In eukaryotes, A- and B-type cyclins are thought to be required for mitotic events such as chromatin condensation, reorganization of the microtubule cytoskeleton into a bipolar spindle and degradation of the nuclear envelope. However, whether cyclins exert specific functions and what cellular targets are regulated remains unknown. To address cyclin specificity during cell division, we are examining the role of the B-type cyclins in C. elegans. We have identified four B-type genes (
cyb-1,
cyb-2,
cyb-3,
cyb-4) in C. elegans. Sequence analyses showed
cyb-2 and
cyb-4 share 96% nucleotide identity with each other and 65% identity with
cyb-1. These B-type cyclins are significantly divergent from either the
cya-1 or
cyb-3 genes. To address the roles of B-type cyclins during cell division we used RNA interference and followed the first divisions of these RNAi embryos by Nomarski microscopy. Injection of
cyb-1,
cyb-2,
cyb-3 or
cyb-4 dsRNA each resulted in an embryonic lethal phenotype, indicating distinct essential functions for B-type cyclins in C. elegans. The
cyb-3(RNAi) phenotype was most severe. In
cyb-3 defective embryos cells entered mitosis, however, sister chromatids failed to separate, cytokinesis was initiated in the absence of a completed nuclear division and cell division arrested. In contrast, in
cyb-1(RNAi) embryos chromosomes were separated successfully but mitosis often resulted in the formation of multinucleated cells. Interference of both
cyb-1 and
cyb-3 resulted in an arrest upon meeting of the pronuclei at the one-cell stage, following abnormal meiosis, pseudocleavage and/or pronuclear migrations. This more dramatic phenotype suggests that
cyb-1 and
cyb-3 exert overlapping functions in addition to their distinct functions. The high degree of sequence identity among
cyb-2 and
cyb-4 prevents conclusions about specific functions. However, injection of either
cyb-2 or
cyb-4 dsRNA resulted in multinucleated cells, clearly distinct from
cyb-3(RNAi). We conclude that
cyb-3 is specifically required for an essential step in chromosome segregation.