Spore release in hypogeous, gasteroid and agaricoid Russulales

Abstract

Late basidiospore development was examined in seven heterotropic and orthotropic species of Russulales. In Lactarius, Russula, Macowanites, Elasmomyces, Martellia , and Zelleromyces , a transverse electron-transparent region developed in the sterigmal apex that separated sterigmal cytoplasm from basidiospore cytoplasm. A plug-forming body descended into the hilar appendix, and electron-transparent plugging material, composed of up to three distinct regions was deposited. An electron-transparent plug also formed in the sterigmal apex. A moderately electron-dense layer that formed between sterigmal and hilar appendix plugs became a septum or abscission zone. Spore wall layers near the hilar appendix of Lactarius lignyotellus , a heterotropic species, became discontinuous from wall layers in the sterigma, and developed striations prior to spore discharge. Hilar appendix wall layers also became hydrolyzed near the point of liquid droplet formation. An adaxial liquid droplet was observed in Macowanites luteolus with TEM for the first time. Late spore development in orthotropic species was similar to that described for heterotropic species, however, little hydrolysis of the hilar plug, septum, or spore wall layers in the vicinity of the hilar appendix occurred before or after spore release. Spore release in orthotropic spores was the result of a degeneration or hydrolysis of the sterigma. Because orthotropic spores retained the septum and variable lengths of sterigmal wall after release, a functional distinction was made in this study between the septum in orthotropic spores and the abscission zone in heterotropic spores. Similarly, spore release and spore discharge are thought to be separate processes in the Russulales. Lack of hydrolysis in the hilar plug and absence of an adaxial liquid droplet in orthotropic spores are considered to be significant in the evolution of spore release mechanisms in the Russulales.