The ‘tree of life’ model of evolution may turn out to be more tangled and overgrown than thought.
Researchers at Vanderbilt University have discovered that a large cluster of genes appears to have jumped directly from one species of fungus to another – making a mosaic a better metaphor, they say.
“The fungi are telling us something important about evolution… something we didn’t know,” said assistant professor of biological sciences Antonis Rokas.
The team discovered that millions of years ago, a cluster of 23 genes jumped from one strain of mold commonly found on starchy foods like bread and potatoes, Aspergillus, to another strain of mold that lives in herbivore dung and specializes in breaking down plant fibers, Podospora.
The findings came as a big surprise, as this type of gene transfer between organisms, known as horizontal gene transfer, has only rarely been reported in complex cells like those found in plants, animals and fungi.
“Because most people didn’t believe that such large gene clusters could be transferred horizontally, they haven’t looked for them and they haven’t been found,” Rokas said.
The jumping gene cluster codes for a toxic compound called sterigmatocystin. Cells produce this type of compound to attack competing organisms or to protect themselves from attacks. As a result, these compounds are the source of a number of important drugs, like penicillin and cyclosporin, as well as a number of natural poisons.
“Fungi produce an astonishing variety of drugs and poisons. Our discovery that one of the largest gene clusters responsible for making such a poison moved intact between species suggests that horizontal transfers of wholesale pathways may have contributed significantly to the generation of this diversity,” Rokas said.
Horizontal gene transfer was first discovered in bacteria, where it’s been recognized as largely responsible for the problem of drug resistance. But it was assumed that it remained relatively rare among complex organisms like plants and animals.
“The thinking has been that there is very little horizontal gene transfer among plants and animals except for a few big, ancient events and maybe the occasional transfer of a single gene here or there,” said research associate Jason Slot.
“Our discovery suggests that the horizontal transfer of gene clusters may have been a big player not only in the evolution of bacteria but also in more complex organisms.”