title: Explosively launched spores of ascomycete fungi have drag-minimizing shapes.
abstract: The forcibly launched spores of ascomycete fungi must eject through several millimeters of nearly still air surrounding fruiting bodies to reach dispersive air flows. Because of their microscopic size, spores experience great fluid drag, and although this drag can aid transport by slowing sedimentation out of dispersive air flows, it also causes spores to decelerate rapidly after launch. We hypothesize that spores are shaped to maximize their range in the nearly still air surrounding fruiting bodies. To test this hypothesis we numerically calculate optimal spore shapes-shapes of minimum drag for prescribed volumes-and compare these shapes with real spore shapes taken from a phylogeny of >100 species. Our analysis shows that spores are constrained to remain within 1% of the minimum possible drag for their size. From the spore shapes we predict the speed of spore launch, and confirm this prediction through high-speed imaging of ejection in Neurospora tetrasperma. By reconstructing the evolutionary history of spore shapes within a single ascomycete family we measure the relative contributions of drag minimization and other shape determinants to spore shape evolution. Our study uses biomechanical optimization as an organizing principle for explaining shape in a mega-diverse group of species and provides a framework for future measurements of the forces of selection toward physical optima.
authors.affiliation: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA. mroper@seas.harvard.edu
authors.forename: Marcus
authors.initials: M
authors.lastname: Roper
authors.forename: Rachel E
authors.initials: RE
authors.lastname: Pepper
authors.forename: Michael P
authors.initials: MP
authors.lastname: Brenner
authors.forename: Anne
authors.initials: A
authors.lastname: Pringle
mesh_terms: D001203:Ascomycota; D048690:Fruiting Bodies, Fungal; D008954:Models, Biological; D009038:Motion; D013172:Spores, Fungal