Understanding animals’ molecular mechanisms of adaptation to variable dietary availability is an essential issue in animal science studies. The mechanistic target of rapamycin (mTOR) is a candidate nutrient-sensing pathway mediating a form of fitness-related responses when individuals encounter constraints in their energy budget. Despite the fundamental importance of this process, how nutritional variability is regulated through the expression of genes governing this pathway and its consequential effects on fitness remain understudied, particularly in birds. Therefore, this doctoral study is designed to investigate the mechanisms by which different dietary conditions affect the expression of mTOR pathway genes and their links with life-history traits, mainly body weight and reproduction. To achieve this goal, three consecutive experiments has been conducted on Japanese quails (Coturnix japonica).

1. Quantitative feed restriction: Quails of both sex were subjected to feeding at 20% (DR20), 30% (DR30), and 40% (DR40) restriction levels or ad libitum (ADL) for 2 weeks. Female birds grouped under all restriction levels showed a significant reduction in body weight compared to the ad libitum-fed birds, whereas in males, only the FR40 group proved a statistically significant reduction, suggesting sex-specific effect. Concerning egg production, the FR40 group laid significantly fewer eggs than the ADL group, while other restriction levels had no effect. Egg weight was significantly reduced in the FR30 and FR40 groups. Considering the above result, we analysed resource allocation strategy among reproduction and body mass and found that the FR20 and FR30 groups re-allocate more resources to reproduction on a proportional reduction in investment to body weight, while the FR40 re-allocated resources to body weight on a proportional reduction in investment to egg weight. Feed restriction reduced the expression of mechanistic target of rapamycin (mTOR) and insulin-like growth factor 1 (IGF1) while increased the expression of autophagy-related genes (ATG9A and ATG5), growth hormone receptor (GHR), and ribosomal protein S6 kinase 1 (RPS6K1). The effect was slightly different in females and males. Circulating IGF-1 levels were more than 64% higher in females than in males. Resource allocation strategy was also related to mTOR expression. Feed restriction noticeably reduces the level of plasma triglycerides in a sex-specific manner.
2. Unpredictable feeding: Daily unpredictable feeding (30%-170% feed intake), constantly restricted feeding (40% restriction), and full feeding treatments were applied for 16 days on both female and male Japanese quails. Unpredictable feeding notably reduced body weight of female birds, while male individuals did not respond. Unpredictability did not affect egg production traits. This indicates that female birds reallocate resources from self-maintenance to reproduction under unpredictable conditions. We found characteristic and sex-dependent gene expression patterns of the mTOR pathway genes: while the unpredictable treatment did not affect gene expression in males, it downregulated mTOR and IGF1 and its receptor (IGF1R) and upregulated Unc-51-like autophagy activating kinase-1 (ULK1), ATG9A, and ATG5 in females. Additionally, expression of mTOR pathway genes mediated the effect of treatments on body weight and egg weight, highlighting their role in linking nutritional variability with fitness traits.
3. Macro- and micronutrients manipulation: The third experiment tested how supplementing with 20% leucine (FR+L), 20% methionine (FR+M), or combination of both (FR+ML) on top of restricted feeding (FR) affected the mTOR pathway. The effects of a 20% protein (PR) or 20% energy (ER) restriction under full feeding conditions were also examined. Combined supplementation of leucine and methionine supports partial increase in body weight gain compared to the FR group. Supplementing leucine, methionine, or both partially increased the expression of liver GHR, IGF1, and mTOR genes and decreased the expression of the ATG9A gene. Methionine supplementation showed a significant increase in SOD2 gene expression compared to all groups. Energy restriction significantly affected body weight, similar to the balanced feed restriction group. However, the effect was not coupled with an effect on the expression of the anabolic genes. Protein restriction did not affect body weight or the expression of mTOR pathway genes. Over all the study revealed that quantitative and qualitative dietary manipulation regulates the expression of mTOR pathway genes and in turn mediates the effect on production triats.