Roles of M-phase cell cycle regulatory paralogs in the urochordate Oikopleura dioica

Abstract

Cell cycle progression is controlled by a series of regulators with complex spatialtemporal dynamics, ensuring faithful transitions of different phases. The urochordate Oikopleura has systemically employed diverse cell cycle variants, including proliferative mitotic cycles, numerous endocycle variants and reproductive meiotic cycles, to support its rapid growth and reproduction in an extraordinarily short chordate life span. It also possesses an expanded complement of some mitotic regulators that are variably expressed during development. This study focuses on Mphase events and investigates functional diversification and specialization of CDK1- Cyclin B complements during female meiosis and of duplicated chromosomal passenger complex (CPC) components in embryonic mitosis. The molecular mechanisms involved in meiosis resumption during oogenesis echo those in G2/M transition of mitosis, whereas coenocystic oogenesis in the semelparous Oikopleura is characterized by the unique feature that vitellogenesis precedes the timing of oocyte selection. We revealed dynamic deployment of two specialized CDK1 paralogs during oogenesis. Both CDK1a and CDK1d are localized at Organizing Centres (OCs) before oocyte selection. CDK1a relocates to selected meiotic nuclei at the beginning of phase 4 of oogenesis, and CDK1d then relocates to selected meiotic nuclei, concomitant with its binding partner Cyclin Ba. Targeted RNAi knockdowns revealed that CDK1a is required for coenocystic vitellogenesis, whereas CDK1d- Cyclin Ba is required for meiosis resumption and meiotic spindle assembly during prometaphase I. Additionally, Cyclin B3a plays a minor, dispensable role in promoting CDK1d activity, implying Cyclin B3 has undergone divergent evolution in the urochordate subphylum. Oikopleura embryonic divisions are fast and synchronized. Within the CPC, two INCENP paralogs always co-exist, with significant expression levels during mitosis, and display distinct localizations and subfunctionalizations. INCENP paralog switching on centromeres modulates Aurora B kinase activity and localization, thus regulating CPC functional transitions during fast embryonic divisions. The INCENPa paralog regulates early mitotic events whereas the INCENPb paralog is required for abscission during mitotic exit.