New Species Evolves From Two Invasive Aliens…Twice

The picture of invasive species is often painted in broad strokes: plant or animal gets introduced to a new environment, explodes in numbers, causes problems for species that were already there. But look a little closer, as a researcher at the University of Stirling in the UK did, and ecological curiosities may come to light. The researcher, Mario Vallejo-Marin, discovered a Scottish species of monkey-flower that evolved from two invasive species – twice.

Monkey-flowers get their name because the flowers of some species have a shape resembling a monkey’s face. Back in the early 19^th Century, two species of monkey-flower, Mimulus guttatus and and M. luteus, were introduced to Britain from North America and South America, respectively. The yellow-petaled plants soon escaped into the wild and spread widely along river- and stream banks and in patches of soggy ground – so far, so typical, as invasive species go.

Scientists have known for a while that the two species hybridize to form a third kind of monkey-flower, known as M. x robertsii, which is sterile but spreads vegetatively, sometimes forming clumps thousands of stems strong.

The surprises started in 2012, when Vallejo-Marin was working in South Lanarkshire, in southern Scotland, and found a yellow monkey-flower that looked very much like M. x robertsii but had fertile pollen. Even odder, two years later he noticed the same flower nearly 350 miles away, on the Orkney Islands off the northern Scottish coast. As far as anyone knows, this new species is endemic to Scotland, meaning it lives nowhere else in the world.

The monkey-flower, which Vallejo-Marin christened M. peregrinus, meaning “the foreigner,” evolved independently at least twice, he and his colleagues reported May 1 in the journal Evolution.

To reach this conclusion, the researchers analyzed the genomes of 16 plants, representing all four species of this troop of monkey-flowers collected from various locations. The distribution of alleles, or different forms of a gene, across these species supports the story that M. x robertsii is a hybrid of M. guttatus and M. luteus.

Moreover, each of the M. peregrinus specimens in the study is genetically most similar to M. x robertsii specimens collected nearby, strong evidence that the two populations of M. peregrinus evolved from M. x robertsii independently. “It shows that when the conditions are right, the origin of species is a repeatable phenomenon,” Vallejo-Marin says.

Previous work had shown that M. peregrinus has twice the number of chromosomes as M. x robertsii, suggesting that it evolved by genome duplication. This process, called polyploidization, plays a major role in plant evolution. About 15 percent of plant species are believed to have originated in this way, including wheat, tobacco, and cotton.

When the formation of a new species involves both hybridization and genome duplication, the process is called allopolyploidization. Scientists know of only a few other allopolyploid species that have formed within the last 200 years, so M. peregrinus represents a rare opportunity to study the evolution of a very young species.

But just because a new species evolves in a particular place doesn’t mean it fits harmoniously into its environment. Like M. peregrinus, the cordgrass Spartina anglica evolved through allopolyploidy involving exotic species. It arose on the English coast in the late 19^th Century and now chokes out native vegetation in salt marshes not only in Britain but across Asia, Australia, New Zealand, and North America as well.

Whether M. peregrinus will take a similar course is unknown. For now it’s a Janus-faced monkey, simultaneously exotic and endemic, but its ultimate ecological role in Scottish wetlands remains uncertain. –