Selection for floral integration and trait variation in zygomorphic flowers of Aconitum japonicum ssp. subcuneatum (Ranunculaceae)

Abstract

Pollinator-mediated selection might lead to among-trait differences in the degree and pattern of floral integration and intra-flower variation. To examine the patterns of intra-flower variation in floral traits, including nectar volume, we performed a field study using the zygomorphic flowers of Aconitum japonicum ssp. subcuneatum. We investigated (1) correlations between the sizes of the left and right sepals and petals, (2) variation in floral traits among plants, within plants and within flowers, (3) effects of sexual phases on floral integration variation in floral and nectar traits, and (4) the effect of size and intra-flower variation in traits of the left and right sepals and petals on pollen removal by pollinators. Lateral sepal area, but not lower sepal area, was highly correlated between the left and right sepals. Floral traits were more integrated during the male phase than during the female phase. Nectar standing crop in male-phase flowers correlated with helmet height and lateral and lower sepal area, but in female-phase flowers it only correlated with spur length. While intra-flower variance in lateral sepal area accounted for approximately 10% of the overall variance in these traits, the variance in lower sepal area accounted for 70% of the overall variance. Lateral sepal area had a negative effect on the number of pollen grains remaining after pollinator visits. Low variance in lateral sepals within flowers and measurements of pollen removal suggest that lateral sepals play a more important role in pollen export than the other traits. Left and right sepals may be the targets of selection for symmetry in zygomorphic flowers.