# A precise relationship among Buller's drop, ballistospore and gill morphology enables maximal packing of spores within gilled mushrooms

## TL;DR

This study uncovers a precise relationship between Buller's drop, spore, and gill morphology in gilled mushrooms, revealing how spores are maximally packed by optimizing drop size relative to spore size, with implications for understanding spore dispersal.

## Contribution

We identify a specific morphological relationship that maximizes spore packing efficiency in gilled mushrooms, advancing understanding of the physics and biology of spore ejection.

## Key findings

- Buller's drop volume scales with spore volume
- Drop radius is about half of spore radius
- Real species conform to the predicted optimal relationship

## Abstract

Basidiomycete fungi eject spores using a surface tension catapult; a fluid drop forms at the base of each spore and after reaching a critical size, coalesces with the spore and launches it from the gill surface. Although basidiomycetes function within ecosystems as both devastating pathogens and mutualists critical to plant growth, an incomplete understanding of ballistospory hinders predictions of spore dispersal and impedes disease forecasting and conservation strategies. Building on a nascent understanding of the physics underpinning the surface tension catapult, we first use the principle of energy conservation to identify ejection velocities resulting from a range of Buller's drop and spore sizes. We next model a spore's trajectory away from a basidium and identify a specific relationship among intergill distances and Buller's drop and spore radii enabling the maximum number of spores to be packaged within a minimum amount of gill tissue. We collect data of spore and gill morphology in wild mushrooms and we find that real species lie in a region where, in order to pack the maximum number of spores with minimum amount of biomass, the volume of Buller's drop should scale as the volume of the spore, and its linear size should be about half of spore size. Previously published data of Buller's drop and spore size confirm this finding. Our results suggest that the radius of Buller's drop is tightly regulated to enable maximum packing of spores.