Microorganisms subsist in the guts of some insects and help with the breakdown of cellulose and lignin and with nitrogen fixation. These particular insects and microorganisms developed a need for each other somewhere along evolutionary lineages, and now they actually benefit from each other in symbiotic relationships. Termites are one example of insects with protists, fungi, and bacteria living in their hindgut. Because termites diverged from cockroaches about 250 million years ago, exploring the similarities and differences of these two insects helps shed light on the evolution of microbial communities in their hindguts and the state of symbiosis in which they exist.
First, it is necessary to understand the digestion of these insects. Termites and cockroaches contribute to the degradation of wood because they have the ability to digest cellulose, a complex carbohydrate that makes up the cell walls of plants and is therefore the major component of wood. The diet of termites is usually high in cellulose because their food is mainly woody material. Cellulose is the most abundant form of carbon on Earth, excluding fossils (Martin, Jones, and Bernays 1991). Termites digest cellulose more efficiently than any other animal, and most animals are not able to digest this bountiful resource. Martin, Jones, and Bernays propose four ways that cellulose is digested in termites.
The first proposal states that bacteria in the hindgut are solely responsible for breaking down the cellulose that termites ingest. Higher family termites seem to rely more heavily on bacteria than protozoa in the breakdown of cellulose (Martin, Jones, and Bernays 1991). This proposal, however, lacks strong evidence because bacteria usually work with other microorganisms, such as other bacteria or protists, in this process. Bacteria do not usually possess the mechanisms to break down cellulose on their own.
The second proposal states that fungi living in wood help digest cellulose (Martin, Jones, and Bernays 1991). However, the Macrotermitinae is the only Termitidae subfamily that has developed a mutualistic relationship with fungi (Aanen et al 2002). These types of termites are referred to as fungal-growing termites because they help the fungi grow in their nest by providing them with nutrients (Martin, Jones, and Bernays 1991). In this relationship, the fungi genus Termitomyces helps termites degrade wood before the wood is ingested by the termite (Aanen et al 2002).
The third proposal states that the termite midgut epithelial cells secrete cellulolytic enzymes, such as cellulase, to digest cellulose (Martin, Jones, and Bernays 1991). This is seen is higher termites such as Trinervitermes trinervoides and Nasutitermes walkeri, as well as in the wood-eating cockroach Panesthia cribata. In these species, protozoa do not exist in the hindgut (Martin, Jones, and Bernays 1991).
The fourth proposal states that lower termites such as Reticulitermes flavipes use protozoan symbionts, or helpers, to break down the cellulose. Protists in the hindgut are aided by other microorganisms. To understand the evolution of this digestive process, this proposal must be examined in detail.
Termites have both prokaryotic cells, or bacteria, and eukaryotic cells, such as protists, living in their hindguts. These microorganisms can survive extreme environments, including anoxic conditions. The protozoa flagellates in the hindgut have the unique ability to degrade cellulose and produce acetate as a source of carbon for the termite to absorb and use for energy (Ohkuma 2001). The cellulolytic protozoa hydrolyze, or add water to, cellulose, which breaks down this complex carbohydrate into individual glucose molecules. Each molecule of glucose ferments into two acetate compounds, two carbon dioxide molecules, and four hydrogen molecules (Brauman et al 1992). One experiment showed that R. flavipes had high concentrations of hydrogen in their guts, which proves that a large number of protozoa live in the gut of these termites (Ebert and Bruce 1997).
Bacteria referred to as ectobionts, or epibionts, also live in or on these protozoa (Ohkuma 2001). Some of these bacteria are responsible for converting the hydrogen and carbon dioxide products from the fermentation process into another acetate compound. The three acetate compounds made per glucose molecule are oxidized by the termite to produce carbon dioxide and water (Brauman et al 1992). The water aides in the respiratory system of the termite, and the carbon dioxide is used and reduced by methanogens (Brauman et al 1992).
Methanogens are bacteria that are members of the domain Archaea, the oldest cells on earth. Methanogens live in anaerobic conditions and are killed by the presence of oxygen (Ohkuma 2001). The termite hindgut creates a perfect anaerobic environment for methanogens, as these organisms either live free in the guts of termites or inside the protozoa that co-exist there (Ohkuma 2001). The genus Methanobreuibacter is an example of a methanogen that thrives in the hindgut of a termite (Ohkuma 2001). These methanogens use the extra hydrogen molecules that are produced in the hindgut by the protozoa to reduce the carbon dioxide (Ohkuma 2001).
It has been shown that termites are more attracted to areas in the soil with a constant supply of carbon dioxide (Bernklau et al 2005).This may actually be due to the methanogens in the termite gut that need a constant supply of carbon dioxide as the carbon source to make methane.
The relationship between acetogenesis, the process of making acetate, and methanogenesis, the process of making methane, could potentially be important in understanding the evolution of the termite hindgut. This relationship may help researchers explain the divergence of different types of termites and determine which types of cells were the first to inhabit the termite gut. It may also help researchers determine which process the termites prefer in different environmental situations. Brauman et al. devised an experiment to determine whether acetogenesis or methanogenesis is the major hydrogen-consuming reaction of the termite hindgut. After testing the rates of both processes in 14 wood-consuming termites, the researchers concluded that the rate of acetogenesis was approximately three times the rate of methanogenesis (Brauman et al 1992). However, methanogenesis seems to dominate acetogenesis among fungus growing and soil feeding termites or in most anoxic habitats (Brauman et al 1992).
This research shows how the microorganisms and the termite demonstrate co-evolution and may shed light on the history of divergence. As termites adapted to new environments, the microorganisms were also able to adapt quickly to the new surroundings. Research has proved that other insects and bacterium do have the ability to adapt to one another at high rates (Degnan et al 2004). This seems to be the case for termites as well.
In addition to digesting cellulose and producing acetate and methane, the termite also utilizes nitrogen from its diet to enrich soil in their environment (Breznak and Potrikus 1981). The termite diet usually includes low levels of nitrogen, so conserving nitrogen is important. The termite hindgut is responsible for cycling this nitrogen back into the ecosystem, which is known as nitrogen fixation (Ohkuma 2001).
One waste product from the nitrogen in the termite diet is the chemical uric acid. R. flavipes has the ability to recycle this nitrogenous waste product for use in its body cells (Breznak and Potrikus 1981). Experimentally, researchers fed termites antibiotics to kill the bacteria in the gut and then monitored the levels of uric acid. The uric acid levels in the gut increased because the bacteria were absent. This led them to conclude that certain types of bacteria in the hindgut helped break down the uric acid and return it to the fat tissue of the termite (Breznak and Potrikus 1981).
These different processes occur simultaneously in the termite hindgut and all require different microorganisms. It has been shown that termites depend on this symbiotic relationship for survival. Eutick et al. demonstrated that lower termites like R. flavipes cannot survive very long if protozoa spirochetes are eliminated from their guts (1978). These termites can only secrete cellulase, the enzyme that breaks down cellulose, when in contact with protozoa (Cleveland 1994). If the termite lineage is traced back far enough, the origin of this relationship can be understood. Since termites and cockroaches are thought to have shared a common ancestor toward the end of the Paleozoic period about 250 million years ago, it is important to look at how they developed this need for microorganisms and what characteristics termites and cockroaches share (Buchner 1965).
Some cockroaches have cellulose-digesting protozoa in their gut, but these protozoa are not necessary for survival because of the cockroaches’ lower cellulose intake and the ability to secrete cellulolytic enzymes from their gut. On the other hand, lower termites cannot survive without these protozoa (Martin, Jones, and Bernays 1991). The termite Mastotermes darwiniensis is considered to be the most primitive termite and is supposedly the transitional form between cockroaches and the lower termites (Martin, Jones, and Bernays 1991).
One particular experiment compared different bacteria in cockroaches and M. darwiniensis to the show similarities between the two insects (Bandi et al 1995). M. darwiniensis is the only termite that has endosymbiotic bacteria in fat body cells just like cockroaches (Bandi et al 1995). After looking at the bacterial ribosomal RNA subunits, the researchers found that the endosymbiotic bacteria in M. darwiniensis were similar to seven species of cockroaches (Bandi et al 1995). This research suggests cockroaches and termites shared a common ancestor, and M. darwiniensis was the link between the two. Other families of termites lost this specific relationship and gained new relationships with symbionts.
M. darwiniensis shares not only biochemical characteristics with cockroaches, but physiological and morphological characteristics as well. M. darwiniensis is the only termite that oviposits an egg case just like cockroaches (Wier et al 2002). As all other termites evolved, this ability must have been lost or was no longer favored.
Also, M. darwiniensis, like cockroaches, attacks food other than wood, including sugar cane and vegetables (Martin, Jones, and Bernays 1991). Research shows that when M. darwiniensis workers are fed starch instead of cellulose, the protists in the hindgut are eliminated (Veivers et al 1983). This suggests that the protozoa symbionts in the hindgut of the termites have developed a need for cellulose and die when there is no cellulose available. M. darwiniensis, unlike most other species of termites, can live without the cellulolytic protozoa.
The types of microbial species in the hindgut of different termites and cockroaches are also important in understanding the evolution of this symbiotic relationship. For example, researchers isolated two different forms of spirochetes, types of protozoa, from the guts of the termite Zootermopsis angusticollis (Leadbetter et al 1999). The 16S ribosomal DNA sequences in these specific strains of spirochetes were 98% compatible (Leadbetter et al 1999). These spirochetes were also found to be similar to spirochetes of the genus Treponema (Leadbetter et al 1999). Treponema spirochetes are known to be common in termite hindguts (Ohkuma 2001). This shows that different species of termites have many of the same microorganisms.
Other research has shown that similar protists exist in termites. M. darwiniensis, the most primitive termite still in existence, has been preserved in amber from the Miocene period (Wier et al 2002). This preservation allowed researchers to observe a 20 million-year-old microbial community. Fossils of the extinct M. electrodominicus from the Dominican Republic were examined and compared to M. darwiniensis fossils from Australia from the same period. The microbes in the intestines of each species were very similar (Wier et al 2002). These findings indicate very little evolutionary change in 20 million years. Also, the protozoa from these termites were covered with bacteria (Wier et al 2002). These findings are consistent with other findings concluding that types of bacteria actually attach to protists by embedding themselves into the outer membrane of the protists (Ohkuma 2001).
These similarities may suggest that protists and bacteria infected termite ancestors. As termites evolved and diverged from each other, the organisms modified and changed to suit the environment. As higher termites evolved from lower termites, the higher termites became less reliable on these symbionts and more reliable on their own body cells.
The protists in termite guts have also been shown to exhibit some traits of primitive eukaryotes (Ohkuma 2001). The members of these protists may be direct descendents of the earliest cells with nuclei (Wier et al 2002). Since the nucleus is thought to have evolved from bacteria living on and within protozoa, the termite gut may be a perfect tool for understanding the evolution of eukaryotic cells.
Several theories have attempted to explain the development and evolution of symbiosis between insects and microorganisms. One theory suggests that this association allowed both the host insects and microorganisms to fill new niches in their particular environments (Margulis and Foster 1991). According to Margulis and Foster, the divergence of termites from cockroaches was extremely advantageous because it allowed the termites to consume large amounts of cellulose. Since no other insects had that ability, this niche in the ecosystem had not yet been filled. After this divergence, competition between termites may have increased the rate of this symbiotic evolution (Allen 2003). The termites that could consume the most cellulose would be better nourished, and therefore would have an advantage in the battles.
These theories are consistent with the research that has been discussed. Termites and cockroaches share a common ancestor. At some point in history, this common ancestor was infected with microorganisms, which became advantageous for both the host and the microorganisms. The first sign of divergence was the presence of the termite M. darwiniensis, which has many characteristics of modern day cockroaches. As the evolution of termites proceeded, the microorganisms adapted with the termites. Both the microorganisms and the termites have become so dependent on each other that this symbiotic state is absolutely necessary for their survival.
Studying the relationship between microorganisms and termites is valuable for several reasons. First, this relationship helps illustrate the anatomy and physiology of the individual organisms involved. The fields of microbiology and entomology can both benefit from research in this area. Also, scientists may be better able to understand the evolution from prokaryotic cells to eukaryotic cells and also from lower, more primitive insects to modern day insects. Finally, and probably of great interest to nonscientists, research in this area may allow for more efficient prevention and control of termite infestation, a major pest problem in the United States. The more researchers understand about the co-existence of microorganisms and termites, the more they are potentially able to help the general public guard against these insects in homes, worksites, and other areas needing protection.
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