aphid/bacteria coevolution (was: speciation)

R. Joel Duff (Virkotto@intrnet.net)
Sat, 13 Dec 1997 08:45:34 -0600 (CST)

Hi all,

I had this example in mind yesterday but didn't have the papers. This
isn't OBSERVED speciation but I think is a good example of inferred
speciation. Aphids are truly combination organisms. They exist as a
combination of aphid and host bacterium of which neither can live
independently. Aphids have a diet that is lacks particular amino acids
which the bacteria provides. In fact the bacteria have recently been
found to contain plasmids which are nothing but a circle of repeated copies
of genes involved in the tryptophan bioshythetic pathway. The bacteria
lives in specialized cells that are found in very specialized organs in the
aphid. As many as 5 million cells of Buchnera (the bacteria) may live in a
single aphid.

Recently (Baumann et al. 1997) a molecular phylogeny of 11 GENERA of aphids
and their endosymbiots was constructed from small subunit (SSU) rDNA
sequences from the aphid (18S) and the bacteria (16S). The result was
perfect congruence between the estimated phylogeny of both organisms. This
suggests that the bacteria was "a single ancient infection of a common
ancestor of all aphids and has since been transmitted vertically through
the variaous lineages of aphids as they diversified."

To me the indentical phylogenies are independent results that suggest
decent from common ancestor of the eleven GENERA and all the bacterial
lineages. If the aphids were created separately I can accept that there
could be an appearance of relationship among them based on gene sequence
similarities but here we have another organism intimately associated that
just happens to have gene sequence differences that suggest the identical
relationships. If we suggest that all species and genera of aphids came
from a single stock them we have witnessed speciation even at the generic
level since creation.

I would also point out that the bacteria in each aphid, though related,
show dramatic difference even between those in two species of aphids. The
number of copies of trpE and trpG genes is different in each species looked
at. Some have plasmids with multiple copies of these genes while in other
bacteria these genes are integrated into the chomosome. Other large
insertions and deletions of the bacterial genome are identified and
bacteria in some aphids have become specialized to produce a particular
amino acid while another bacteria in another species of aphid has become a
"factory" to produce another. These changes may be due to selection
pressures imposed by the aphid or more stochastic factors or a combination.
Studies of these bacteria have lead to real examples ofthe evolution of
use and disuse of various biosnthetic pathways.

The whole relationship between these aphids and bacteria is fascinating and
is by no means unique in nature as many if not most insects have symbiotic

Joel D.

Rouhbakhsh, D., Clark, M. A., Baumann, L., Moran, N. A., and P. Baumann.
1997. Evolution of the Tryptophan biosynthetic pathway in Buchnera (Aphid
endosymbionts): Studies of plasimid-associatied trpEG withing the genus
Uroleucon. Molecular Phylogenetics and Evolution 8: 167-176

Moran, N., and P. Baumann. 1994. Phylogenetics of cytoplasmically
inherited microorganisms of arthropods. Trends in Evolution and Ecology
Jan. 15-20.

Baumann, D. N. A. Moran, and L. Baumann. 1997. The Evolution and Genetics
of Aphid Endosymbionts. Bioscience 47: 12-20

Brul S., and C. K. Stumm. 1994. Symbionts and organelles in araerobic
protozoa and fungi. Trends in Evolution and Ecology pg. 319-322

Baumann, P., Baumann, L., Laui, C.-Y., Rouhbakhsh, D., Moran, N. A., and M.
A. Clark. 1995. Genetics, physiology, and evolutionary relationships of
the genus Buchnera: Intracellular symbionts of aphids. Annual Review of
Microbiology 49: 55-94.

Joel and Dawn Duff / | ' \ Spell Check?
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