Like most birds build nests in spring, deer always rut to compete for access to females in the fall, various animals show seasonal reproductive patterns finely tuned to their habitats to maximize their reproductive success and fitness outcomes.

While many species rely on seasonal cues like photoperiod to initiate reproduction, some gradually reduce or even lose this seasonality. This shift can lead to temporal mismatches of breeding, reducing inter-population gene flow and potentially driving ecological speciation. Therefore, reproductive seasonality is a fundamental life-history trait that shapes population dynamics, species interactions, and evolutionary trajectories. Though the diversification of reproductive seasonality has been widely documented across the animal kingdom, significant gaps remain in our understanding of “how do some organisms reproduce seasonally while others do not?”, especially the underlying molecular regulatory mechanisms.

To address this question, we used the three-spined stickleback, a species known for its divergent reproductive patterns across ecotypes/populations, as a model. Our previous study revealed the photoperiodic response of thyroid-stimulating hormone β2 (TSHβ2) in the pituitary played a key role in controlling seasonal reproduction. To further understand the regulatory network behind reproductive seasonality, we performed co-profiling of single-cell transcriptomic and epigenetic landscapes from pituitary cells under both short- and long-day conditions. Here we identified a novel cell type that exclusively expressed TSHβ2 (hereafter TSHβ2-producing cells). Under short-day conditions, this cell type exhibited increased TSHβ2 expression and enhanced chromatin accessibility approximately 3kb upstream of TSHβ2, indicating a photoperiod-responsive transcriptional and epigenetic regulation. Additionally, cell-specific expression of some opsin genes was detected in TSHβ2-producing cells. Here, we proposed a comprehensive model that explains photoperiodic perception and subsequent physiological modulation, with TSHβ2-producing cells acting as a pleiotropic hub that coordinates reproductive seasonality in sticklebacks.