Basidiosporogenesis, meiosis, and post-meiotic mitosis in the ectomycorrhizal fungus Pisolithusmicrocarpus.

Pisolithus microcarpus (Cooke and Massee) G. Cunn. is a model organism for the studies on the ecology, physiology, and genetics of the ectomycorrhizal associations. However, little is known about the basidiosporogenesis in this species and, in particular, the nuclear behavior after karyogamy. In this work, the events involved in basidiosporogenesis and meiosis in P. microcarpus were analyzed using fluorescence and scanning electron microscopy. The basidia are formed inside peridioles by the differentiation of the cells along the whole hyphae. Basidial cells measure 12-18 microm in length and 6-7 microm in diameter. P. microcarpus produces eight basidiospores per basidium imbibed in a gelatinous matrix in the basidiocarp. The basidiospores are globose, equinate, with blunt spines, and measure 6-8 microm. Karyogamy can take place inside basidia as well as in undifferentiated hyphal cells followed by nuclear migration to a newly developed basidium where meiosis takes place. After the formation of the meiotic tetrad, one round of post-meiotic mitosis occurs, resulting in the production of eight nuclei per basidium. The newly-formed nuclei migrate into the basidiospores asynchronously, resulting in the production of eight uninucleate spores. This corresponds to pattern A of post-meiotic mitosis. This work is the first report on meiosis and post-meiotic mitosis during basidiosporogenesis in P. microcarpus and contributes to clarify some aspects of the biology and genetics of this ectomycorrhizal species.