The left–right-handedness of the cylindrical spathe correlates with the phyllotactic spiral direction in Arisaema (Araceae)

A left–right asymmetry of plants has attracted attention in various study areas (e.g., developmental biology, ethology, and evolutionary biology); however, the underlying mechanisms remain unclear. In the genus Arisaema (Araceae), each plant possesses a cylindrical spathe that exhibits the left–right-handedness in the folding direction. As the spathe is a modified leaf, a phyllotactic spiral (clockwise or counter-clockwise directions) in vegetative organs may affect this dimorphism. In this study, we aimed to examine (1) interannual changes in the spathe folding direction in the same individuals, (2) the effect of phyllotactic spirals on the left–right-handedness of the spathe, (3) the abundance ratio of left- and right-folded spathes in several Arisaema species, and (4) the relationship between the coiling direction of young folded leaves and phyllotactic spirals in other aroid genera. The spathe folding direction changed annually and was significantly related to the phyllotactic spiral direction. The left–right-handedness prevailed among the 27 tested Arisaema species, and frequencies of left- and right-folded spathes were mostly similar. In the five aroid genera, the phyllotaxis was clearly correlated with the leaf coiling direction. These results imply that the phyllotactic spiral affects the spathe folding direction of Arisaema and the leaf coiling direction of aroids. Considering developmental studies on asymmetric leaves in other taxa, the left–right-handedness of the spathe in Arisaema is possibly a consequence of the phyllotactic pattern formation.