Clock genes, such as cryptochrome 1 (CRY1), exhibit rhythmic expression in the reproductive organs. This gene is a key component of the circadian clock and is involved in various physiological processes, including reproduction, suggesting that it may be linked to ovarian activity via neuroendocrine pathways. However, the association between CRY1 variants and ovarian traits in dairy cows is unclear. In this study we used qRT-PCR to assay the mRNA expression of this gene, and found that CRY1 expression was highest in oocytes in several tissues; subsequently, we found that CRY1 expression was highest in the ovary in Chinese Holstein dairy cattle, followed by muscle and spleen tissues (p < 0.01). Next, we identified a six bp insertion/deletion (indel) locus within the CRY1 gene in Chinese Holstein dairy cattle. Only two genotypes were detected: II (n = 854, 83.7%) and ID (n = 166, 16.3%). Association analysis revealed that the identified indel was significantly associated with dominant follicle and corpus albicans diameters (p < 0.05) during metestrus. Specifically, individuals with the ID genotype exhibited smaller dominant follicles and corpus albicans than those with the II genotype. Our preliminary findings suggest that CRY1 is associated with specific ovarian morphological traits and may serve as a basis for further investigation of its potential role in bovine reproductive function.
