More g's than the Space Shuttle: ballistospore discharge

Abstract

Ballistospores of basidiomycete fungi form at the tips of spear-shaped projections called sterigmata that extend from basidia. At maturity, a spherical drop of fluid appears at the base of each spore, and a few seconds later the spore is propelled into the surrounding air. The development of the fluid drop was first reported in 1889, but a century of innovative research was necessary to solve the mechanistic link between the drop and spore discharge. Through an extraordinary series of experiments the composition of the drop has now been established, its development is explained, and an effective solution to the relationship between drop appearance and spore discharge has been proposed. Drop formation is initiated when a femtomole quantity of mannitol and hexoses is excreted from a specific site at the base of the spore, forming a hygroscopic nucleus upon which water condenses from the surrounding air. Discharge of the spore occurs when the drop fuses with a film of liquid that curves over the adjacent spore surface. This rapid coalescence results in a decrease in surface free energy within the liquid and displaces the center of mass of the spore. The change in weight distribution exerts a force that is opposed by the pressurized sterigma, and the spore is shot away from the basidium into the surrounding air. The mechanism is described as a surface-tension catapult. During discharge, ballistospores are subjected to an acceleration of 25 000 g, which is about ten thousand times the acceleration experienced by astronauts during the launch of the Space Shuttle! Even more impressive is the fact that while the Shuttle consumes 50% of its weight in fuel in the first 2 min of flight, ballistospore discharge is fueled by the mannitol and hexoses that cause water to condense on the spore surface, and these solutes represent only 1% of the mass of the spore.

Key Words:

• basidiomycete
• basidiospore
• biomechanics