A new study reveals how the Andes have shaped the history of aquatic plants

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The Bogotá River drops into a dramatic 132-metre waterfall known as the Salto de Tequendama, southwest of Bogotá City, Colombia. Almost 2,000 metres above sea level, one can imagine Humboldt and Bonpland wiping mist off their faces while plucking a tiny aquatic plant with lacy leaves from between the rocks. That plant now is known as Marathrum foeniculaceum and, for the untrained eye, can easily be mistaken for algae.

“We found it only once on the rocks of Salto de Tequendama, on the verge of a very dangerous precipice of nearly 195 metres,” the pair of naturalists wrote in 1806.

The Salto de Tequendama is the result of the pushing and lifting of the Earth’s crust giving rise to the Andes in an ongoing process that began nearly 11 million years ago. Plant scientists have known that the hoist of the Andes was a melting pot for biodiversity, but water-loving plants, like Marathrum foeniculaceum and its sister species Marathrum utile, remained overlooked. Now, a study shows how the history of South America’s longest continental mountain range has determined the fate of this pair of plants.

Both species diverged around 17 million years ago, unable to exchange genes because of blockage from the rising of the Andes, researchers from the University of Washington report in New Phytologist.

The new findings show the interplay of biology and geology as “changes in the landscape result in changes in the history of these organisms,” says evolutionary biologist and study main author Ana María Bedoya.

What’s more, groups of the same species also split, the team found. Populations of M. foeniculaceum in the Sierra Nevada de Santa Marta and in the Magdalena basins split around 12 million years ago, while M. utile from those two valleys diverged only roughly 4 million years ago.

Previous research of Andean plant diversity had focused on land species. In the new study, Bedoya and her colleagues add information about the evolution of aquatic plants in an area of high diversity.

“The rivers were shaped by the Andes and there are plants which are only found in rivers,” the scientist says. “To understand how the flora of the most diverse area of the world came together we cannot ignore water.”

Bedoya travelled to her native Colombia, navigated and kayaked to collect 75 individuals of M. foeniculaceum and 40 of M. utile from 21 sites across the Andes and the Sierra Nevada de Santa Marta. Bedoya says that many of these collections are new records.

“[Our collections make up] more than 50% of all Marathrum specimens collected since 1802,” she explains.

Part of the reason why these water-loving plants might be poorly represented in herbaria is because most botanists don’t like getting their feet wet, Bedoya explains paraphrasing the Swiss botanist Christopher D.K. Cook.

And it’s not only their wet habitat what keeps researchers from studying them. Bedoya says that when selecting her study sites, she opted not to visit the Salto de Tequendama, the type locality of M. foeniculaceum, as it is now highly polluted. Things are not good in the south either. Their sampling in Boyacá, in the Orinoco basin, only included two individuals partly because the populations of Marathrum are quickly disappearing because of the construction of hydroelectric plants, which divert the river flow.

“The future of these populations depends on the future of these rivers,” Bedoya says.  

Boyacá is also special in terms of ancestry because the individuals they collected are admixed, meaning that their DNA shares parts with both species from the two northern basins: the Sierra Nevada de Santa Marta and Magdalena.

“This is super interesting because we find there is no gene flow among basins and dispersal is very limited, so then the question is what is the origin of the Boyacá population?” Bedoya wondered.

Even when their sampling was limited, their phylogenetic analyses and their associated inheritance probabilities, show that these hybrids result from the merging of the two northern ancestral lineages, something known as a reticulation event.

The group of researchers suggest two alternatives to explain the mixed genetic identity of the Boyacá population. On one hand, it is possible that seeds from northern populations could have been transported by what a group of geologists have called the Trans-Andean Portal, a fluvial highway that used to connect the Orinoco and Magdalena basins between 13 to 4.1 million years ago. Another alternative is that birds might have dispersed the sticky seeds of both species.

Bedoya is passionate about her research and doesn’t mind having her feet wet; water transports her to a distant past, one in which plants are venturing into new territory.

“The information contained in the DNA of a plant that one collects in a river can lead us to generate hypotheses of how the landscape was different in the past,” she says.

RESEARCH PAPER:

Bedoya, A.M., Leaché, A.D. and Olmstead, R.G. (2021), Andean uplift, drainage basin formation, and the evolution of plants living in fast-flowing aquatic ecosystems in northern South America. New Phytologist. Accepted Author Manuscript. https://doi.org/10.1111/nph.17649