Ecology and evolution of the diaspore "burial syndrome"

Hygroscopically active awns or "bristles" have long intrigued scientists. Experimental evidence shows that they are important for diaspore burial in the correct orientation, thereby increasing successful seed germination and seedling survival. Despite these ecological advantages, 38 of the 280 species of grasses in Danthonioideae lack awns. We provide the first study of awns in a phylogenetic context and show that although the awnless state has arisen ca. 25 times independently, the ecological disadvantage of not having an awn also applies in an evolutionary context. Only in Tribolium and Schismus have awnless ancestors diversified to form a clade of primarily awnless descendents. Several of the awnless species in these genera are annual and we find a significant correlation between the evolution of awns and the evolution of life history. A suite of other diaspore traits accompany the awned or awnless states. We interpret the awn as being the visible constituent of a compound "burial syndrome," the two ecological extremes of which may explain the correlation between awns and life history and provide an explanation why awnless species in Tribolium and Schismus persist.