Trees may be solid and static, but their seeds and genes move. James Nicholls and colleagues have examined how Inga trees, a genus of around 300 species, moved from the rain forests of the Amazon to the Atlantic coastal rainforests of Brazil. Their DNA analysis has uncovered that the trees have made multiple migrations from the Amazon, starting over twelve million years ago. Yet to reach the Atlantic their offspring had to cross a thousand kilometer barrier of dry woodland and savanna.
Inga trees are at home in the rainforest. Indeed, their seeds require the forest’s constant moisture and shelter to germinate. Their canopies provide shelter for other rainforest species, while their roots can enrich the soil through partnerships with nitrogen-fixing bacteria. Some species, like Inga edulis, are cultivated for their edible pulp and have been used as food plants by indigenous peoples for generations. Their seeds, wrapped in a sweet, fleshy pulp, are dispersed by monkeys, so the monkeyless drylands to the east of the Amazon are quite a barrier to travel. Yet, the march of the Inga to the sea is captured in their DNA.
The research team collected DNA from 453 tree samples in 164 different Inga species from both regions to compare them. They then used advanced DNA sequencing techniques to analyse 810 genes from each sample, creating detailed family trees showing how different populations are related. Statistical analysis of these family trees could then tell the scientists how long ago the trees arrived.
The study found that there were around sixteen different migrations of Inga trees from the Amazon to the Atlantic. This is a lot more than might be expected. Previously, it was thought that plants could only cross between the forests when climate conditions allowed temporary forest connections. The continual migration points to the Inga trees using riverside forest corridors to travel, rather than changes in climate.
If the trees are using rivers, then presumably Inga trees could also make the trip from the Atlantic to the Amazon. But the genetic data says this is only likely to have happened once or twice. The explanation for the largely one-way direction of travel could be the Amazon’s role as a generator of biodiversity. In the case of Inga trees, these have been seen as an “Andean centred” group, so Nicholls and colleagues seem to have found a species leaving their ancestral home.
Nicholls JA, Ringelberg JJ, Dexter KG, Loiseau O, Stone GN, Coley PD, Hughes CE, Kursar TA, Koenen EJM, Garcia F, Lemes MR, Neves DRM, Endara MJ, de Lima HC, Kidner CA & Pennington RT. 2025. Continuous colonization of the Atlantic coastal rain forests of South America from Amazônia. Proceedings of the Royal Society B. https://doi.org/10.1098/rspb.2024.1559
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Cover: Canva.
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