Symbiotic relationships

Aargh! I was almost finished with a post responding to your last question (evidence of termite/microbe coevolution), and now you've gone and asked a MUCH broader one!Just very quickly: the word "symbiosis" in scientific circles can refer to different sorts of relationships, including parasitism, commensualism, amensualism, and mutualism. As I said before, mutualism (where both species benefit from the interaction) is what we're looking at with termites and their gut microbes, in particular obligate mutualism, in which termites and their intestinal microbes need each other to survive.I'll finish my post on the research into termite/microorganism coevolution. If you just wanted evidence of coevolution of obligate mutualism in general, I could have talked much more knowledgeably about bark beetles and ambrosia beetles with their fungal symbiotes, and saved myself the literature search! I can still go into that later, if you like.DinoBoy93 asked:

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Could you show me evidence that co-evolution is how [termite/microbe obligate mutualism] got this way?

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The short answer to this question is, "yes." However, a complete description of the evidence would be beyond the scope of what could be covered in a forum post. The coevolution of termites and their gut microorganisms has been the subject of a huge amount of research. I will attempt to explain how it has been studied, and bring up some key lines of evidence, especially ones with sources accessible on the web.As I said in my previous post (and as is very well covered in MrDarwin's blog article), evidence from fossils and molecular (DNA) analyses indicate that termites share a common ancestry with cockroaches and mantids, and the three groups are now placed by most entomologists within the superorder Dictyoptera. Existing dictyopterans apparently developed from a group of insects (well-represented in the fossil record) that have been termed "roachoids," which strongly resembled modern cockroaches, except that they had long ovipositors. The close relatedness between modern roaches and termites is well-illustrated by termite-like roaches (wood roaches, Cryptocercus spp.) and roach-like termites (Mastotermes darwiniensis) that still exist today (see also Lo et al., 2000 for an outline of molecular evidence for this relationship).Another important thing to keep in mind is that the termite group itself (order Isoptera) is very diverse, with several distinct lineages showing different feeding behaviors and interactions with different groups of symbionts. These are separated into the "higher" termites (the more recently evolved and highly variable family Termitidae) and the "lower" termites (Figure 1). Many of the higher termite species don't feed exclusively on wood (such as the subfamily Macrotermitinae, which has species that cultivate and feed on fungi), and these lack the eukayrotic flagellate microorganisms that help lower termites digest cellulose. However, like all termites, their guts are host to a wide variety of other symbiotic microorganisms. New microbial symbionts have been picked up and old ones lost by different termite lineages in the course of evolutionary history (Ohkuma et al., 2001).Figure 1. Phylogeny of Isoptera, with feeding habits of different families. Termite species characterized by their gut symbionts are indicated. Taken from Ohkuma et al., 2001. A primary way in which coevolution between mutualists can be investigated is by looking at DNA for evidence of "cocladogenesis." What this means is that if the host and symbiont have been evolving with each other for a long time, then the genetic relatedness of the host termite species should correlate with the relatedness of the microbial symbionts; i.e. the more closely related two termite species are, the more closely related their gut microbe species should be, assuming that different termite species don't directly transfer the symbionts to each other. Many molecular studies have been carried out studying cocladogenesis between lineages of termites (and other dictyopteran lineages) and various symbiotic gut microorganisms, and they have all supported the conclusion of coevolution.Here's an example. All cockroaches have symbiotic bacterium in the genus Blattabacterium in their guts, hosted within specialized cells in the gut lining and transmitted from mother to egg. These are not cellulose-digesting bacteria, but they help to recycle waste products and increase nutrient availability. The most primitive (and most genetically similar to roaches) of living termites, Mastotermes darwiniensis, also has specialized cells hosting this same bacterium genus, unlike other termites species (which have other gut microbes serving the same purpose). When DNA from different roach species and M. darwiniensis were analyzed along with their Blattabacterium species, the relatedness of the termites lined up very well with that of the bacterial symbionts (Figure 2). Furthermore, a "molecular clock" was identified in the Blattabacterium that allowed researchers to estimate when the different lineages diverged, resulting in predictions that lined up well with fossil evidence (Lo et al. 2003).Figure 2. Phylogeny of dictyopteran species and a comparison with the phylogeny of Blattabacterium spp. Taken from Lo et al. 2003. So, this sort of study is evidence that the gut microbes have been with these insects for a very long period of time, and that they have been evolving in parallel with their hosts. Similar studies have been done with termite symbionts specifically, and with similar results, but I don't want to get too bogged down in that because I'm not sure that it satisfies what you're really looking for: evidence that the mutualistic relationship could have evolved from a point where termites and their symbionts didn't need each other.I think that the most surprising evidence of this comes from the somewhat recent discovery that (contrary to long-held belief), lower termites DO independently produce a cellulose-digesting enzyme (cellulase) which is responsible for a great deal of their wood digestion. In fact, a recent study of M. darwiniensis and its symbiotic flagellate found something even more surprising: although these Archaezoan flagellates do possess genes for producing cellulase, apparently they don't use them in the termite gut. Rather, they take in the termite's own cellulose-digesting enzymes along with the wood particles that the termite ingests, and use these termite-produced enzymes in the digestion process. The flagellates' own cellulase genes are apparently unused in the process, and the researchers speculate that the symbionts may eventually lose these genes altogether (Li et al., 2003). This cellulase-producing ability also suggests that the xylophagous (wood-eating) roach ancestor may have fed entirely without the aid of cellulose-digesting microorganisms, as do some of the xylophagous "higher" termites today. Literature Cited

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Li, L., J. Froehlich, P. Pfeiffer, and H. Koenig. 2003. Termite gut symbiotic Archaezoa are becoming living metabolic fossils. Eukaryot Cell 2(5): 1091-1098. Link.

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Lo, N., G. Tokuda, H. Rose, M. Slayter, K. Maekawa, C. Bandi, and H. Noda. 2000. Evidence from multiple gene sequences indicates that termites evolved from wood-feeding cockroaches. Current Biology 10(13):801-804. Link (.pdf).

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Lo, N., C. Bandi, H. Watanabe, C. Nalepa, and T. Beninati. 2003. Evidence for cocladogenesis between diverse dictyopteran lineages and their intracellular endosymbionts. Mol. Biol. Evol. 20(6):907-913. Link.

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Ohkuma, M., S. Noda, Y. Hongoh, and T. Kudo. 2001. Coevolution of symbiotic systems of termites and their gut microorganisms. Riken Review 41:73-74. Link (.pdf).Postscripts:As an aside, I would add that termites and roaches are not unusual in hosting symbiotic microorganisms in their guts. Indeed, the Blattabacterium I wrote about earlier is in the same order (Bacteroidetes) as bacteria that are found in many other insects - and in humans, too! Bacteroidetes in our guts help us get more nutrients from our food than we would otherwise. Alternatively, they can also be disease-causing pathogens under certain circumstances.You may be wondering: why doesn't Kent Hovind find this sort of evidence convincing? It's likely that he doesn't know about it. In spite of how he presents himself, "Dr." Kent Hovind is not a scientist; he got his PhD in Christian Education from a religious mail-order university.