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Symbiotic fungus does not depend on fungus-farming ants for reproduction, researchers say

Fungus-farming ants around the world cultivate essentially the same fungus and are not as critical to the reproduction of the fungi as previously believed, biologists at The University of Texas at Austin have discovered.

Fungus-farming ants are dependent on cultivating fungus gardens for food, and it has been widely believed the fungi also evolved dependence on the ants for their dispersal and reproduction. When young ant queens establish new colonies, they take a start-up crop of fungi with them from their parental garden.

A leaf-cutting ant queen (Acromyrmex coronatus) is sheltered in a chamber deep inside of her fungus garden, made of leaf fragments and strands of a symbiotic fungus. The garden was grown from a few strands brought by the queen on her mating flight from the maternal garden. After the mating flight, the queen never leaves her garden metropolis. Credit: Alexander Mikheyev and Barrett Klein

Graduate student Alexander Mikheyev and Dr. Ulrich Mueller, professor of integrative biology, have now found that the fungi reproduce sexually and disperse widely without the aid of their ant farmers.

Different genera of the ants, it turns out, are essentially cultivating the same fungus across wide geographical areas.

The scientists' finding provides a new perspective on coevolutionary processes. Coevolution, like that between honeybees and the flowers they pollinate, occurs when two or more species influence each other's evolution over time. Mikheyev says that two species don't necessary need to have a very specific, one-to-one relationship in order to coevolve.

"This shows that coevolution can proceed without specificity at the species level," said Mikheyev. "It has been believed that mutualistic interactions, as well as parasitic ones, are very specific and one-to-one. We are beginning to realize that this is not necessary for long-term coevolutionary stability, with the leaf-cutting ants being a dramatic example."

The research was published June 26 in Proceedings of the National Academy of Sciences.

"Previously, the fungi were thought to be passive players, moved around by the ants," said Mikheyev. "We show that the power of fungal dispersal is probably beyond ant control."

Observations of the fungi reproducing sexually--producing a fruiting body or mushroom--deep inside an ant nest are extremely rare. So the scientists, in collaboration with Dr. Patrick Abbot at Vanderbilt University, read the signatures of sexual reproduction in the fungal genes. They studied fungi cultivated by leaf-cutting ants, well known fungus farmers that bring pieces of leaves back to their nest to use as a growth medium for their fungi.

Genetic analysis revealed that the fungi are still using the cellular machinery necessary for sexual reproduction, which would have been lost or changed had the fungi become completely dependent on asexual, clonal reproduction through their ant farmers.

Studying the fungus gardens of an introduced population of leaf-cutting ants on the French Caribbean island of Guadeloupe, the scientists also found that the fungal genes have been recombining, a sign they are reproducing with one another.

"The fungi are not completely domesticated and under the strict control of the ants," said Mueller. "Instead, the fungi occasionally have a life of their own, dispersing independently of the ants and exchanging genes with other ant-cultivated fungi."

Mikheyev, Mueller and Abbot also compared leaf-cutter ants and their fungal crops from Cuba to populations of ants and fungi from Central and South America. Though the Cuban ants have been isolated from mainland populations for many years, genetic analysis showed their fungal crops have been exchanging genes with mainland fungi populations.

"The fungi are able to cross geographical boundaries too great for the ants, intermingling genes between mainland and Cuban populations, possibly through airborne spores," said Mikheyev.

Ulrich Mueller | EurekAlert!
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