Wednesday, 18 September 2013

Selfish Genes and Termite Indigestion

Creationist pseudo-scientists will assure their credulous customers that genetic evolution alone can only lead to selfish organisms. Maybe this mistake comes from assuming their own greed and selfishness is a result of their own genetic evolution. It isn't of course, it results from a sociopathic personality disorder.

But one only need to look at nature to see that cooperation is actually the norm - so common in fact that we either take it for granted or it's operating at a level which is too small for us to see easily. Just one example is the termite - if one can accurately even speak of them in the singular. Termites only exist as part of a cooperative colony.

But it's not the obvious cooperation in the termite colony that I'm talking about here.

Termites are a very old order of insects which branched off the group which gave rise to cockroaches about 150 million years ago so have been evolving into their specialised niche for a very long time. They are not at all closely related to the other social insects like the many hymenopterans like ants, wasps and bees. They live exclusively on decaying wood which presents them with very special digestive problems because decaying wood is almost completely cellulose and lignin (with fungal hyphae) and cellulose is notoriously stable and hard to break down. It is the main structural substance for plants and the last thing plants, especially long-lived ones like trees, need is for their structural material to break down.

In fact, very few animals can digest wood. Those which eat lots of plant matter have a specialised digestive system which normally contains a sizeable fermentation vat - which is one reason that herbivore mammals tend to be comparatively large. Termites are no different in this respect but have evolved a gut which achieves the same thing on a very small scale.

Termites depend entirely on cooperative symbiotic organisms living in their gut - and of course these organisms depend entirely on termites.

Interaction between the microbes in the termite gut is highly mutual, usually beneficial for both microbes.
Prokaryotes are closely associated with protists as symbionts, either attached to the cell surfaces or live within the cytoplasm or nucleus of the protists. For instance, Treponema spirochete bacteria are attached to the special bracket-like structures on the plasma membrane of mixotricha and contributes to the movement of the host protist known as “motility symbiosis”. Treponema also benefits by living on and within the protist, easily accessible to nutrients H₂ and CO₂ produced by mixotricha and utilize them to synthesize acetate and obtain energy for their own growth as well.

Another mutual relationship shown between Methanobrevibacter and parabasalids protist, H₂ plus CO₂ produced by protists also can be used by methanogens as energy source but they form methane,CH₄ in this case. Successful elimination of produced H₂ by endosymbiont’s H₂ evolution activity enables the protists to maintain optimal pH and stimulate its decomposition activity. These two groups of microorganism interact and work together to digest cellulose and enhance the cellulose fermentation.

Although most of the microbes act mutually, there is one exception between the relationship of methanogens and acetogens. Both take up H₂ and CO₂ as their substrates, thus they are likely to be in a compete relationship. Acetogenesis dominates methanogenesis from the same substrate, H₂ plus CO₂, because acetogenesis requires less energy loss of the termite by absorbing acetates but not methane as the energy source.

The amazing thing is that not only do termites depend on microbes in their gut but that some of these microbes depend on symbiotic bacteria and archaea which live inside them. One protista, mixotricha, even has special brackets on its surface which the the flagellated spirochete, treponema fits into so they can move it about. It didn't need to evolve motility because it lived in close association with a spirochete which provided it for them, and it was in the spirochete's interest to cooperate.

This system evolved by natural selection, by genes appearing to mimic selfishness because the ones which produced the most descendents was the one which predominated in the genepool. There was no choice in the matter - whatever produced the most descendants won the race.

And by that process termites evolved together with their gut flora to become one of the most successful insect orders on the planet, and by utilising a food source which few other creatures could use.

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