AP2X-1 is a negative regulator of sexual commitment.

is a widespread protozoan with a complex life cycle, characterized by transitions between various hosts and developmental stages, each tailored to a specific niche within its host. However, the regulatory mechanisms governing these life cycle transitions are not well understood. In this study, we investigated the AP2 factor AP2X-1, which is expressed during the tachyzoite and bradyzoite stages but decreases in the mature merozoite stage. Knockout of significantly impaired tachyzoite invasion and replication while increasing the frequency of bradyzoite differentiation. As a component associated with the HDAC3/MORC complex, loss of led to the upregulation of bradyzoite- and sexual stage-specific genes. Single-cell sequencing revealed that the knockout strain exhibited a mixed population of tachyzoite-, bradyzoite-, merozoite-, and sporozoite-like parasites. Cleavage under targets and tagmentation analysis revealed a substantial overlap between AP2X-1 and the HDAC3/MORC complex at the promoters of bradyzoite- and sexual stage-specific genes. Additionally, assay for transposase-accessible chromatin with high-throughput sequencing analysis demonstrated that AP2X-1 influences chromatin compaction and accessibility, suggesting that AP2X-1 may modulate the function of the HDAC3/MORC complex to facilitate the repression of bradyzoite differentiation and sexual commitment. Loss of resulted in significant attenuation of virulence and decreased brain cyst formation . These findings identify AP2X-1 as a critical negative regulator of sexual development.IMPORTANCE undergoes a complex life cycle characterized by alternating developmental stages. The genetic reprogramming mechanisms driving these stage transitions remain largely unknown. In this study, we identified the AP2 factor AP2X-1 as a critical regulator important for growth and life cycle progression. Our findings suggest that AP2X-1 functions as a repressor by modulating the function or influencing the association of the HDAC3/MORC complex at the promoters of bradyzoite- and sexual stage-specific genes, leading to chromatin compaction, restricting DNA accessibility and thereby repressing the transcription of genes required for bradyzoite formation and sexual commitment. Deletion of significantly reduced virulence and its ability to form brain cysts. These findings reveal a previously unknown regulatory pathway controlling sexual development in , providing new insights into its underlying mechanisms.