A common European weed has turned its compact flower into an expansive, yellow-petalled blossom by borrowing a couple of genes from a close Italian relative. Researchers say the exchange is a rare documented example of beneficial genetic flow between species. It also challenges the notion that higher organisms must rely on their own genes to evolve.
The story starts 300 years ago, when botanists introduced a yellow Sicilian flower called Senecio squalidus to Oxford, U.K. At the time, there was only one variety of a British weed known as the common groundsel (S. vulgaris), its 10-millimeter floral head packed with tiny tubular flowers. But as the Sicilian flower--which eventually became known as Oxford ragwort--spread, a new variety of groundsel emerged that is ringed with large yellow petals.
Enrico Coen, a geneticist at the John Innes Centre in Norwich, U.K., and Richard Abbott of the University of St. Andrews, also in the U.K., wondered if the Oxford ragwort had anything to do with the transformation. They knew of genes in snapdragons that control the symmetry of that flower's blossoms, and they used the sequence of those genes to find the equivalent genes in the groundsel. Two genes, Ray1 and Ray2, seem to be involved in forming ray flowers. The sequences of these genes in the showy groundsel differed from those in the original groundsel, but they were almost identical to the ones in the ragwort, Coen's team reports tomorrow in Science. They entered the groundsel's genome when the two species hybridized, says Coen.
Coen says the petals likely provided the showy groundsel with an evolutionary advantage. Groundsel is usually self-fertilizing, but the ray flowers tend to be pollinated from neighboring plants, increasing the diversity of the resulting seeds and, with that, presumably their ability to cope with a wider variety of conditions. That may explain why the showy groundsel--once rare--has rapidly spread.
Borrowing genes may also have been a way for the groundsel to regain petals it had lost during evolution, suggesting that evolution is not a one-way street. The fact that genes--and a lost trait--may be reincorporated into a species "may represent a common theme in plant evolution," says Loren Rieseberg, an evolutionary biologist at the University of British Columbia in Vancouver, Canada.
The work illustrates that "evolution is more of a web of life than a tree of life," in that genes can flow between branches, says Michael Arnold, an evolutionary biologist at the University of Georgia, Athens. Such gene flow has been seen before, primarily in microbes, "but this is probably, to date, the best example in eukaryotes where they nail down what the gene is," he says. "That's a key, key finding."