Goldilocks mushrooms: How ballistospory has shaped basidiomycete evolution

Ballistospory has been a governing factor in mushroom diversification. Modifications to fruit body morphology are subject to a series of fundamental constraints imposed by this uniquely fungal mechanism. Gill spacing in lamellate mushrooms, tube width in poroid species, and other configurations of the hymenium must comply with the distance that spores shoot themselves from their basidia. This reciprocal relationship between the development of fruit bodies and spores may have been maintained by a form of evolutionary seesaw proposed in this article. The necessity of the accurate gravitropic orientation of gills and tubes is another constraint on mushroom development and physiology, along with the importance of evaporative cooling of the hymenium for successful spore discharge and the aerodynamic shaping of the fruit body to aid dispersal. Ballistospory has been lost in secotioid and gasteroid basidiomycetes whose spores are dispersed by animal vectors and has been replaced by alterative mechanisms of active spore discharge in some species. Partnered with the conclusions drawn from molecular phylogenetic research, the biomechanical themes discussed in this review afford new ways to think about the evolution of basidiomycetes.