STUDIES IN HETEROBASIDIOMYCETES .74. MEIOSIS, SPINDLE POLE BODY CYCLE AND BASIDIUM ONTOGENY IN THE HETEROBASIDIOMYCETE AGARICOSTILBUM-PULCHERRIMUM

Meiosis, the meiotic spindle pole body cycle and aspects of basidial and basidiospore development of the heterobasidiomycete Agaricostilbum pulcherrimum were studied electron microscopically. The extranuclear interphase and prophase spindle pole body consists of a large middle piece and two lateral, multilayered discs. In late prophase I the spindle pole body sinks into a close-fitting depression in the nuclear envelope and becomes perpendicular to the long axis of the nucleus. During the transition from prophase I to prometaphase I, the spindle pole body enters the nucleus and the discs become positioned at the ends of a bipolar spindle. In metaphase-anaphase I and II, the spindle apparatus is entirely intranuclear. During post-telophase I to interphase I, a new spindle pole body appears on the nuclear face of the original disc, extending at an acute angle from the thin electron-dense layer of the original disc. During early meiosis II, the two discs of a nucleus are unequal in size. In prometaphase II the two intranuclear discs lie on the same side of the nucleus and generate two nearly parallel half spindles. In late telophase II/early interphase II each nucleus has an extranuclear spindle pole body with a spherical and a discoidal component. The spherical component forms a new spindle pole body with two discs at interphase II. The ultrastructural characters of nuclear division and the spindle pole body cycle show unique features. The phylogenetic position of Agaricostilbum pulcherrimum is discussed with special reference to meiotic nuclear division.