Dorstenia

The most striking characteristic of Dorstenia is their reproductive structure, called pseudanthium (Greek for "false flower") or in Moraceae hypanthium, which is composed of clusters of tiny unisexual flowers on a disc- or cup-shaped receptacle that are often adorned with bracts of various sizes and shapes. The pseudanthiums can be planar, convex, concave, round, oval, square, lobed, twig, star, boot, or tongue-shaped. Their color varies from green to yellowish and reddish to violet and brown. Beneath the pseudanthium, there are usually bracts, scattered or in rows, sometimes carrying appendages. Sometimes the bracts are absent and only their remaining tooth-shaped, awl-like, spatula-shaped or band-shaped appendages are recognizable.

Dorstenia urceolata from Brazil.

The globular, tapered, or warty flowers are unisexual. The female flowers within the receptacle mature first. The male flowers are either scattered between the female flowers or are concentrated on the outer edge of the receptacle or are separated by a flower-free zone at the outer edge. They are stalked and carry one to four (usually two to three) free or almost free tepals and one to four (usually two to three) stamens. The sunken female flowers carry tubular tepals and a free fruit node with one or two, then mostly unequal scars. The stone fruits are embedded in the broadened inflorescence axis and when mature are scattered by a centrifugal mechanism . Like most members of the Moraceae, Dorstenia species have drupe like fruits that are embedded in the receptable. However, a special feature of Dorstenia drupes is that they explode to release and scatter the seeds by way of a centrifugal mechanism. The stone seeds are usually small with a minuscule endosperm.[2]

Fossils of Ficus and Morus fruits have been found on the African continent, and are used to approximate the origin of family Moraceae to a maximum of 135 million years ago. In a recent study using fossil fruits, Bayesian molecular dating, and maximum likelihood, researchers attempted to reconstruct the ancestral history of Dorstenia with ITS (internal transcribed spacer) sequences from ribosomal DNA of 35 Dorstenia species and seven out-group species from the different tribes within the Moraceae. The goal was to resolve a long-standing issue within Dorstenia of if this genus diverged and radiated prior to the split of Africa and South American about 105 mya, and members of this genus are on separate continents by vicariance, or if this genus diverged post split and Dorstenia became established in the Neotropics by seed dispersal.[5]

This study produced an interesting phylogeny that revealed an initial old world divergence around 112.3 mya, divergence and radiance of new world Dorstenia at 67.2 and 30.3 mya respectively, and an old world group nested within the new world that radiated around 13.6 mya. The results of this phylogeny do not determine if vicariance or seed dispersal is the cause behind the old and new world populations of Dorstenia; however, it does pose several hypotheses regarding how the new world population came about as well as how three old world Dorstenia species are nested in the new world clade of the phylogeny. Due to the small endosperm that is typical of Dorstenia seeds, it is unlikely that seed dispersal by animals is the reason for the new world and reemerged old world species. However, it is possible that the new world population emerged by transport over the Beringia, established populations all throughout the North and South Americas, and when climate conditions changed and North America was no longer tropical or subtropical that the North American population died out, leaving only the New World population seen today in South America. This idea also allows for the Old World species nested within the new world by Dorstenia populations established in America returning to Africa by Beringia. For this hypothesis to receive more credence, fossil Dorstenia plants in North America would be needed.[5]