Millions of Years of Movement: New data shows tropical mallow plants heading out of Madagascar
The history of botanical evolution is full of slim chances: Some fruits or seeds get swept up in a storm or lodged in a bird’s feathers and carried across an ocean. In some instances, this long-distance dispersal can lead to rapid diversification as new species evolve from the founding ancestor(s). On islands like Madagascar, a particularly high proportion of plants have been introduced via long-distance dispersal. While islands have long been recognized as important “sinks”—or places where plants are introduced via dispersal—a new paper in Molecular Phylogenetics and Evolution improves on our current knowledge by demonstrating that Madagascar has also been a prominent source for dispersal of plants to other places.
Co-first authors Cynthia Skema, botanical scientist at Morris Arboretum & Gardens, and Timothée Le Péchon, research scientist at Meise Botanic Garden in Belgium, along with Lucile Jourdain-Fievet and Jean-Yves Dubuisson, both of Sorbonne University in Paris, studied the evolutionary and biogeographic history of the Dombeyoideae. Dombeyoideae is a group of tropical plants that are part of the cotton family (Malvaceae), falling somewhere between cotton and cacao. The researchers found that this group dispersed from Madagascar to Africa five times more than from Africa to Madagascar.
“This paper completely underlines this sort of percolating idea that Madagascar is really a source [for plant dispersal] too, and a source for [plant dispersal to] continents, which is really interesting,” explains Skema.
For the project, Skema and Le Péchon sequenced DNA from members of Dombeyoideae, and then, by looking at the similarities and differences across the DNA, they generated hypotheses about how the members of Dombeyoideae are related to each other. They represented these hypotheses as phylogenies, which are tree-like depictions of evolutionary relationships. The phylogeny they created has the most complete sampling of Dombeyoideae to date.
Still, opportunities for inquiry remained: “Can you actually understand evolutionary change without understanding evolutionary movement?” asks Skema.
In response to this question, the team capitalized on their newly built tree, using it in conjunction with software programs to model how the ancestors of modern dombeyoids spread from place to place. Their analyses produced a compelling story.
In science, there’s a general notion that species tend to flow from large, continental landmasses to surrounding islands. While this isn’t entirely wrong, the full story is more complicated, explains Skema. “By their definition, volcanic islands are a clean slate. They have to be; the biota has to come from somewhere if [a volcano] just erupted [to form an island]... but that doesn't pertain to Madagascar.” Instead of being formed by volcanic activity, Madagascar is a small chunk of continent that was wedged between the African continent and the Indian subcontinent, splitting off from the latter over 80 million years ago. When it broke off, it brought an already diverse flora with it.
While the group found strong evidence of one long-distance dispersal from Africa to Madagascar, they found five going the opposite direction. Skema explains the significance of their finding.
“It’s that newness of the idea that more things are moving from Madagascar to Africa than vice versa,” she says. “It goes against the grain of what we’ve seen in the majority of the papers so far.”
In total, the analyses showed that Madagascar served as the source for nearly 70 percent of the dispersal events in dombeyoids, comparatively a very high percentage.
In addition to identifying patterns of dispersal, Skema and Le Péchon also found that most of the species diversity of dombeyoids in Madagascar evolved on the island in sympatric speciation. “Sympatry is when you have species diverging from one another in a shared area,” Skema notes. “It’s not that something comes in from somewhere else and you get a new species, [they’re] actually there living together on the island and diversifying.” This process can occur when flowering times drift apart, or pollinators start to differ. Anything that could stop plants within a population from interbreeding, thus limiting gene flow, could lead to speciation eventually. What has led to sympatric speciation in Dombeyoideae? That is a question that remains to be addressed. “For this group, we don’t know yet,” says Skema. “But we have some ideas.”
Skema notes that future research could look at geographic patterns in Dombeyoideae on a much smaller scale, for example, within Madagascar. By looking at how Dombeyoideae has evolved across the diverse habitats on the island, researchers may be able to uncover the drivers of sympatric speciation in the
Thinking about the impact of their work, Skema stresses that understanding the evolutionary and geographic history of one group of plants helps botanists understand and steward plants all around the world: “Everything is interrelated. If we want to understand how to conserve diversity, we also have to
understand the processes of diversification."