INTRODUCTION

Indigenous chickens, typically characterized as slow-growing breeds, are increasingly recognized as valuable genetic resources for sustainable poultry production, particularly in tropical and low-input systems where tolerance to heat stress, endemic diseases, and variable management conditions is essential [1–3]. Thai native chickens represent such adaptive genetic resources, exhibiting high survival and strong environmental fitness under hot and humid conditions [4, 5]. Despite these adaptive advantages, Thai native chickens consistently exhibit lower reproductive efficiency than commercial layer lines, as reflected in moderate growth rates, delayed sexual maturity, and reduced egg production after peak lay [6, 7]. These limitations constrain overall productivity and hinder their wider adoption in sustainable and dual-purpose production systems. These performance patterns reflect inherent biological and genetic trade-offs among growth, reproduction, and environmental adaptation rather than management limitations alone [8, 9]. Selection strategies that emphasize rapid early growth or total egg number in isolation may disrupt physiological equilibrium and reduce long-term reproductive efficiency, particularly in slow-growing genotypes adapted to tropical environments [10, 11].

Egg-laying persistency is a key reproductive trait that reflects a hen's capacity to maintain stable egg production beyond peak lay [12, 13]. In contrast to cumulative egg number, persistency captures the temporal stability of laying performance and is closely associated with clutch length, pause duration, and neuroendocrine regulation [13, 14]. Evidence from commercial layer populations indicates that persistency is genetically and biologically distinct from total egg production, as high egg number does not necessarily translate into sustained laying performance over time [12, 14]. Recent studies investigating the genetic parameters of clutch length and hen-day egg production in Thai native chickens under heat stress have demonstrated that egg-laying persistency is an important trait to incorporate into breeding programs [15]. Moreover, most quantitative genetic studies in indigenous and slow-growing chickens have primarily emphasized growth traits and cumulative egg production, generally reporting low-to-moderate heritability estimates and, in some cases, unfavorable genetic correlations between early growth and reproductive performance [16, 17]. Although recent research has adopted multi-trait animal models and selection indices to enhance overall performance [6, 11], egg-laying persistency has rarely been explicitly evaluated, and its genetic relationships with growth and sexual maturity remain poorly quantified. Furthermore, while principal component analysis (PCA) has proven effective for resolving complex trait interrelationships and identifying biologically meaningful trait dimensions in poultry, its application to integrated analyses of growth, maturity, egg production, and persistency in Thai native chickens remains limited [18].

Candidate gene approaches continue to provide valuable insights into the biological mechanisms underlying complex reproductive traits, particularly those that are difficult or costly to measure phenotypically [19, 20]. Genes involved in neuroendocrine regulation, including dopamine receptor D2 (DRD2), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), and melatonin receptor 1C (MTNR1C), play central roles in appetite control, circadian rhythm, prolactin secretion, and oviposition behavior in birds [21–23]. Although polymorphisms in these genes have been associated with egg production traits in poultry [24–27], their associations with egg-laying persistency and EBV-based reproductive performance in slow-growing Thai native chickens remain largely unexplored. Nevertheless, recent evidence suggests that persistency-related traits, including clutch length and sustained laying performance under heat stress, are important indicators of long-term productivity and adaptation in tropical poultry populations [15].

Genes involved in neuroendocrine regulation play critical roles in controlling reproductive performance in poultry by modulating hormonal pathways that govern ovulation, feeding behavior, and circadian rhythms. NPY is an important neuromodulator influencing gonadal function, feed intake, and the secretion of key reproductive hormones, and has been associated with age at first egg (AFE) and egg production rate through its role in regulating ovulation [28]. Dopaminergic signaling also contributes to reproductive control, where dopamine receptor D2 (DRD2) inhibits prolactin secretion at the pituitary level, thereby reducing incubation behavior and promoting egg production; polymorphisms in DRD2 have been linked to variation in egg production in Thai native chickens [29]. Similarly, VIP is a major stimulator of prolactin release in birds, and its gene expression and protein levels are closely associated with circulating prolactin concentrations, with several polymorphisms reported to affect total egg number in chickens [23]. In addition, melatonin and its receptor MTNR1C, which is unique to avian species, play important roles in regulating circadian rhythms and reproductive physiology, with evidence linking melatonin receptor polymorphisms to AFE and reproductive traits through endocrine pathways involving estradiol and gonadotropin-inhibitory hormone [22].

Estimated breeding values (EBVs) derived from mixed model genetic evaluations provide robust measures of additive genetic merit by integrating phenotypic, pedigree, and environmental information [6, 11]. However, few studies have jointly examined EBVs and candidate gene polymorphisms within a multivariate framework that explicitly accounts for growth–maturity–egg production trade-offs. Previous studies in Thai native chickens have mainly focused on cumulative egg production, test-day records, or heat stress responses, while candidate genes such as NPY and MTNR1C have generally been evaluated individually for their effects on egg number. Consequently, the biological relationships among growth, sexual maturity, total egg output, and egg-laying persistency remain incompletely understood. Moreover, no previous study has simultaneously applied multivariate PCA to distinguish these traits as separate biological dimensions and integrated this approach with animal-model genetic analyses and EBV-based associations involving neuroendocrine candidate genes (DRD2, VIP, NPY, and MTNR1C) throughout an entire 365-day laying cycle in a slow-growing Thai native chicken population. Addressing these knowledge gaps is essential for developing dual-purpose breeding strategies to improve long-term productivity, reproductive stability, and adaptation to tropical low-input production systems.

Therefore, the present study aimed to investigate the multivariate relationships among growth performance, sexual maturity, egg production, and egg-laying persistency using PCA and to estimate the genetic parameters and correlations among these traits in slow-growing Thai native chickens. In addition, the study evaluated the associations between polymorphisms in the neuroendocrine candidate genes DRD2, VIP, NPY, and MTNR1C and EBVs for growth, reproductive, and persistency traits. By integrating multivariate phenotypic analyses, animal-model genetic evaluation, and candidate–gene associations, this study provides a biologically informed framework for sustainable breeding strategies and supports the development of resilient dual-purpose Thai native chicken lines optimized for productivity and environmental adaptation in tropical production systems.