To awn or not to awn: Revisiting the evolution of awnedness during rice domestication and improvement

Abstract

Awns play multifaceted roles in the physiological and ecological processes that facilitate plant adaptation. Awnless varieties predominate in contemporary rice cultivation. This review synthesizes historical, agricultural, and biological evidence demonstrating that awns remained prevalent in Asia until the first half of the last century. Notably, the Geng (japonica) and Xian (indica) subspecies exhibit distinct evolutionary trajectories, with awn reduction in Geng occurring considerably later than in Xian. Awns may have provided ancient farmers with distinct advantages in rice cultivation, contingent upon environmental conditions and agricultural practices. These findings suggest that awn reduction is unlikely to have been strongly selected during rice domestication. This assumption is particularly evident when considering the long-standing presence of awnless alleles within the gene pool, although these alleles are not fixed in the population. The widespread elimination of awns since the early 20th century has coincided with breeding practices aimed at achieving high yield. A substantial body of research has identified a negative correlation between awn development and seed number. This review provides novel insights into the evolution of awns in rice and may also inform related research in other species. Key words: rice domestication, awnedness, awn reduction, molecular evolution, domestication syndrome, crop breeding