A systematic evaluation of basidiospore symmetry and tegumentation in hypogeous and gasteroid Russulales

Abstract

Spore wall architecture and ontogeny of ornamentation in several genera and species of hypogeous and gasteroid Russulales are similar to those described previously for agaricoid Lactarius lignyotellus. Spore walls are composed of four layers, each differing in thickness and electron density. Layer 2 is electron transparent and corresponds to a dark blue, amyloid layer when mounted in Melzer's iodine reagent and viewed with the light microscope. Establishment of spore symmetry may be regulated by the hilar appendix body, which is a poorly differentiated cytoplasmic region in the hilar appendix of asymmetric spores of Macowanites luteolus, Elasmomyces russuloides, and Zelleromyces versicaulus but which is absent in symmetric spores of Z. sculptisporus, Martellia subochracea, and Gymnomyces yubaensis. A continuum in spore morphology from truly symmetric to asymmetric is evident in spores from individual sporocarps of many species of the Russulales. The variation in spore symmetry and spore surface ornamentation has clouded taxonomic concepts in the Russulales. Systematically, development of orthotropic and heterotropic spores has been regarded as two distinct end points of evolution, when they are likely terms describing degrees of the same phenomenon. The current circumscription of families and genera in the Russulales based on spore symmetry, therefore, appears to be artificial.