Developmental constraint underlies the replicated evolution of grass awns

Replicated evolution, in which traits repeatedly evolve in distantly related species, can provide insights into the mechanisms underlying evolution and how structures are generated. One example of replicated evolution is the grass awn, an organ elaboration in inflorescences. We focus on lemma awns in the Pooideae, the largest grass subfamily with high variation in awn morphology.

Using ancestral state estimations, we discovered that awns emerge about twice as often as they are lost in the Pooideae, and have been independently derived at least 25 times. Independent derivations share similarities in micromorphology and anatomy, but awn function is not conserved. Differences in partition development can explain diversity in morphology at maturity, despite a shared underlying pattern of development.

In two species with independently derived awns and different awn morphology, we find a role for the YABBY-family transcription factor gene DROOPING LEAF in awn initiation. Together, these data support a role for developmental constraints in the replicated evolution of awns in the Pooideae.