<i>Fzr</i> knockdown disrupts cell cycle transition in ovarian follicle cells of <i>Nilaparvata lugens</i>

During insect oogenesis, follicular cells (FCs) typically undergo the endocycle to become polyploid, thereby supporting oocyte development. The brown planthopper (BPH, Nilaparvata lugens), one of the most destructive rice pests, exhibits remarkable fecundity. However, the polyploidization of FCs and its regulatory mechanisms remain poorly understood. Here, we demonstrate that 92.3% of FCs become binucleate via endomitosis at stage 4 of oogenesis, followed by a significant increase in DNA content through endoreplication at stage 5. Knockdown of fizzy-related protein (Fzr), a key regulator of the mitosis-to-endocycle transition, disrupted the expression of cell cycle-related genes and caused a marked reduction in both binucleate FC numbers and DNA content, resulting in ovarian malformation and impaired egg development. Gene expression analyses revealed that Fzr knockdown caused aberrant expression and alternative splicing of genes related to spliceosome function and energy metabolism. Furthermore, these alterations appear to be at least partially independent of the polyploidization process. Our findings reveal a two-step polyploidization mechanism in BPH reproduction, offering insights into the evolutionary adaptation of reproductive strategies in insect pests. Additionally, this work advances our understanding of the molecular mechanisms underlying cell cycle transitions and establishes a foundation for future studies on insect reproduction and pest management strategies.