Centriole Replication and Nuclear Division in Saprolegnia

SUMMARY: Paired centrioles in the hyphae and young sporangia of Saprolegnia ferax and Dictyuchus sterile were aligned end to end at 180° to each other. At the end of interphase new centrioles were replicated on the proximal end of each parent centriole. Each pair of centrioles was associated with a characteristic region of the nuclear envelope, termed the pocket. As the centriole pairs moved apart the mitotic spindle developed between these pockets. Kineto-chores, initially found at the equator of the spindle, became polarized as the nucleus elongated. A characteristic association between the nuclear envelope and astral microtubules is described and its role in nuclear division discussed. Mitochondria frequently lay along tracts of microtubules. The processes of centriole replication, spindle formation and nuclear division are discussed in relation to reports of the ultrastructure of these processes in other organisms.