lpetrich

Aphids have a very interesting diet: plant sap. However, there is a certain problem: its nutritional value. Aphids, like all of the rest of the animal kingdom that has been tested for nutritional needs, are unable to make certain cofactors ("vitamins") and amino acids ("essential amino acids"). However, plant sap is very lacking in these, with mostly carbohydrates dissolved in it -- making it a sort of junk food.

However, aphids have an interesting adaptation: symbiotic bacteria live inside of them in, inside of special cells, and make the EAA's and vitamins for their hosts. This relationship has started about 200-250 million years ago, and the bacteria and their hosts have branched in parallel.

Recently, the genome of the symbiotic bacterium (Buchnera) of one species of aphid (the pea aphid Acyrthosiphon pisum (Harris)). was sequenced (Nature 407, 81 - 86 (2000)). Here are some results:

Buchnera is closely related to the enteric bacteria, the likes of Escherichia and Salmonella, but its genome has become much reduced, to 641 thousand base pairs, the second-smallest bacterial genome known (the world's record, Mycoplasma genitalium, is at 580 Kbp).

It biosynthesis capabilities are complementary to its host's, being able to make the "essential" amino acids, but not the non-essential ones; it also makes some vitamins, like pantothenate.

It has much-simplified gene regulation, it is very short on cell-surface proteins and membrane-transport proteins, and it is non-motile and without chemotaxis genes (it can't move, and it would not know where to move to) -- unlike free-living bacteria like Escherichia coli. All this reduction is very typical of internal symbiotes and parasites.

Aphid symbionts pose a serious difficulty for creationism. One favorite creationist theme is the excellence of various adaptations. But if aphids were created to live off of plant sap, then why were they not created with essentially complete biosynthetic abilities? Why do they have the same biosynthesis defects as the rest of the animal kindgom? Why do they need bacteria living inside of them to fill in the nutritional gaps of their junk-food diet?

Also, the family trees of aphids and their symbiont bacteria closely parallel each other. Were they created like that or was this a result of microevolution?

But an imaginative creationist may be able to think of a way out of these difficulties; one way is, of course, the Philip-Gosse "creation that looks like evolution" hypothesis.

However, evolutionary biology explains this necessity quite naturally. The biosynthesis pathways for producing the EAA's require several steps, and if they are gone, then it would be difficult to re-evolve them, because of a lack of intermediate molecules to work from. However, in the Primordial Soup, there would have been an abundance of intermediate molecules, which would have allowed EAA biosynthesis pathways to evolve in step-by-step fashion in some early microbe.

Furthermore, the animal-kingdom ancestors had lived at least a few hundred million years before land plants had come into existence, meaning that there was no way that they could have become adapted to eating land plants, with their abundance of cellulose and their sugary sap.

The ultimate origin of this animal-kindgom biosynthesis deficiency is likely to be a side effect of eating fresh organism flesh, with its nutritional completeness.

However, many decay organisms, like many bacteria and fungi, are much less choosy; this is because others may often beat each individual to the choiciest molecules, meaning that they have to subsist on whatever is available. And indeed we find that some such organisms can live off of any of a variety of organic-carbon sources.

Duvenoy

I found that very interesting. Many thanks for the food for thought.

d

Duvenoy

Hmm. I wonder if this is also true of cicadas, the big, noisey insect that you hear in summer. These, as a nymph, live for as long as 17 years, depending on species, underground feeding on the juices of tree roots. When they emerge, they only live long enough to breed. They can't feed as the adult has virtually no mouth parts. Adult may flies are pretty much the same way, although the larva are aquatic and not sap suckers.

The deity must have been either seriously hung over or off it's meds when it thought this one up.

Incidently, I have a standing bounty of twenty five cents for every live cicada brought to me. My Copperheads love them and my grand kids have gotten very good at finding them as they lie on the ground beginning to die.

What a strange and marvelous world we live in!

d