May 12, 2007

Intertwined lives: symbiosis

Some actinomycetes are known for establishing symbioses with other organisms. A typical example is the formation of nodules on the roots of certain plants by soil bacteria of the genus Frankia. Although these microbes can also be found as free forms in the soil, the nodule constitutes a comfortable home for Frankia, with abundance of carbon sources. Additionally, it's an adequate environment for an activity that greatly benefits the plant: nitrogen fixation. This is a process by which atmospheric nitrogen is converted into ammonia, nitrate and other compounds. Hence, the actinomycete fixes nitrogen and fertilizes its host plant. Recently, the genomes of three Frankia strains have been sequenced, which will help to understand how different strains are able to select and colonize certain plant hosts but not others.
(Image: Nodule from Alnus incana subsp. rugosa, about 1.5 cm diameter; D. R. Benson)

In other actinomycetal symbioses, the second partner is an insect, for instance a beewolf. Beewolves are not wolves, but a type of wasps that hunt honeybees to feed their larvae. After digging a nest in sandy soil, the female beewolf deposits an egg together with one or several paralyzed bees. But the underground nest is humid and warm, and the wasp larva may easily get infested by pathogenic microorganisms. As an strategy to diminish larva infestation, beewolves cultivate and use their own antibiotic-producing actinomycetes. Antennae of female beewolves have specialized glands housing symbiotic Streptomyces bacteria. The wasp applies a secretion from these glands all over the nest before leaving its egg. Later, the larva takes the bacteria and applies them to its cocoon, resulting in lower risk of fungal infestation. Sequencing DNA from both symbiotic partners is beginning to yield interesting results.
(Image: Philanthus triangulum, a European beewolf)

But the story can get more complicated. Imagine a symbiosis with four co-evolving partners: three of them are engaged in a mutualistic relationship, while the fourth one is a parasite. That's the beautiful case of fungus-growing ants. In their underground nests, the ants grow a mushroom-like fungus by feeding it with plant materials or other organic matter. In turn, the fungus serves as food for the ants (yes, this is agriculture!). But every garden has its pests, and the ants' farm is home for the Escovopsis mold. Escovopsis is a specialized pest, found only on the crop of farming ants. To battle the parasite, the ants combine special behaviors and microbial symbionts. These insects carry a bunch of antibiotic-producing actinomycetes in elaborate cuticular crypts, supported by unique exocrine glands. The symbiotic bacteria produce substances that specifically inhibit Escovopsis growth. Although initially identified as Streptomyces, the actinomycete symbionts appear to belong to the Pseudonocardia genus. The case of the fungus-growing ants has become a textbook example for teaching evolution and symbiosis (educational materials are available from the University of Nebraska State Museum or from the PBS Evolution project)
by Grey Wulf: leaf-cutter ants [a type of fungus-growing ants])

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