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Monday, August 23, 2004 Peering Inside Nature's Nesting Doll Termites, the bane of homeowners everywhere, cause a couple billion dollars of damage in the U.S. alone. A colony of 60,000 can devour a foot of a 2-by-4 in five months. But we would have never known what termites were eating were it not for Joseph Leidy. Leidy was a naturalist of the nineteenth century--a passionate observationist and "the founder of American vertebrate paleontology, parasitology, and protozoology"--yet he has fallen into obscurity. His curiosity drove him to find out the diet of the "white ants" living in a rotting log in New Jersey. These "white ants" were the eastern subterranean termites, Reticulitermes flavipes. Under the microscope, the termite hindguts revealed a teeming soup of partially digested rotten wood and microbes that streamed out "like citizens leaving a crowded meeting hall." Joseph Leidy's paper in 1891 was the first to describe the termite endosymbionts. Three of the most common endosymbionts are archaeoprotists: Trychonympha agilis, Pyrsonympha verteus, and Dinenympha gracilis. And they all exhibit a dizzying array of strange forms. Although Leidy deduced correctly that these protists were entirely dependent on the termite for providing a safe niche, he thought that these microbes were unnecessary for the termite. Today, we know that the protists are essential for termite survival by producing enzymes to digest wood cellulose. Without their gut flora, the termites would starve to death. But it doesn't stop there. The protists themselves are literally beautiful examples of symbiosis in motion. Take for instance the protist Mixotricha paradoxa, first identified by J.L. Sutherland in 1933, which lives in the hindgut of the Australian termite Mastotermes darwiniensis. One M. paradoxa is an amazing half a millimeter in length (a giant compared to other microbes in its family) and has four small andulipodia which act as rudders to change direction. Swimming is another matter. In the 1950s, A.V. Grimstone and L.R. Cleveland examined the protist via electron microscope and discovered at least four different kinds of bacteria (epibionts) living in or on the protist which made some people start calling it the beast with five genomes. Three of the bacteria live on M. paradoxa's surface: a large spirochete which might be a parasite, a small hair-like spirochete approximately 250,000 to 500,000 per cell which act as the protist's propulsion system via synchronized swimming, and a rod-shaped bacterium that anchors onto the host membrane. The fourth member of the quintet is a round bacterium living inside M. paradoxa that acts as the energy factory--just like mitochondria. And on top of that, M. paradoxa breaks down the cellulose ingested by the termite! Another unusual epibiont was recently described by Stingl et al. It is also a rod-shaped bacterium, but it is only distantly related to the one found on M. paradoxa. This bacteria, from the lineage Bacteroidales, is found on the protists of the Staurojoenina species which live inside the dry wood termite Neotermes cubanus. Under a scanning electron micrograph, Staurojoenina looks like a strawberry with a bad toupee. If you look even closer, the rod-shaped bacteria come into view with occasional spirochetes attached like a couple fobs hanging off a sequined dress. But these bacteria aren't getting a free ride on a gentle elephant. Staurojoenina was also observed to phagocytose many of the bacteria on the surface. It's obvious that these epibionts aren't used for locomotion such as M. paradoxa, but they could very well interact with the protist host to help digest cellulose by producing fiber-degrading enzymes. Or, since Staurojoenina has been observed to be "eating" the bacteria, they may just as well be a convenient snack for the protist. But how on earth did these unusual partnerships form? We're still not sure although researchers are slowly moving toward the answer to a different question. Wier et al. examined the ancient Miocene termites Mastotermes electrodominicus from the Dominican Republic which were preserved in amber approximately 20 million years ago. M. electrodominicus is not only an ideal fossil to study because it is related to the above mentioned M. darwiniensis, but these early termites also contain M. paradoxa and similar protists that may help us understand how the modern eukaryote came about. The guts of the fossilized termites were extracted and examined by electron microscopy. By comparing the morphology of the ancient protists and their epibionts to their modern counterparts, the researchers hypothesize that the first eukaryotes depended on their epibionts for the evolution of their nuclei. [posted by S. Y. Affolee on 6:17 PM : ]
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