Extinction of the Fittest

/ by Adnaan Wasey /

A fungus' lack of libido illustrates how a strong species could become extinct.

Think of Penicillium marneffei as the Morrissey of the fungus world.  It’s perfectly happy shunning what comes naturally to four out of five fungi: sex.  That celibate lifestyle is putting this modern eukaryote at risk of extinction.

P. marneffei, as it’s known to epidemiologists, is a pathogen often found in the compromised immune systems of AIDS patients. It’s endemic to Southeast Asia, where it thrives in its niche and is very adept at dispersing its spores over long distances. Darwin might say the fungus is successful because it has out-competed other organisms.

But Mat Fisher, a lecturer at Imperial College London and the lead author of a paper published in the October 2005 issue of PLoS Pathogens, said that the fungus has become so specialized that it’s shut off its sexual reproductive pathways— much to its own detriment.

“They’re not doing well against their sexual relatives,” Fisher said. “We can see they’re dispersing but they’re not initiating new live colonies.” It looks like the fungus is quarantining itself in the environments in which it is flourishing.

The researchers mapped genetic variation in the Penicillium across Thailand by looking at “microsatellites,” small regions of DNA that are prone to mutation. Using bioinformatics software and geographical information, Bill Hanage, a research associate at Imperial College London and a coauthor of the paper, said they found a high degree of adaptation to local environments but also a high degree of dissimilarity between fungi at opposite ends of the country. Even though the fungi could spread, the data showed they weren’t doing so.

So how can a species be both “fit” and “unfit” for survival?

Organisms that reproduce by having sex are able to exchange genes and create offspring that are genetically hedged against changes in the environment—whether it’s a climate shift, a new predator or a change in resources.  Fungi can reproduce sexually—and most of them do—but a minority opts for asexual reproduction because of the method’s increased efficiency.

“Asexuality, while it may have short-term benefits, may lead to a long-term extinction,” said Fisher. “Truly asexual fungi are extremely rare; in fact, P. marneffei may be the only fully asexual fungus found in the environment”

The researchers believe that P. marneffei used to be sexual, but has since lost that ability. Because the fungus doesn’t recombine its genes, it’s put itself at a disadvantage. A slight shift in its niche could be catastrophic.

The fungus is expected to lose Darwin’ game because of its relatively non-diversified genome. P. marneffei‘s only hope is genetic mutation—but this happens only once every 50,000 generations.  It’s a very inefficient process and it’s too slow to compensate for new pressures.

This is bad news for P. marneffei but it has implications for other heavy-hitting endemic pathogens that remain confined to certain parts of the globe.  The research reported in PLoS Pathogens suggests that the endemicity of any microbe, caused by genetic specialization, could be a marker for future extinction.

Fisher said the fungus is on a one-way road to extinction, albeit a long one.“We’re talking about time scales of thousands to tens of thousands of years. It may have been very successful once but this is the end game for it.”

Originally published October 31, 2005

Tags contagion ecology research resilience

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