Sunday, April 22, 2012

Farming beetles: Ambrosia beetles and their fungus

Different ambrosia beetles from tropical rainforests.
As we have seen with the Attine ant and Leucoprini fungus, animals have beaten humans to the act of farming. Another example of this would be the ambrosia beetles and ambrosia fungi. Ambrosia beetle is a general term used to describe several species of beetles from the two weevil subfamilies Scolytinae and Platypodinae [1]. One of the most rapidly diversifying, most widely distributed, and probably most ecologically and economically important group of ambrosia beetles is the tribe Xyleborini [1]. Ambrosia beetles can be found in all regions with tropical vegetation and even in some temperate areas [1].

Beautiful pink Fusarium repeatedly isolated from mycangia of Xylosandrus crassiusculus.

Since there are several species of ambrosia beetles, it comes with no surprise that there are many species of fungi that have a long established relationship with these beetles. The most common are polymorphic asexual anamorphs from the general Ambrosiella, Raffaelea, Ambrosiozyma and Dryadomyces [1]. Most ambrosia fungi are not capable of living independently of the beetles [1].

Description of the relationship:
The relationship between the beetles and the fungus has been describes as one of the most successful and widespread cases of insect-fungus symbiosis, ambrosia beetles are the product of sixty million years of intricate mutualism between the faster evolving beetles and some of the most bizarre fungi [2].

Surface pit mycangia. Left: Female of Treptolatypus solidus with glandular pit mycangia on top of the pronotum. Right: Simple nonglandular pit mycangium on the head of Scolytodes unipunctatus. Arrow on the right points to a fungus spore.
Ambrosia beetles have evolved a variety of morphological structures to carry their fungal symbionts from tree to tree. These structures are generally called mycangia [1]. As the beetles leave their initial home, they carry spores of the fungus in their mycangia. Once they find a suitable tree, they then create tunnels (galleries)  inside the wood which will make small strings of compacted sawdust protrude from the tree [3].

Sawdust stacks on a dead tree - typical sign of ambrosia beetles at work.
The beetles will then cultivate the fungi inside the galleries within dead or decaying wood and feed on their mycelium. Once the "fungus farming" has started, the beetles can lay their eggs that will produce larva. The beetles will care for their young larva in the trees where they will become beetles and carry fungal spores to their next habitat.

Cost/benefit analysis:
     The beetles are dependent upon the fungi, from which they require amino acids, vitamins, and sterols [5]. Certain immature stages of the beetles are unable to complete development in the absence of the fungus [6], At the same time, the activities of female beetles have been hypothesized to control the growth of composition of ambrosial gardens. If the female dies, the garden is quickly overgrown by contaminating fungi and bacteria, which ultimately results in the death of the brood [5].
     The interaction is clearly mutualistic. The symbiosis allows the beetles to exploit a nutritionally poor resource (wood) and reduce interspecific competition, while providing the fungi consistent transport to a relatively rare and ephemeral resource (a new host of the appropriate condition and successional stage) [5].
     Although both the beetle and the fungi are benefited by this relationship, there are also cost to pay. The beetle could die if the fungus is suddenly removed from its habitat as the beetles are too specialized to be able to use another species of fungus. The fungus has also evolved to where it does not reproduce sexually, thus reducing genetic diversity and ability to explore different niches without the help of the beetle.


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