This beautiful image of synchronized spore discharge from an ascomycete fungus comes from a book published in 1791. A cross-disciplinary group of researchers led by Marcus Roper and Agnese Seminara have now brought twenty-first-century approaches, including algorithms used to model the behaviour of droplets in clouds, to bear on study of this phenomenon (M. Roper et al. Proc. Natl Acad. Sci. USA doi:10.1073pnas.1003577107; 2010). They find that simultaneous discharge in itself creates an air flow, a cooperatively generated wind, that allows the spores to travel much farther than if ejected alone — so enhancing their prospects of wafting farther afield.

Roper, Seminara and colleagues used a combination of simulations, analytical models and experiments to investigate spore release from species of ascomycetes, in which spores develop in sacs (asci) in a cup-shaped structure called the apothecium. Their subjects included species of Sclerotinia (a plant pathogen) and Ascobolus (a dung fungus).

Spores of Sclerotinia, for example, have to rise from the fruiting body on the ground to infect plant flowers. In experiments, confirmed by simulations, spores riding a cooperative wind behaved much like “frictionless projectiles”. They travelled 10 centimetres or more, with the range probably being limited by gravity, compared with the 3 millimetres of those ejected on their own, which are soon halted by viscous drag. Moreover, if the cooperative spore plume hit an obstacle (which in experiments was mimicked by a glass slide, but in a natural setting might be a leaf), pressure differences in the plume resulted in spore movement around it.

Credit: COURTESY FARLOW LIBRARY OF CRYPTOGAMIC BOTANY, HARVARD UNIV.

The authors also used high-speed imaging to see how spore release is coordinated, and looked at various apothecial species. Their data show that the process is self-organized. In Ascobolus, the process of ejection is initiated in a few asci, perhaps by a highly local change in air pressure. A wave of spore discharge across the apothecium then ensues, possibly driven by an alteration in elastic stress, that may arise from changes in the turgor pressure of cells that are interspersed among the asci.

As well as the practical aspect of providing insight into the dispersal dynamics of a plant pathogen (the species studied, Sclerotinia sclerotiorum, infects and damages many different crops), there is another angle to this line of research. The authors point out that synchronized spore discharge might catch the fancy of biologists interested in the evolution of self-organized cooperative behaviour.