Abstract

The oviduct of R. dybowskii undergoes significant pre-brumation hypertrophy, not pre-spawning. • 1,25-(OH)₂D₃ levels and vitamin D metabolic pathway components are upregulated in the pre-brumation oviduct. • Oviducal 1,25-(OH)₂D₃ correlates with steroidogenic and cell proliferative gene expression. • Vitamin D may act via local autocrine/paracrine mechanisms to support the seasonal adaptation. The R. dybowskii oviductal tissue undergoes significant expansion during pre-brumation rather than the pre-spawning period, reflecting a unique physiological adaptation for brumation. While vitamin D is known to support growth and reproductive functions in various organisms, its role in seasonal oviductal hypertrophy remains unclear. In this study, the vitamin D metabolic pathway's role in regulating this process was investigated. Morphometric measurements revealed a significant increase in oviductal weight and luminal diameter during pre-brumation compared to pre-spawning, while histological examination showed marked glandular cell hypertrophy in the ampullary region. Correspondingly, 1,25-(OH)₂D₃ was significantly elevated in the oviduct during pre-brumation. Immunohistochemistry revealed that vitamin D metabolic enzymes Cyp2r1, Cyp27b1, Cyp24a1, the vitamin D receptor (Vdr), and the reductase Dhcr7 were localized in both epithelial cells and glandular cells during pre-spawning, but became predominantly restricted to epithelial cells during pre-brumation. qRT-PCR and western blot analyses confirmed higher expression of Cyp2r1, Cyp27b1, Cyp24a1, and Vdr, but reduced expression of Dhcr7 during pre-brumation. To further evaluate potential functional associations, 25-(OH)D₃ and 1,25-(OH)₂D₃ levels were found to be positively correlated with the mRNA expression of steroidogenic ( star , cyp11a1 ) and proliferative ( mki67 , and pcna ) markers across the two periods. Transcriptome analyses further supported these findings, identifying enrichment of differentially expressed