Ultrastructure of developing basidiospores in Rhizopogon roseolus (= R. rubescens)

The ultrastructure of developing basidiospores in Rhizopogon roseolus is described. When viewed in the fruiting body chamber using scanning electron microscopy, basidiospores appear narrowly ellipsoid and have smooth walls. Eight basidiospores are usually produced on the apex of each sterigma on the basidium. Transmission electron micrographs showed that basidiospores formed by movement of cytoplasm (including the nuclei) via the sterigmata, and then each basidiospore eventually became separated from its sterigma by an electron-lucent septum. The sterigma and basidium subsequently collapsed, resulting in spore release. Freshly released spores retained the sterigmal appendage connected to the collapsed basidium. After spore release, the major ultrastructural changes in the spore concerned the lipid bodies and the spore wall. During maturation, lipid bodies formed and then expanded. Before release, the spore wall was homogeneous and electronlucent, but after release the spore wall comprised two distinct layers with electron-dense depositions at the inner wall, and the dense depositions formed an electron-dense third layer. The mature spore wall complex comprised at least four distinct layers: the outer electron-lucent thin double layers, the mottled electron-dense third layer, and the electron-lucent fourth layer in which electron-lucent granular substances were dispersed.