How Low Can You Go—Again?

by Dr. Georgia Purdom on November 8, 2006

Two articles make claims of having sequenced even smaller bacterial genomes.

In an article I wrote back in April, scientists claimed to be on the path to determining the minimal number of genes bacteria need to survive (not in the wild, of course, but in the nutrient-rich, stress-free, competitor-free environment of the lab). At that time, Mycoplasma genitalium held the record for the smallest genome at almost 600 kb with 480 genes. Of those genes, 382 are thought to be absolutely essential (as of last count!).

Two articles published recently in Science1,2 make claims of having sequenced even smaller bacterial genomes. Carsonella ruddii, an endosymbiont (an organism that lives inside the body or cells of another organism) of the psyllid Pachypsylla venusta, has a genome size of approximately 160 kb with 182 genes. Buchnera aphidicola, an endosymbiont of the aphid Cinara cedri, has the smallest known genome size of its genus at approximately 422 kb with 362 genes. (The articles did not address the number of genes that are essential, but even so, the numbers of total genes of these bacteria are lower than the number of “essential” genes in M. genitalium.)

Both C. ruddii and B. aphidicola are endosymbionts of insects in the order Homoptera. This is one of the most destructive orders of insects, which includes cicadas. The bacteria are assumed to have evolved with the insects over millions of years. In the process of the bacteria becoming dependent on the insects the bacteria have “lost” genes. Their genomes are now smaller than when they were free-living (before they became endosymbionts).

Endosymbiotic Relationships of Bacteria and Insects

Many organisms, from insects to humans, have symbiotic relationships with bacteria. In humans, bacteria residing in our large intestine help to digest food components that would otherwise be impossible for us to digest. They also provide necessary vitamins. In return, the bacteria live in a protected environment rich in nutrients.

In insects, many times bacteria reside in specialized structures called mycetocytes (bacteriocytes), with each individual bacterium surrounded by a membrane that forms a vesicle called a symbiosome (bacteriome).3 The bacteria supply the insects with amino acids and other substances that fail to have been supplied by the insect’s diet. In return, the insects give the bacteria a safe, nutrient-rich place to reside. Psyllids and aphids mainly feed on plant sap. This diet is rich in sugars but low in amino acids; thus, the insect gets needed amino acids from the endosymbiotic bacteria.3 Even when the necessary amino acids are supplied by diet to aphids that are aposymbiotic (having no endosymbiotic bacteria), the aphids do not grow and reproduce as well as aphids with endosymbiotic bacteria.4

The relationship is complex and specific, with many aspects still poorly understood. As one article stated,

The implication is that the bacteria contribute directly to amino acid homeostasis in symbiotic aphids, i.e. control over the profile of amino acids provided by the bacteria is so fine-tuned that it maintains the optimal amino acid titers of the aphid body fluids.4

Evolutionists often try to argue that nature shows lots of “poor design,” and then say this is exactly what evolution would predict. If so, how does it also give sophisticated design and this type of “find tuning” (pardon the pun)?

Problems With “Evolving” Endosymbiotic Relationships

A major question for evolutionary theory is how psyllids and aphids would have survived while waiting for endosymbiotic relationships with bacteria to form. The bacteria live in specialized structures inside the insects—where did the information to form those structures come from? Some might say the insects initially had genes for making the necessary amino acids and lost them after they obtained the endosymbiotic bacteria. But then why did these insects need endosymbiotic bacteria if they could make amino acids themselves? (This is a similar problem to the endosymbiotic theory of mitochondria and plastid origin in eukaryotic cells.) Many of the genes necessary for mitochondria and plastids to function are contained in the nuclear genome. Where did the information come from to form the pathways necessary to transport the proteins back to the organelles so they could function? If the organelles aren’t functioning (still waiting for those transport pathways to form), where is the selection pressure for the eukaryotic cell to keep them?

In both Science articles the repeated mantra is that the endosymbiotic bacteria had “lost” genes. One article stated, “Genome reduction in endosymbiotic bacteria is a continuous process derived from their adaptation to intracellular life.”1 (emphasis added). This assumption is based on genomic comparisons between different Buchnera in many species of aphids. The species selected for comparison are based on evolutionary trees of aphids and bacteria, and evolutionists’s beliefs about when the two became associated. One of the striking things seen in this comparison is the amount of genomic stasis. It has been proposed that the genomes of endosymbiotic bacteria have not undergone chromosome rearrangements or acquired new genes.5 This is presumed to be the case because they have “lost” the genes necessary for rearranging their chromosomes as well as genes necessary for other life processes while becoming endosymbiotic. Although natural selection should favor the bacteria keeping certain genes, genes are purportedly lost at a rate of one every 5 to 10 million years.6 The transfer of genes from the bacteria to the host and/or acquisition of a secondary endosymbiont are suggested as possible compensatory mechanisms. The other possibility is that the bacteria never had certain genes in the first place. The bacterial genomes are small because God created the bacteria to live in the insect in a symbiotic relationship. It is also possible that many of these symbiotic relationships quickly formed following the curse—and more may have occurred in the new post-Flood environment. Gene loss within the endosymbiotic bacteria may have occurred (or may still be occurring) as a result of the formation of these new symbiotic relationships.

Some fear that the endosymbiotic relationships between the insects and bacteria may one day end: “However for the smallest of small endosymbionts [referring to C. redii and B. aphidicola], the future seems gloomy. It is a dead end from which there is no escape.”6 Again, this is based on the evolutionary assumption that the bacteria are “losing” genes that are no longer needed and will someday not have enough to support their own life. Of course, this would also mean the end of the insect as well unless it can compensate (as mentioned above). But how long would it take for the compensatory mechanisms to develop and how would the insect survive until then? Evolutionary theory leaves us with more questions than answers. If these symbiotic relationships were a part of God’s original design or as a mechanism of survival in a post-Fall, post-Flood world then most likely they would not cease to exist. The truth given to us in the Bible provides the proper perspective for understanding the wonderful and amazing relationships that God has designed among His creation.


  1. Pérez-Brocal, Vicente, et al.,
    A Small Microbial Genome: The End of a Long Symbiotic Relationship,” Science, 314:312–313, 2006.
  2. Nakanachi, Atsushi, et al.,
    The 160-Kilobase Genome of the Bacterial Endosymbiont Carsonella,” Science, 314:267, 2006.
  3. Joerg Graf, “The Nutritional Symbiosis of Buchnera and Aphids.”
  4. Douglas, A.E., “Nutritional Interactions in Insect-Microbial Symbioses: Aphids and Their Symbiotic Bacteria Buchnera,” Annual Review of Entomology, 43:17–37, 1998.
  5. Tamas, Ivica, et al., “50 Million Years of Genomic Stasis in Endosymbiotic Bacteria,” Science, 296:2376–2379, 2002.
  6. Andersson, Siv, “The Bacterial World Gets Smaller,” Science, 314:259–260, 2006.


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