[Genome sequencing in the sea urchin embryo: what is new concerning the cell cycle?]

Sea urchin is a classical research model system in developmental biology; moreover, the external fertilization and growth of embryos, their rapid division cycle, their transparency and the accessibility of these embryos to molecular visualization methods, made them good specimens to analyze the regulatory mechanisms of cell division. These features as well as the phylogenetic position of sea urchin, close to vertebrates but in an outgroup within the deuterostomes, led scientists working on this model to sequence the genome of the species S. purpuratus. The genome contains a full repertoire of cell cycle control genes. A comparison of this toolkit with those from vertebrates, nematodes, drosophila, as well as tunicates, provides new insight into the evolution of cell cycle control. While some gene subtypes have undergone lineage-specific expansions in vertebrates (i.e. cyclins, mitotic kinases,...), others seem to be lost in vertebrates, for instance the novel cyclin B identified in S. purpuratus. On the other hand, some genes which were previously thought to be vertebrate innovations, are also found in sea urchins (i.e. MCM9). To note is also the absence of cell cycle inhibitors of the INK type, which are apparently confined to vertebrates. The uncovered genomic repertoire of cell-cycle regulators will thus provide molecular tools that should further enhance future research on cell cycle control and developmental regulation in this model.