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Posted 04 December 2008

Secret sex life of killer fungus discovered

Scientists at University College Dublin and the University of Nottingham have identified the sexual behaviour of a common microscopic fungus called Aspergillus fumigatus which is a major cause of death in people with weakened immune systems. Until now, the fungus, which was first described over 145 years ago, was thought to have no sexual cycle and to reproduce only by asexual spores.

The discovery was made by Céline O’Gorman, Hubert Fuller and Paul Dyer and is published in the leading scientific journal Nature.

Picture far right: Sexual structures formed where two compatible strains of Aspergillus fumigatus have mated.

The mould, which causes aspergillosis, is also responsible for a range of allergic diseases including asthma and sinusitis in humans.  Birds, cattle and horses are also susceptible to infection by the fungus.

“Aspergillus fumigatus is a common fungus worldwide, growing on decaying organic matter such as compost heaps and damp hay,” says Hubert Fuller, a mycologist from the UCD School of Biology and Environmental Science.

“Like all fungi it produces enormous numbers of spores which become airborne and spread the fungus. Its spores are very common in the air and it has been estimated that everybody inhales several hundred Aspergillus spores each day that are normally eliminated by the immune system in healthy individuals.”

“However, due to its ability to cause infections in hosts with a weakened immune system, this fungus has become the most prevalent airborne fungal pathogen causing more than a 50% mortality rate in those susceptible.”

4% of patients in modern EU teaching hospitals have invasive aspergillosis – some 5,000 cases per year in the UK. It is the leading infectious cause of death in leukaemia and bone marrow transplant patients.

The fact that the fungus can sexually reproduce means it can develop genetic variations that could enable it to become resistant to antifungal drugs. In addition, the sexual spores are better able to survive harsh environmental conditions, allowing the fungus to spread over wider terrain.

“Although this discovery seems to represent a further threat, there is also a more positive side to the story,” says Céline O’Gorman who conducted the research as part of her PhD at University College Dublin. “The sexual cycle can be used as a valuable aid in laboratory experiments to try to work out how the fungus causes disease and triggers allergic reactions. Once the genetic basis of disease is understood researchers can then look to devising methods to control and overcome the fungus.”

The strains of Aspergillus fumigatus used in the experiments were isolated from the air in Dublin and the first observations of the sexual stage were made in a laboratory at University College Dublin.

O’Gorman’s research was funded through an IRCSET Postgraduate Research Scholarship. An EC Marie Curie Training Fellowship and a grant from the British Mycological Society enabled her to travel to a laboratory in the University of Nottingham run by Paul Dyer, who is a co-author of the paper.

The newly discovered sexual stage of the fungus has been named Neosartorya fumigata O’Gorman, Fuller & Dyer.

Almost one-fifth (ca 20,000 species) of all fungi have no known sexual stage – these include many Aspergillus, Penicillium, Coccidioides and Malassezia species which are of major economic and medical importance. However, some of these species have apparently functional sex-related genes and this discovery could lead to a sexual revolution for many other of these supposed ‘asexuals’.

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Sexual structures formed where two compatible strains of Aspergillus fumigatus have mated.