Fruit evolution in Hydrophyllaceae

Fruit type and morphology are tightly connected with angiosperm diversification. In Boraginales, the first-branching families, including Hydrophyllaceae, have one- to many-seeded capsules, whereas most of the remaining families have four-seeded indehiscent fruits. This fact argues for many-seeded capsules as the ancestral condition. However, little is known about the evolution of fruit dehiscence and seed number. The present study investigated the gynoecium and fruit development and morphology and the evolution of seed-numbers in Hydrophyllaceae.

Gynoecium and fruit development and morphology were studied using scanning electron microscopy and x-ray microcomputed tomography. Ancestral character state reconstruction of seed number was performed using a broadly sampled phylogeny of Boraginales (ndhF and ITS) with an emphasis on Hydrophyllaceae.

Our ontogenetic studies not only demonstrate parallel developmental trajectories across Hydrophyllaceae, but also a striking diversity regarding the internal organization of the gynoecium. Ovule number appears to determine ovary structure. Many-seeded capsules are retrieved as the ancestral state of Hydrophyllaceae. At least seven transitions to fruits with (one to) four seeds and four reversals (i.e., from four- to many-seeded fruits) were reconstructed in Hydrophyllaceae.

Several shifts in seed number from "many" to "four" and back to "many" have taken place in capsular-fruited Hydrophyllaceae, a strikingly high number considering that seed number is virtually conserved across the rest of the order. The groups with a conserved seed number of four are characterized by indehiscent schizocarps or drupes and by seeds that are integrated into mericarps. This functional integration probably acts as an evolutionary constraint to shifts in seed number.