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Fiendish fungus

发布时间:2019-03-08 07:04:05来源:未知点击:

By Stephanie Pain AN UNNATURAL union between two fungi has created an aggressive new disease that is felling hundreds of thousands of alder trees along Europe’s rivers. Two species of Phytophthora, relatives of the potato blight, have hybridised to create lethal offspring that destroy the bark around the base of the trees, eventually killing them. So far, the disease has claimed around 10 per cent of the alders in southern England and Wales, and is steadily killing up to 2 per cent of the population each year. The blight has also had a catastrophic impact in parts of Sweden, France and Holland, and is spreading in Germany and Austria. Forest researchers warn that this might be the first of a rash of new pathogens to strike as the world trade in plants accelerates the evolution of new species. “I’m concerned that there’s a lot of this going on,” warns Clive Brasier of the Forestry Authority Research Station in Farnham. Epidemics of fungal diseases are usually caused by introduced pathogens, which make short work of native species with no resistance to them—as with Dutch elm disease in the 1970s. But until now, there has been no evidence that introduced fungal species were exchanging genes with either resident fungi or other exotic fungi. When the alder disease first struck in 1993, Brasier and his colleagues at the Scottish Crop Research Institute in Invergowrie tried to identify the fungus by sequencing a region of its DNA called the internal transcribed spacer, a stretch of genes that provides a signature of the species. They were surprised to find signatures from two different species, suggesting it was a hybrid. “That’s very unusual,” says Brasier. Fungi rarely hybridise in nature, because species that live in the same environment have evolved barriers to prevent it. “But fungi that are geographically isolated don’t accumulate barriers to reproduction. So if you bring them together there is more risk of hybridisation,” says Brasier. Further studies have identified one parent as Phytophthora cambivora, which infects some trees, but not alders. The other parent is close but not identical to P. fragariae, a blight of strawberries and raspberries (Proceedings of the National Academy of Sciences, vol 96, p 5878). Both species have been introduced to Europe. The hybrids of these species found in different parts of Europe vary greatly in structure and behaviour and often have reproductive abnormalities. This suggests that the pathogen is a very recent creation and is still evolving. “It hasn’t stabilised into a single entity yet,” says Brasier. “We don’t know where selection will take it.” Alders are a key species in wetlands and along rivers, where they stabilise the banks, says John Gibbs, who has surveyed the damage in Britain for the Forestry Authority. If the fungus reaches the US, which has many native alder species, the losses could be immense. “We don’t know yet if the American species are susceptible,