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Join Date: Mar 2002
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Quadpartite arms race
This article was linked to on ARN, but it fits well with the discussion on MDT. From the NY Times article:
Quote:
At the time of that research, in 1994, the ants and their mushroom fungus were the only two members of the symbiosis known. Cameron R. Currie, then a graduate student at the University of Toronto, felt sure that a clonal monoculture, especially one so widespread as that of the ants' fungus gardens, had to be highly vulnerable to parasites, even though none had been found by generations of ant biologists.
His intuition was right. He discovered the Escovopsis mold, which can wipe out a whole garden in a couple of days, and the antibiotic-producing bacterium that the ants use to keep it in check.
In his article in the current Science, Dr. Currie, now at the University of Kansas, and colleagues report they have analyzed the DNA of Escovopsis molds from ant fungus gardens in many different countries.
They find that all the molds that attack attine ant gardens are sprigs of the same family tree, indicating a single origin in the distant past. Escovopsis is found just in the ants' fungus gardens, but it is related to the dreaded "green mold" that is well known to commercial mushroom farmers.
Is Escovopsis also a clone? Does the symbiosis between mold and mushroom work because this is a true battle of the clones, each mutating too slowly to overwhelm the other? Dr. Currie said he had not yet found Escovopsis in a sexual stage and did not know whether it was a clone. His guess is that it is not.
But if Escovopsis is sexual and can evolve quickly, then how does the fungus stay one step ahead of its deadly pursuer? Dr. Douglas Futuyma of the University of Michigan suggested that the antibiotic-producing bacterium might hold the key to the puzzle. The mushroom has been forced to quit the sexual race, but it could have handed over its evolutionary defense to the bacterium, as deployed by the ants.
"The bacterium is a major player," Dr. Futuyma said. "Bacteria do have a certain amount of sex and can evolve very rapidly."
Dr. Currie and his colleagues said they believed that an evolutionary arms race had occurred between the mold on one side and the fungus, the ant and the bacterium on the other. The ants could play an important role because they are known to remove assiduously all foreign microbes from their gardens and will abandon a garden with any uncontrollable strain of Escovopsis. When an Escovopsis strain begins to escape the ants' control, the bacteria may be able to evolve a new brew of antibiotics to even the balance.
[...] In nature's impassive eyes, this may be just a protracted chess game among four species. But except for those who side with molds and microbes, it is hard not to admire the ant's achievements. They developed two remarkable inventions — agriculture and antibiotics — some 50 million years before people did.
Beyond that, they have learned how to handle technologies more skillfully than the bumbling civilization above their heads. They can grow a monoculture — a genetically homogeneous crop, something that in human hands generally leads to disasters like the Irish potato famine — and they have also learned how to deploy an antibiotic without the target pest's becoming resistant to it.[...]
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From the corresponding Science article:
Quote:
Currie CR, Wong B, Stuart AE, Schultz TR, Rehner SA, Mueller UG, Sung GH, Spatafora JW, Straus NA.
Ancient tripartite coevolution in the attine ant-microbe symbiosis.
Science. 2003 Jan 17;299(5605):386-8
[...]Phylogenetic analyses of nuclear DNA sequence data (20) indicate that Escovopsis parasitism of attine fungus gardens likely had a single evolutionary origin (Fig. 1). Four lines of evidence support an ancient origin of Escovopsis. First, Escovopsis is a monophyletic group found in association with the entire attine ant clade (Fig. 1). Second, a comparison of the phylogeny of Escovopsis with previously published phylogenies of attine ants and their fungal cultivars (5, 6, 8, 21, 22) indicates that, at the deepest phylogenetic levels, the evolution of the Escovopsis parasites parallels the evolution of both the ants and their fungal cultivars (Fig. 2) (20). Third, Escovopsis is phylogenetically (Fig. 1) and morphologically (12) diverse, suggesting a long evolutionary history. Fourth, we found no correlation between Escovopsis phylogeny and geographic distribution (Fig. 1), indicating lineage mixing across large geographic areas over extensive time periods. Taken together, these findings suggest that Escovopsis originated in the early stages of fungus cultivation by ants (7). [...]
Known host-pathogen arms races involve two symbiont lineages engaged in an escalating series of adaptations and counter-adaptations (24). In contrast, the attine ant-microbe system involves three mutualists--the ant, mutualistic bacterium, and cultivar--that all depend on successful fungal cultivation and are therefore aligned in their opposition to Escovopsis. The fungus garden is defended by the ants, which use specialized behaviors to remove the Escovopsis (16), and by the bacterium, which produces antibiotics that specifically inhibit Escovopsis (10, 17). The direct involvement of three diverse mutualists in defending the fungus garden against Escovopsis, in conjunction with our finding that Escovopsis has a long coevolutionary history within this symbiosis, indicates that this mutualism has been shaped by an arms race involving four symbiont lineages. Empirical and theoretical investigations into the evolutionary dynamics of this multi-symbiont arms race will inform a general model of the evolution of host-pathogen associations and parasite virulence.
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02-23-2003, 01:20 PM
