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theoretical minimal genome size for any living organism (Maniloff, 1996). In addition, the amino acid
and pantothenateÏcoenzyme A biosynthetic pathways of Buchnera are inextricably linked to aphid
metabolism (Shigenobu et al., 2000). As a consequence, Buchnera cannot be isolated into long-term
axenic culture, even though viable and metabolically active preparations of Buchnera can be maintained
for several hours in vitro (Whitehead and Douglas, 1993b). To quote Shigenobu et al. (2000), Ñthe gene
repertoire of the Buchnera genome is so specialized to intracellular life that it cannot survive outside
the eukaryotic cell.Ò
No accessory bacteria have been brought into axenic culture. There could be several reasons
for this. First, the correct conditions (especially media) may not have been tested yet. Second, the
accessory bacteria may be among the >90% of all bacteria that apparently are not amenable to
current culture methodologies. Finally, these bacteria, like Buchnera , may have an absolute require-
ment for conditions or resources provided by aphids. To date, virtually all research on the relation-
ship between aphids and their accessory bacteria has been conducted from the perspective of the
aphid (see below), and as a consequence we are ignorant of the importance of aphids as a habitat
for these taxa. We lack the information to answer such simple questions as: What proportion of
the global population of accessory bacteria is in aphids? And if all aphids went extinct today, would
their accessory bacteria go extinct too? The answers to these questions may vary among the different
taxa of accessory bacteria.
T HE A PHID
Aphids require their association with bacteria. When young larval aphids are administered
antibiotics at a dose that disrupts the bacteria but has minimal direct effects on aphid metab-
olism or behavior, the aphids grow very poorly and usually produce either no offspring or,
less commonly, a few offspring that are dead at birth or die within a few days without growing
or developing. Adults treated with antibiotics produce bacteria-free offspring that, in turn,
grow slowly and are reproductively sterile. The effect of antibiotic treatment on embryo growth
is particularly severe. As Figure 2.4 illustrates, the embryos in a young adult of A. pisum (11
days old) account for 65% of the total protein of aphids containing their bacteria but just 12%
of the protein in bacteria-free aphids. There is, however, no evidence that embryogenesis is
halted at a speciÝc developmental stage (Douglas, 1996). This dependence of aphids on their
bacteria has been attributed to Buchnera because only Buchnera , and none of the accessory
bacteria, is universal.
It is widely accepted that the requirement of aphids for Buchnera has a nutritional basis.
Aphids experimentally deprived of their bacteria are deÝcient in protein, but not lipid, and have
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FIGURE 2.4 Protein growth of pea aphids Acyrthosiphon pisum and their embryos. Total protein growth
(closed symbols) and embryo protein growth (open symbols) are shown for aphids containing (circles) and
experimentally deprived of their symbiotic bacteria (triangles). [Redrawn from Douglas, A.E. (1996). J. Insect
Physiol. 42: 247Ï255: Fig. 2.]
 
 
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