Nuclear cycle of Saprolegnia ferax

The mitotic nuclear (equivalent to cell) cycle of the oomycete fungus, Saprolegnia ferax, was analysed by quantitative serial-section electron microscopy of hyphal nuclear populations synchronized by inhibition of DNA synthesis by fluorodeoxyuridine (FdUrd). Following telophase and karyokinesis, kinetochore mitrotubules persist into G1 stage as a single group of approximately 42 per nucleus (2n = 42 for this species). During G1 the centrioles replicate and kinetochore microtubules separate into 2 groups of approximately 21, a configuration they retain through S and G2. During metaphase a new population of kinetochore microtubules are formed, each one of an amphitelic pair connecting to the opposite pole to that associated with the persistent microtubule from the previous division. Thus, by the end of metaphase, there are approximately 42 kinetochore microtubules per half spindle. FdUrd, applied for 2 h with uracil, completely blocks DNA synthesis yet permits centriole replication and causes nuclei to accumulate with 2 pairs of centrioles, 2 arrays (each of 21) of kinetochore microtubules, and apparently enlarged nucleoli. Removal of FdUrd permits rapid (within 30 min) DNA synthesis followed by successive rounds of decreasingly synchronous nuclear cycles. These post-FdUrd cycles are 2.5 times longer than normal at 2.5 h, with S plus G2 being more extended than other phases. Calculated durations of a normal nuclear cycle are: G1, 33 min; S, 7 min; G2, 10 min; metaphase, 8 min; anaphase, 0.5 min; and telophase, 4 min.