ULTRASTRUCTURE OF MITOSIS AND CLAMP FORMATION IN THE SOMATIC HYPHAE OF PLEUROTUS-OSTREATUS

An electron microscopic study of clamp formation and conjugate nuclear division was made of the somatic hyphae of Pleurotus ostreatus. Cytop lasmic organization of the growing clamp apex resembles that of the pa rent hyphal tip. One nucleus migrates into the clamp when the clamp ha s reached approximately half its mature size. A biglobular spindle pol e body, associated with numerous microtubules, is present at the leadi ng edge of the migrating nucleus. Separation of the globular elements occurs during migration. Mitosis of both nuclei is initiated when the clamp is fully formed. While the length of the spindle within the clam p is about half that of the spindle in the parent hypha, the paired mi toses are approximately synchronous. Septum formation delimiting the c lamp cell precedes septum formation delimiting the progeny nuclei in t he parent hypha. The clamp cell fuses with the subterminal hyphal segm ent. The subsequent migration of the progeny nucleus within the clamp cell into the subterminal hyphal segment reestablishes the dikaryon. M icrotubules were not associated with the spindle pole body of this mig rating nucleus.