber diameter, impacting meat texture and taste [44].

Crude Protein Content
Protein is a fundamental component of the muscles and tissues in chicken meat, directly influencing its nutritional value and overall quality.However, variations in protein composition among different chicken meat products lead to discrepancies in production and consumption.Consequently, researchers have explored genes associated with crude protein content in chicken meat to enhance its protein levels.Leveraging advancements in genomics and bioinformatics, researchers have identified candidate genes linked to crude protein synthesis and metabolism in chicken meat [32,33].Bordini et al. [32] suggested that breast muscle width and total crude protein concentration can indicate enhanced breast yield and reduced aberrant composition in contemporary broilers.Hub genes within interconnected modules, such as THRAP3, PRPF40A, and BIRC2, play roles in oxidative stress adaptation and apoptosis inhibition, potentially influencing protein levels.
Moreover, genes encoding collagen variants (COL5A2 and COL6A3) and SPARC and MMP2 correlate strongly with crude protein content, indicating their significance in chicken meat production [32].The ATGL gene, associated with chicken growth and fat traits, is strongly correlated with body weight and pectoral muscle crude protein content [33].Similarly, studies suggest a relationship between FABP gene polymorphism and chicken meat crude protein content, potentially mediated through fatty acid-protein interactions [45].Additionally, research indicates a correlation between IGF1 gene polymorphism and chicken meat's crude protein content, emphasizing its role in protein regulation [46].
Investigating the interplay between candidate genes and chicken meat's crude protein content offers novel pathways for regulating protein synthesis and metabolism.However, current research on candidate genes in chicken meat remains nascent and requires further experimentation.Future studies, particularly utilizing gene editing techniques, can elucidate these genes' functionality and regulatory mechanisms, contributing to enhanced chicken meat quality and increased protein content to meet the demand for high-quality meat [47].

Shear Force
The shear force of chicken meat, which represents the resistance encountered when cutting through the meat, is a crucial indicator for assessing the quality and tenderness of poultry meat.It is a significant parameter in determining meat quality, reflecting the strength and chewiness of muscle fibers.However, this force varies among different breeds and individuals, influenced by genetic factors.Hence, researchers have initiated studies on candidate genes associated with chicken meat's shear force to unravel its genetic regulatory mechanisms [48].
Understanding the shear force of chicken meat can enhance food texture and cater to diverse consumer preferences.Piórkowska et al. [48] emphasized its importance in determining tenderness, a critical quality criterion.They identified genes like ASB2, associated with meat conversion by promoting protein breakdown, and genes involved in lipogenesis and collagen synthesis, potentially impacting shear forces.For instance, in regulating shear force, the CAST gene exhibited differential expression in lean and fat muscle [48].Zhang et al. [49] highlighted its potential role in muscle fiber development and chicken shear force.Similarly, the MSTN gene encoding Mystatin, a muscle growth suppressor, showed associations with muscle growth and satellite cell proliferation, potentially influencing shear force [50].
Moreover, comprehensive gene expression analyses revealed essential genes like PRKAG3, ATP2A2, and PPARGC1A implicated in muscle fiber composition and transition [51].PPARGC1A, in particular, emerged as crucial in muscle fiber dynamics in chickens.However, understanding these candidate genes' functions and regulatory mechanisms is still in its infancy, necessitating further experimental validation.Future research employing gene editing and transgenic technology could unravel the mechanisms of these genes, optimizing the shear force and texture of chicken meat.Large-scale genome association studies and candidate gene screening hold promise in identifying additional genes related to chicken meat's shear force, paving the way for enhancing meat quality and meeting consumer demands [47].
The IMF content significantly influences chicken meat quality, with optimal levels enhancing tenderness, though excessive fat content can lead to greasiness.Therefore, controlling intramuscular fat content during breeding is crucial to ensuring favorable meat quality traits in chickens [55].
In conclusion, genetic factors are pivotal in determining meat quality traits in chickens, influencing characteristics such as flesh color, water-holding capacity, pH value, tenderness, muscle fiber diameter, crude protein content, shear force, and IMF content.By unraveling the genetic basis of these traits, researchers can develop targeted breeding strategies and management practices to enhance meat quality, meet consumer preferences, and drive advancements in the poultry industry.

Future Perspectives
Research into chicken meat quality traits has advanced significantly in recent decades, yet noteworthy challenges and opportunities warranting further exploration remain.Future investigations can delve into the foundational aspects of chicken meat quality traits, particularly by understanding their genetic underpinnings.By elucidating the genetic mechanisms governing these traits, breeders can enhance chicken meat's quality and nutritional value through targeted breeding programs.Additionally, emphasis can be placed on refining feeding techniques and nutritional management practices, including optimizing feed composition, utilizing growth promoters, and enhancing feeding environments to maximize chicken meat quality.
The progression of science and technology opens up new avenues for studying chicken meat quality traits, with emerging technologies offering fresh opportunities.Modern biotechnological tools such as gene editing technology and metabolomics can be harnessed to analyze chicken meat quality traits, ultimately contributing to quality improvement.For instance, Tizard et al. [56] demonstrated the potential of gene editing and transgenic technology in controlling disease-related traits and removing allergens, showcasing the transformative impact of modern technologies in various production scenarios.Moreover, studies like that of Xu et al. [57] utilizing gene editing technology to elucidate the role of specific genes in chicken meat development further underscore the potential of these technologies.
However, integrating various analytical approaches can provide deeper insights into chicken meat quality development [20,58].Research on chicken meat traits should encompass genetic exploration, muscle structure, physiological processes, feeding management, and advancements in nutritional technology.Such holistic investigations will be instrumental in elevating chicken meat quality to meet evolving consumer demands.

Summary
This article provides a comprehensive overview of the advancements in understanding the factors influencing chicken meat quality.The studies discussed have identified numerous genes associated with crucial broiler meat quality traits, such as meat lightness (COL1A2 and ACAA2), meat yellowness (BCMO1 and GDPD5), fiber diameter (myostatin and LncIRS1), meat pH (PRDX4), tenderness (CAPN1), and intramuscular fat content (miR-24-3p and ANXA6).These findings underscore the importance of investigating candi-date genes to enhance chicken meat quality further and establish a solid foundation for future research.

Table 1 .
Genes associated with meat quality attributes in chickens.