Cell cycle regulation of Oikopleura dioica. A study of the cyclin CDK-complement
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Regulation of the eukaryotic cell cycle is a fundamental biological process which controls proliferation of all eukaryote cells. Progression through the cell cycle is highly dependent on its core regulators; Cyclins and associated Cyclin-dependent kinases (CDKs), which orchestrate a coordinated series of events through growth in the first gap phase (G1), initiation of DNA synthesis (S), the second gap phase (G2) and mitosis (M). Variations of the cell cycle include the canonical mitotic cell cycle, giving rise to identical sister cells, meiosis, giving rise to haploid gametes, and various endoreduplicative cycles, which increase ploidy of cells through repetitive S-phases without intervening cytokinesis. Although limited to a very few specialized cell types in vertebrates, endoreduplication is widespread amongst invertebrates. The marine urochordate Oikopleura dioica, deploys somatic endocycling as a main developmental strategy, which facilitates rapid growth during a very short life cycle. O. dioica females also take advantage of the elevated transcriptional capacity of endocycling nurse nuclei within the coenocyst; a single cell compartment shared by hundreds of nurse and meiotic nuclei. Being a large transparent ovary, the coenocyst provides a unique model to study both endocycling and meiosis within a shared cytoplasm. The urochordates also belong to the closest sister group to vertebrates, which places knowledge about the O. dioica cell cycle in an interesting evolutionary context.
By searching the fully sequenced genome of O. dioica we annotated the Cyclin- CDK complement of O. dioica, which revealed amplified Cyclin D and Cyclin B complements. We also identified a surprising amplification of CDK1, an important Mphase regulator, which is highly conserved from yeast to vertebrates. Interestingly, the majority of somatic cells grow through endocycling during O. dioica development, which should favor conditions with low CDK1 activity. This observation therefore raised the question; why does an organism that develops mainly through a mechanism favoring reduced CDK1 activity have several paralogs of this particular cell cycle regulator? In order to dissect possible explanations, we analyzed expression of odCDK1 paralogs throughout O. dioica development revealing diverse expression throughout mitotic and endocycling proliferation, in addition to male- and femaleiv specific expression during gametogenesis. We also assessed functions amongst the odCDK1 paralogs, which displayed variations within the highly conserved PSTAIRE motif. Because the PSTAIRE motif is decisive in Cyclin interaction and thus indirectly affects substrate specificity, functional variation amongst odCDK1 paralogs might occur. Targeted knockdown of odCDK1 expression by injection of double stranded RNA (dsRNA) revealed non-redundant and essential functions for two odCDK1 paralogs in producing viable oocytes, representing the first known case in metazoan models where CDK1 paralogs have sub-functionalized in the control of meiosis.