Grass awns: Morphological diversity arising from developmental constraint.

Grasses dominate agriculturally and ecologically. One hypothesized driver of this dominance is grasses' facility for grain dispersal and rapid seedling establishment. Dispersal and establishment are aided by the awned lemma - a modified bract associated with grass flowers. Awns have diverse forms, many proposed functions, and have been gained and lost repeatedly in grass evolution. Here we hypothesize that the evolution of awn emergence is underpinned by deep conservation of developmental genes. Awns are likely homologous to leaf blades. Because leaf blades are essential, every grass species likely has a latent developmental program available for awn development. This developmental program may be repeatedly reactivated in lemmas, resulting in the frequent appearance of awns. Because awns are inessential, they can be lost and modified without dire consequences to fitness, resulting in the frequent loss and diversity of awns. Replicated awn evolution reveals how developmental conservation can potentiate the evolution of diversity. Awns also present a powerful opportunity to dissect mechanisms of leaf development.