Coevolution in the termite–protist symbiosis
Gillian H. Gile
T ermites are best known as pests thanks to the damage they inflict on wooden structures, though many species have important ecological roles, especially in the tropics. Most wood-feeding termites, including the destructive pest species, rely on an assemblage of eukaryotic microbes (protists or flagellates) in their hindguts to help digest wood. The termites grind wood into tiny particles using their specialised hardened mandibles. These particles are then engulfed by the protist symbionts and digested, releasing carbohydrates for energy. Termite-symbiotic protists are the only microbes known to engulf and digest wood particles in this way; it is far more common for microbes to digest wood by secreting enzymes out of their cells and then importing the digestion products (as bacteria and fungi do). This nutritional partnership between termites and their protist symbionts is mutually obligate: the protists cannot survive outside their hosts, and the termites will starve to death despite continued feeding if cleared of their protists.
The obligate symbiosis between termites and protists has been remarkably stable over time: it was already well established in the common ancestor of termites and their closest insect relative, the wood-feeding cockroach Cryptocercus , more than 150 million years ago. Since that time, the protists have been faithfully passed on from generation to generation of hosts by a process called proctodeal trophallaxis (anal feeding), allowing them to evolve and diversify in parallel with their hosts. Today there are roughly 800 wood-feeding termite species, each of which harbours its own unique community of symbiotic protists. These protists tend to be much larger and more complex in cellular structure than related protists that live in anaerobic sediments or vertebrate guts, making them the charismatic megafauna of the microbial world (see Figure 1). The fact that termite guts are packed with dense masses of writhing protists, most of them very large, filled with wood particles and covered with flagella, was first noticed and
Figure 1. Termite hindgut protists: the charismatic megafauna of the microbial world. Note the angular birefringent (shiny) particles of ingested wood in (d)–(f). (a) Holomastigotes sp. (b) Eucomonympha sp. (c) Pseudotrichonympha leei . (d) Holomastigotoides sp. (e) Teranympha mirabilis . (f) Trichonympha sp. All scale bars indicate 20 µm. Gillian Gile
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92 Microbiology Today October 2022 | microbiologysociety.org
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