Association Between Accumulated Temperature and Ovarian Development in the Pacific Oyster ( Magallana gigas ): An Integrative Histological and Transcriptomic Analysis

Temperature is a critical environmental factor regulating reproductive activity in bivalves, yet the molecular signatures defining distinct stages of gonadal development during thermal accumulation remain poorly understood. Here, immature female Pacific oysters ( Magallana gigas ) were cultured under three thermal regimes to examine the association between accumulated temperature (AT) and ovarian development using histological and transcriptomic analysis. Histological observations showed that ovaries under the constant 10°C group remained at the early gametogenic stage. They advanced to the late gametogenic stage at an AT of 131.3°C·day and reached the mature stage at 186.5°C·day, indicating a close association between ovarian maturation and AT. Transcriptomic analysis revealed that the number of differentially expressed genes (DEGs) increased from 846 to 2240 across the accumulated‐temperature treatments. Functional enrichment indicated that lipid metabolism and nutrient mobilization predominated at lower ATs, whereas extracellular matrix (ECM) remodeling, glycosaminoglycan biosynthesis, steroid hormone biosynthesis, and retinol metabolism were enriched at higher ATs. Gene expression trend analysis identified continuously upregulated genes, including MERP1 , ASB1 , MOXD2P , and DMBT1 , suggesting their potential involvement in oogenesis. Furthermore, immune‐related genes, such as C1QL2/4 and CLEC4E, were significantly enriched. This expression pattern raises the hypothesis that immune‐related processes may be associated with follicular remodeling or maternal immune provisioning. Exploratory weighted gene co‐expression network analysis (WGCNA) identified modules associated with AT and ovarian development, mainly enriched in vesicle‐mediated transport and membrane organization. Collectively, our results show that contrasting accumulated‐temperature exposures are associated with ovarian maturation and coordinated changes in lipid utilization, tissue remodeling, membrane trafficking, and endocrine‐related processes, providing insights into temperature‐dependent reproductive regulation in bivalves.