Buchnera Bacteria and Other Symbionts of Aphids - Insect Symbiosis - page 32

The Ýnal line of evidence for Buchnera provisioning of essential amino acids to aphids is
genomic. Despite its tiny size, the Buchnera genome includes virtually all the genes coding for
enzymes in the essential amino acid biosynthetic pathways (Shigenobu et al., 2000). The most
parsimonious explanation for this Ýnding is that Buchnera is under intense selection pressure to
retain these genes because Buchnera are consistently net providers of essential amino acids to the
aphid tissues. A further line of genomic evidence comes from the discovery of two plasmids in
Buchnera , one bearing the genes leuA-D coding for enzymes in the dedicated leucine biosynthetic
pathway and the other bearing multiple tandem repeats of trpEG , key genes in the tryptophan
biosynthetic pathway (Lai et al., 1994; Bracho et al., 1995). These genes are ampliÝed relative to
the chromosomal genes of Buchnera by up to 18 fold, varying among and within aphid species
(e.g., Lai et al., 1994; Thao et al., 1998; Birkle et al., in press). It is widely accepted that this gene
ampliÝcation promotes the production of leucine and tryptophan (Douglas, 1998; Moran and
Baumann, 2000), although this remains to be tested deÝnitively.
The strong evidence that essential amino acid production is the principal, and possibly sole,
function of Buchnera generates the prediction that Buchnera -free aphids raised on a diet with
optimally balanced amino acid content would perform as well as aphids containing Buchnera . This
expectation is not fulÝlled. For example, Buchnera -free Aphis fabae perform maximally on a
chemically deÝned diet containing 75% essential amino acids, but these aphids attain an adult
weight and fecundity that are 40% and 80% lower than aphids containing Buchnera (L.B. Minto
and A.E. Douglas, unpublished data). Douglas (1998) has suggested that either Buchnera fulÝll
other functions Ð e.g., provision of a vitamin, riboÞavin (Nakabachi and Ishikawa, 1999) Ð or
aphids have a limited capacity to assimilate dietary essential amino acids, reÞecting their long
evolutionary history with an endogenous (bacterial) source of these nutrients. The latter possibility
was tested by Douglas et al. (2001). Buchnera -free A. fabae showed poor assimilation efÝciency
for the essential amino acid leucine at 38%, but untreated aphids containing their normal comple-
ment of Buchnera had an assimilation efÝciency of 98%. These data suggest that aphids have very
effective essential amino acid uptake systems, but that one consequence of the depressed protein
synthesis in Buchnera -free aphids may be reduced synthesis of these key gut transporter proteins.
Thus, disruption of the bacteria eliminates an endogenous supply of essential amino acids and
impairs the aphid capacity to exploit exogenous sources of these nutrients.
An alternative approach to providing aphids with exogenous essential amino acids is to
inject them into the hemolymph. This treatment also fails to enhance aphid performance
(Wilkinson and Ishikawa, 2000) probably because hemolymph amino acids are turned over
very rapidly such that injected amino acids are cleared rapidly from the system by respiration
(Wilkinson et al., 2001b).
The demonstration that Buchnera provide aphids with essential amino acids raises many
issues that remain unresolved. Chief among them is the nature of the mobile compounds, i.e.,
the form in which the nutrients are transferred from Buchnera cells to the surrounding mycetocyte
cytoplasm and then distributed to the other aphid tissues. The Buchnera genome sequence offers
few clues because few transporter genes are present: a GlpF and OmpF-like porin that may
promote passive diffusion; a few ABC transporter genes and phosphoenolpyruvate-carbohydrate
phosphotransferase systems for transport of small molecules; and SecB, a protein that promotes
the correct conformation in proteins destined for export (Lai and Baumann, 1992; Shigenobu
et al., 2000). Perhaps both free amino acids and Buchnera proteins are released from Buchnera
cells and translocated across the symbiosomal membrane into the mycetocyte cytoplasm. The
recovery of one protein, GroEL, abundant in Buchnera cytoplasm, from aphid hemolymph (van
den Heuvel et al., 1994) suggests that this protein is made available to the aphid, although it is
unknown whether it is released from living Buchnera cells or lysed cells. The ultrastructure of
mycetocytes likewise offers few clues. The mycetocyte cytoplasm is not richly endowed with
ribosomes, suggesting that if Buchnera cells release amino acids at high rates, the mycetocytes
do not have the machinery to incorporate them into aphid protein. Additionally, the cell membrane