The evolution of the sweet potato

While some argue that the presence of the sweet potato (Ipomoea batatas) in Polynesia is proof of early communication between America and Polynesia, a new study suggests the sweet potato seeds simply have floated across the vast Pacific Ocean[1].
The research reveals that the sweet potato evolved just once, probably in central or northern South America, and originated from a single ancestor.

To delve into the sweet potato’s past, scientists analysed genetic material from almost 200 specimens of the plant and its 14 closest related wild species to reconstruct a family tree.

The results suggest that the sweet potato is more closely related to one wild species, known as cotton morningglory (Ipomoea trifida), distributed around the Caribbean, than any other, suggesting that both evolved from a common ancestor.
[Sweet potato and 5 closely related species: A: Ipomoea batatas, B: I. trifida, C: I. triloba,
D: I. ramosissima, E: I. cordatotriloba (South America), F: I. leucantha]

The team adds that this branch in the family tree occured some 800,000 years ago. Moreover, the analysis suggests the sweet potato interbred with the cotton morningglory at some point within 56,000 years of the two species evolving from their common ancestor.

While some have suggested that the plant’s presence points to communication between inhabitants of the two regions, the latest study suggests it is more likely that seeds of the sweet potato simply floated across the Pacific on sea currents.

A wild relative of the sweet potato found in Polynesia, but not America, appears to have split from American species more than a million years ago – ruling out human transport. The seed pods of this plant, the authors note, are very similar to those of sweet potatoes, suggesting that it, too, could have travelled around the world on the waves.

The evidence, so conclude the scientists, against human-mediated transport of the sweet potato to Polynesia is, therefore, extremely strong.

[1] Muñoz-Rodríguez et al: Reconciling Conflicting Phylogenies in the Origin of Sweet Potato and Dispersal to Polynesia in Current Biology – 2018

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