Rhizopogon roseolus (Corda) Th. M. Fr. (=R. rubescens Tul. & Tul.), known as “shoro” in Japan, is a hypogeous basidiomycete that is an important ectomycorrhizal symbiont of the Pinaceae. Rhizopogon roseolus produced a fruiting body with a basic globose to subglobose shape. Basidiospores were encompassed in a glebal chamber in the fruiting body. However, little is known about basidiosporogenesis and nuclear behavior after karyogamy. We treated R. roseolus glebar chambers with Gimsa acid and observed their hymenium microscopically to characterize nuclear behavior and basidiosporogenesis. Our observations revealed the following five characteristics: ⅰ) meiosis and postmeiotic mitosis took place in the basidium; ⅱ) meiosis occurred in the center of the basidium; ⅲ) the sterigma appeared when the first meiotic division occurred; ⅳ) the center of the basidium constricted slightly when the second meiotic division occurred; ⅴ) after postmeiotic mitosis, asynchronous nuclear migration from the basidium to the basidiospores took place, producing eight uninucleate basidiospores. However, unusual nuclear behavior was frequently observed, indicating that regulations of the timing and the way of nucleus entering to the spores were not exact in R. roseolus.

• Kiyomi Sawada(Faculty of Agriculture, Tottori University, 4-101, Koyamacho Minami, Tottori 680-8553, Japan)
• Norihiro Shimomura( Faculty of Agriculture, Tottori University, 4-101, Koyamacho Minami, Tottori 680-8553, Japan) | Norihiro Shimomura
• Tadanori Aimi( Faculty of Agriculture, Tottori University, 4-101, Koyamacho Minami, Tottori 680-8553, Japan) | Tadanori Aimi