We investigated the floral anatomy and development of Macarthuria australis Hügel ex Endl., an unusual genus endemic to Australia, in the context of floral evolution of core Caryophyllales. Flower initiation is spiral, with sepals developing quincuncially. The first two petals continue the sequence of sepal initiation, but the remaining petals arise from common stamen–petal primordia. The androecium develops sequentially as three inner antesepalous and five outer antepetalous stamens. The globular ovary is trimerous with a short symplicate zone and two arillate ovules per locule. The rapid emergence of the androecium leads to a partial absorption of the petal primordia within the androecial tissue. The two first-formed petals have more room for development and precede the androecium, supporting the fact that petals are not staminodial in origin. This heterochronic shift correlates with an inversed developmental sequence of the antesepalous stamens. The constraint caused by the spatial occupation of sepals and carpels leads to the loss of two stamens, and the re-arrangement of stamens and petals along the flanks of the carpels. The floral development of Macarthuria anticipates a syndrome of stamen and petal development in other core Caryophyllales and culminating in the Caryophyllaceae.
RATES OF FLORAL EVOLUTION: ADAPTATION TO BUMBLEBEE POLLINATION IN AN ALPINE WILDFLOWER, POLEMONIUM VISCOSUM
