characterization of btype cyclins in the smut fungus ustilago maydis roles in morphogenesis and pathogenicity pathogenesis morphogenesis and cell cycle are connected in the fungal pathogen ustilago maydis here we report the characterization of the catalytic subunit of the cyclindependent kinase encoded by the gene cdk1 and the two btype cyclins present in this organism encoded by the genes clb1 and clb2 these cyclins are not redundant and appears to be essential for cell cycle the analysis of conditional mutants in cyclin genes indicates that clb1 is required for g1 to s and g2 to m transitions while clb2 is specifically required for the onset of mitosis both clb1 and clb2 carry functional destruction boxes and expression of derivatives lacking dboxes arrested cell cycle at a postreplicative stage high levels of clb1 generated cells with anomalous dna content that were hypersensitive to microtubuledestabilizing drugs in contrast high levels of clb2 induce premature entry into mitosis suggesting that clb2 is a mitotic inducer in u maydis in addition clb2 affects morphogenesis and overexpression of clb2 induces filamentous growth furthermore we have found that appropriate levels of clb2 cyclin are critical for a successful infection mutant strains with half a dose of clb2 or high level of clb2 expression are impaired at distinct stages in the infection process these data reinforces the connections between cell cycle morphogenesis and virulence in this smut fungus