Devastating diversity

日期:2019-03-07 07:07:01 作者:佘酰胜 阅读:

By Oliver Tickell THE intermingling of different forms of Dutch elm fungus could thwart efforts to fight the disease, a British mycologist told the International Botanical Congress, held in Missouri earlier this month. Dutch elm disease is caused by a fungus carried by Scotylus beetles, which burrow beneath the bark of trees. Since the 1960s, the disease has killed more than 80 per cent of elms in Britain. Clive Brasier of the Forestry Commission Research Agency at Alice Holt in Hampshire warns that a “swarm” of new forms of the fungus could evolve over the next few decades. Ophiostoma ulmi, the original fungus, spread from Europe to North America and Central Asia in the 1920s and 1930s. But in the 1940s, a more virulent species, O. novo-ulmi, began to take over. A North American form of O. novo-ulmi spread from the southern Great Lakes and reached Western Europe in the 1960s in imported timber. At the same time, a European form spread west from the Moldova-Ukraine region. Now the two forms have met in Central Europe (see Map). The hybrids occasionally produced when different fungal forms interbreed can be weak and transient, but they can still act as “genetic bridges”, transferring genes between the two parent forms. In particular, Brasier fears that they might allow an exchange of genes governing vegetative compatibility, known as vc genes. A promising approach being developed to fight the disease relies on viruses called d-factors (New Scientist, 15 February 1997, p 26). These spread from fungus to fungus when the threads of two separate fungi fuse. However, threads can fuse only if both fungi have the same vc genes. If they have different ones, the d-factors cannot spread. Hybridisation may have helped O. novo-ulmi to survive a virus outbreak that checked the spread of O. ulmi in the 1940s. According to Brasier, O. novo-ulmi appears to have gained new vc genes from O. ulmi at that time. These genes allowed it to diversify into numerous vc types and resist the spread of the viruses. History may repeat itself in the current wave of hybridisation between the two subspecies of O. novo-ulmi. “They may be better able to resist the spread of viruses—and this is one of the main ways in which the disease may otherwise go into spontaneous remission,” says Brasier. In the case of alder blight—another fungal disease that attacks trees—hybridisation led to the development of a new pathogen (New Scientist, 15 May 1999,