Ranunculaceae and the origin of petals

There are events that change the course of evolution. In my opinion one of the most important events that have changed the course of evolution in flowering plants was the transition between spiral to cyclic flowers. This episode was obviously gradual as everything in Nature is, but it is still incredible to observe this transition in living plants and there is a group telling us this episode better then any other.

Early diverging eudicots from APG III tree

Early diverging eudicots is the transitional group between basal angiosperms and core eudicots. In fact, plants belonging to this group are inbetweeners, they share characters from both groups, bringing an extreme floral diversity to the plant world. They were the authors of many innovations, such as nectaries, zygomorphy, perianth differentiation in two different whorls (calyx and corolla) and many other novelties. One of the best examples of this diversity is Ranunculaceae, and today I have brought one of the most stunning creations of this family in the form of Aquilegia. In fact, most members of Ranunculaceae have incredible structures where you can almost interpret the gradual formation of cyclic flowers, many of them with extravagant nectaries acting also as petals. Aquilegia is just another example, and the flowers are pretty much incredible!

Side view (left) and front view (right) of Aquilegia flower

In Aquilegia, pentamery is already established, and we can easily find the bipartite perianth with 5 sepals and 5 petals. The petals, however have a very particular character – they are spurred, producing nectar to attract pollinators. This is the reason why many authors prefer using the term “nectar leaves” instead of “petals” in Ranunculaceae.

Left: View of the back of Aquilegia flower; Right: Detail of Nectar leaf of Aquilegia

These nectar leaves are of extreme importance on the evolution of a bipartite perianth, and in Ranunculaceae this transitional situation is still visible. They are not true petals yet, but structurally that’s how we interpret these structures. The tepals, structurally acting as sepals in this Aquilegia are the true perianth members of the flower. The nectar leaves are nothing more than stamens that lost their original reproductive function, becoming staminodes (or sterile stamens) and latter gaining a new function to attract pollinators. This is how true petals were born, they are nothing but modified staminodes that evolved in the direction of pollinator attraction. On the other hand, sepals evolved directly from tepals (leaves) gaining the function of organ protection (the reproductive organs – stamens and carpels).

Origin of petals in Aquilegia. The outer whorl of stamens loosing the reproductive function, and gaining later the function of pollinator attraction, becoming showy  and colourful (like petals) and producing nectar. Tepals at the same time gain the function of flower protection, like sepals.

However, some researchers believe that the origin of petals is not entirely the same for all members of Ranunculaceae. Some members of the family might indeed have their bipartite perianth as a result of the ascension of the bracts to the base of the receptacle, originating the sepals. Likewise, the true perianth (the tepals) gave origin to the petals. Ranunculaceae seems to be a family of flower structure experiments, trying which structure is the best to have. In any case, it seems that after this group pentamery and bipartite perianth were successfully achieved characters, well established in the rest of Eudicots.