Floral Development and Homeosis in Actaea rubra (Ranunculaceae)

Actaea rubra is a member of the Ranunculaceae in which the number and positions of petals are variable. To understand the process by which this variation arises, we used epi-illumination microscopy to study floral development in the species. We found that four sepal primordia initiate in two pairs, one outer and one inner. Then four primordia form in alternation with the sepal primordia, followed by the initiation of six primordia in opposition to the sepal primordia (one primordium opposite each outer sepal primordium, and two opposite each inner sepal primordium). These first 10 small, rounded primordia can develop into either petals or stamens, but they cannot be discerned in early stages because the primordia are very similar in size and shape. Subsequently, numerous primordia that usually develop into stamens initiate in alternation with previously formed primordia, and the floral apex terminates in a crescent-shaped gynoecial primordium. Petal and stamen primordia can be distinguished in later stages when petal primordia are small and flattened in appearance, whereas stamen primordia have anther sacs. At maturity petals are small, white, and spatulate-shaped, and stamens have long, slender filaments and oval anthers. Examination of these later stages showed that the number and positions of petals are not constant. We therefore mapped petal and stamen positions in more mature floral buds and found that petals most frequently occupied the positions that alternate with the sepals. However, a comparison of the positions that petals and stamens occupy in different flowers showed that petals could form in what were usually stamen positions, and stamens could form in what were usually petal positions. We interpret this as an example of homeosis occurring within and among plants of A. rubra. When the floral morphology of Actaea is compared with that of related taxa, various phylogenetic interpretations of homeosis can be hypothesized, depending on the putative outgroup condition.